Macroalgae, or seaweed, are fast-growing marine organisms that use photosynthesis to incorporate carbon from seawater into their living tissue. Macroalgae flourish in coastal and some pelagic environments and a portion of the seaweed produced is exported naturally to the deep sea where it may be buried or remineralized by marine food webs. Slow turnover times in most deep ocean environments will sequester carbon released to bottom waters for centuries or longer. Recently, there have been proposals to cultivate macroalgae and then sink its biomass to the deep ocean to sequester carbon from the atmosphere. In theory, this could amplify the natural process of seaweed carbon sequestration and could remove additional carbon dioxide from the atmosphere,.   The cultivation of seaweed has been practiced for centuries. Asia continues to dominate seaweed mariculture, producing 99.5% of global yields, with the vast majority coming from just four countries: China, Indonesia, the Republic of Korea, and the Philippines. In 2015 the global production of seaweed totaled 30.4 million tonnes, of which over 1.1 million tonnes were wild harvest and 29.4 million tonnes were cultivated (in freshwater, brackish, and saltwater). Chile, China, and Norway were the leading producers of wild species while China, Indonesia, the Republic of Korea, and the Philippines led in cultivated species. Today, the global seaweed industry is worth over USD 6 billion per annum. In 2018, 32.4 million tonnes of aquatic algae were wild-collected and cultivated with a majority of macroalgal farming taking place in coastal waters. Macroalgae is used in a vast array of food and non-food items globally but has yet to be farmed specifically for CDR purposes (although a growing number of start-ups are proposing this. While historically dominated by a small number of countries, algae farming is becoming increasingly popular in Europe and North America, with a particular interest in climate change mitigation and contribution to the blue economy. There are wide knowledge and technology disparities across the globe with Asian countries, the European Union, and the United States leading the way with the most advanced farming practices including, integrated multi-trophic aquaculture. History shows successful long-term cultivation and farming of seaweeds, and a revived interest in this aquatic organism promises great potential for innovation and climate change mitigation solutions.

Macroalgae, or seaweed, are fast-growing marine organisms that use photosynthesis to incorporate carbon from seawater into their living tissue. Macroalgae flourish in coastal and some pelagic environments and a portion of the seaweed produced is exported naturally to the deep sea where it may be buried or remineralized by marine food webs. Slow turnover times in most deep ocean environments will sequester carbon released to bottom waters for centuries or longer. Recently, there have been proposals to cultivate macroalgae and then sink its biomass to the deep ocean to sequester carbon from the atmosphere. In theory, this could amplify the natural process of seaweed carbon sequestration and could remove additional carbon dioxide from the atmosphere.   The cultivation of seaweed has been practiced for centuries. Asia continues to dominate seaweed mariculture, producing 99.5% of global yields, with the vast majority coming from just four countries: China, Indonesia, the Republic of Korea, and the Philippines. In 2015 the global production of seaweed totaled 30.4 million tonnes, of which over 1.1 million tonnes were wild harvest and 29.4 million tonnes were cultivated (in freshwater, brackish, and saltwater). Chile, China, and Norway were the leading producers of wild species while China, Indonesia, the Republic of Korea, and the Philippines led in cultivated species. Today, the global seaweed industry is worth over USD 6 billion per annum. In 2018, 32.4 million tonnes of aquatic algae were wild-collected and cultivated with a majority of macroalgal farming taking place in coastal waters. Macroalgae is used in a vast array of food and non-food items globally but has yet to be farmed specifically for CDR purposes (although a growing number of start-ups are proposing this. While historically dominated by a small number of countries, algae farming is becoming increasingly popular in Europe and North America, with a particular interest in climate change mitigation and contribution to the blue economy. There are wide knowledge and technology disparities across the globe with Asian countries, the European Union, and the United States leading the way with the most advanced farming practices including, integrated multi-trophic aquaculture. History shows successful long-term cultivation and farming of seaweeds, and a revived interest in this aquatic organism promises great potential for innovation and climate change mitigation solutions.

Macroalgae, or seaweed, are fast-growing marine organisms that use photosynthesis to incorporate carbon from seawater into their living tissue. Macroalgae flourish in coastal and some pelagic environments and a portion of the seaweed produced is exported naturally to the deep sea where it may be buried or remineralized by marine food webs^[7]Krause-Jensen, D., Duarte, C. Substantial role of macroalgae in marine carbon sequestration. Nature Geosci 9, 737–742 (2016). https://doi.org/10.1038/ngeo2790 . Slow turnover times in most deep ocean environments will sequester carbon released to bottom waters for centuries or longer{{13}}. Recently, there have been proposals to cultivate macroalgae and then sink its biomass to the deep ocean to sequester carbon from the atmosphere. In theory, this could amplify the natural process of seaweed carbon sequestration and could remove additional carbon dioxide from the atmosphere^[6]GESAMP (2019). “High level review of a wide range of proposed marine geoengineering techniques”. (Boyd, P.W. and Vivian, C.M.G., eds.). (IMO/FAO/UNESCO-IOC/UNIDO/WMO/IAEA/UN/UN Environment/ UNDP/ISA Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). Rep. Stud. GESAMP No. 98, 144 p. {{14}}.   The cultivation of seaweed has been practiced for centuries. Asia continues to dominate seaweed mariculture, producing 99.5% of global yields, with the vast majority coming from just four countries: China, Indonesia, the Republic of Korea, and the Philippines. In 2015 the global production of seaweed totaled 30.4 million tonnes, of which over 1.1 million tonnes were wild harvest and 29.4 million tonnes were cultivated (in freshwater, brackish, and saltwater). Chile, China, and Norway were the leading producers of wild species while China, Indonesia, the Republic of Korea, and the Philippines led in cultivated species. Today, the global seaweed industry is worth over USD 6 billion per annum. In 2018, 32.4 million tonnes of aquatic algae were wild-collected and cultivated with a majority of macroalgal farming taking place in coastal waters (FAO. 2018. The global status of seaweed production, trade and utilization. Globefish Research  Programme Volume 124. Rome. 120 pp. License: CC BY-NC-SA 3.0 IGO.). Macroalgae is used in a vast array of food and non-food items globally but has yet to be farmed specifically for CDR purposes (although a growing number of start-ups are proposing this^[5]"Running Tide". https://www.runningtide.com/removing . While historically dominated by a small number of countries, algae farming is becoming increasingly popular in Europe and North America, with a particular interest in climate change mitigation and contribution to the blue economy. There are wide knowledge and technology disparities across the globe with Asian countries, the European Union, and the United States leading the way with the most advanced farming practices including, integrated multi-trophic aquaculture^[2]Thierry Chopin & Albert G. J. Tacon (2020): Importance of Seaweeds and Extractive Species in Global Aquaculture Production, Reviews in Fisheries Science & Aquaculture, DOI: 10.1080/23308249.2020.1810626 ^[3]García-Poza, Sara, Adriana Leandro, Carla Cotas, João Cotas, João C. Marques, Leonel Pereira, and Ana M. M. Gonçalves. “The Evolution Road of Seaweed Aquaculture: Cultivation Technologies and the Industry 4.0.” International Journal of Environmental Research and Public Health 17, no. 18 (September 8, 2020): 6528. https://doi.org/10.3390/ijerph17186528. . History shows successful long-term cultivation and farming of seaweeds, and a revived interest in this aquatic organism promises great potential for innovation and climate change mitigation solutions^[4]Duarte CM, Wu J, Xiao X, Bruhn A and Krause-Jensen D (2017) Can Seaweed Farming Play a Role in Climate Change Mitigation and Adaptation? Front. Mar. Sci. 4:100. doi: 10.3389/fmars.2017.00100 .

The cultivation of seaweed has been practiced for centuries. Asia continues to dominate seaweed mariculture, producing 99.5% of global yields, with the vast majority coming from just four countries: China, Indonesia, the Republic of Korea, and the Philippines. In 2015 the global production of seaweed totaled 30.4 million tonnes, of which over 1.1 million tonnes were wild harvest and 29.4 million tonnes were cultivated (in freshwater, brackish, and saltwater). Chile, China, and Norway were the leading producers of wild species while China, Indonesia, the Republic of Korea, and the Philippines led in cultivated species.   Today, the global seaweed industry is worth over USD 6 billion per annum. In 2018, 32.4 million tonnes of aquatic algae were wild-collected and cultivated with a majority of macroalgal farming taking place in coastal waters. Macroalgae is used in a vast array of food and non-food items globally but has yet to be farmed specifically for CDR purposes (although a growing number of start-ups are proposing this). While historically dominated by a small number of countries, algae farming is becoming increasingly popular in Europe and North America, with a particular interest in climate change mitigation and contribution to the blue economy.   There are wide knowledge and technology disparities across the globe with Asian countries, the European Union, and the United States leading the way with the most advanced farming practices including, integrated multi-trophic aquaculture ,. History shows successful long-term cultivation and farming of seaweeds, and a revived interest in this aquatic organism promises great potential for innovation and climate change mitigation solutions.

The cultivation of seaweed has been practiced for centuries. Asia continues to dominate seaweed mariculture, producing 99.5% of global yields, with the vast majority coming from just four countries: China, Indonesia, Republic of Korea, and the Philippines. In 2015 the global production of seaweed totaled 30.4 million tonnes, of which over 1.1 million tonnes were wild harvest and 29.4 million tonnes were cultivated (in freshwater, brackish, and salt water). Chile, China, and Norway were the leading producers of wild species while China, Indonesia, the Republic of Korea, and the Philippines led in cultivated species.   Today, the global seaweed industry is worth over USD 6 billion per annum. In 2018, 32.4 million tonnes of aquatic algae were wild-collected and cultivated with a majority of macroalgal farming taking place in coastal waters. Macroalgae is used in a vast array of food and non-food items globally but has yet to be farmed specifically for CDR purposes (although a growing number of start-ups are proposing this). While historically dominated by a small number of countries, algae farming is becoming increasingly popular in Europe and North America, with a particular interest in climate change mitigation and contribution to the blue economy.   There are wide knowledge and technology disparities across the globe with Asian countries, the European Union, and the United States leading the way with the most advanced farming practices including, integrated multi-trophic aquaculture ,. History shows successful long-term cultivation and farming of seaweeds, and a revived interest in this aquatic organism promises great potential for innovation and climate change mitigation solutions.

The cultivation of seaweed has been practiced for centuries. Asia continues to dominate seaweed mariculture, producing 99.5% of global yields, with the vast majority coming from just four countries: China, Indonesia, Republic of Korea, and the Philippines. In 2015 the global production of seaweed totaled 30.4 million tonnes, of which over 1.1 million tonnes were wild harvest and 29.4 million tonnes were cultivated (in freshwater, brackish, and salt water). Chile, China, and Norway were the leading producers of wild species while China, Indonesia, the Republic of Korea, and the Philippines led in cultivated species.   Today, the global seaweed industry is worth over USD 6 billion per annum. In 2018, 32.4 million tonnes of aquatic algae were wild-collected and cultivated with a majority of macroalgal farming taking place in coastal waters.^[1]FAO. 2018. The global status of seaweed production, trade and utilization. Globefish Research Programme Volume 124. Rome. 120 pp. License: CC BY-NC-SA 3.0 IGO. Macroalgae is used in a vast array of food and non-food items globally but has yet to be farmed specifically for CDR purposes (although a growing number of start-ups are proposing this{{13}}). While historically dominated by a small number of countries, algae farming is becoming increasingly popular in Europe and North America, with a particular interest in climate change mitigation and contribution to the blue economy.   There are wide knowledge and technology disparities across the globe with Asian countries, the European Union, and the United States leading the way with the most advanced farming practices including, integrated multi-trophic aquaculture ^[2]Thierry Chopin & Albert G. J. Tacon (2020): Importance of Seaweeds and Extractive Species in Global Aquaculture Production, Reviews in Fisheries Science & Aquaculture, DOI: 10.1080/23308249.2020.1810626 ,^[3]García-Poza, Sara, Adriana Leandro, Carla Cotas, João Cotas, João C. Marques, Leonel Pereira, and Ana M. M. Gonçalves. “The Evolution Road of Seaweed Aquaculture: Cultivation Technologies and the Industry 4.0.” International Journal of Environmental Research and Public Health 17, no. 18 (September 8, 2020): 6528. https://doi.org/10.3390/ijerph17186528. . History shows successful long-term cultivation and farming of seaweeds, and a revived interest in this aquatic organism promises great potential for innovation and climate change mitigation solutions^[4]Duarte CM, Wu J, Xiao X, Bruhn A and Krause-Jensen D (2017) Can Seaweed Farming Play a Role in Climate Change Mitigation and Adaptation? Front. Mar. Sci. 4:100. doi: 10.3389/fmars.2017.00100 .

The cultivation of seaweed has been practiced for centuries. Asia continues to dominate seaweed mariculture, producing 99.5% of global yields, with the vast majority coming from just four countries: China, Indonesia, Republic of Korea, and the Philippines. In 2015 the global production of seaweed totaled 30.4 million tonnes, of which over 1.1 million tonnes were wild harvest and 29.4 million tonnes were cultivated (in freshwater, brackish, and salt water). Chile, China, and Norway were the leading producers of wild species while China, Indonesia, the Republic of Korea, and the Philippines led in cultivated species.

 

Today, the global seaweed industry is worth over USD 6 billion per annum. In 2018, 32.4 million tonnes of aquatic algae were wild-collected and cultivated with a majority of macroalgal farming taking place in coastal waters.^[1]FAO. 2018. The global status of seaweed production, trade and utilization. Globefish Research Programme Volume 124. Rome. 120 pp. License: CC BY-NC-SA 3.0 IGO. Macroalgae is used in a vast array of food and non-food items globally but has yet to be farmed specifically for CDR purposes. While historically dominated by a small number of countries, algae farming is becoming increasing popular in Europe and North America, with a particular interest in climate change mitigation and contribution to the blue economy.

 

There are wide knowledge and technology disparities across the globe with Asian countries, the European Union, and the United States leading the way with the most advanced farming practices including, integrated multi-trophic aquaculture ^[2]Thierry Chopin & Albert G. J. Tacon (2020): Importance of Seaweeds and Extractive Species in Global Aquaculture Production, Reviews in Fisheries Science & Aquaculture, DOI: 10.1080/23308249.2020.1810626 ,^[3]García-Poza, Sara, Adriana Leandro, Carla Cotas, João Cotas, João C. Marques, Leonel Pereira, and Ana M. M. Gonçalves. “The Evolution Road of Seaweed Aquaculture: Cultivation Technologies and the Industry 4.0.” International Journal of Environmental Research and Public Health 17, no. 18 (September 8, 2020): 6528. https://doi.org/10.3390/ijerph17186528. . History shows successful long-term cultivation and farming of seaweeds, and a revived interest in this aquatic organism promises great potential for innovation and climate change mitigation solutions^[4]Duarte CM, Wu J, Xiao X, Bruhn A and Krause-Jensen D (2017) Can Seaweed Farming Play a Role in Climate Change Mitigation and Adaptation? Front. Mar. Sci. 4:100. doi: 10.3389/fmars.2017.00100 .

The cultivation of seaweed has been practiced for centuries. Asia continues to dominate seaweed mariculture, producing 99.5% of global yields, with the vast majority coming from just four countries: China, Indonesia, Republic of Korea, and the Philippines. In 2015 the global production of seaweed totaled 30.4 million tonnes, of which over 1.1 million tonnes were wild harvest and 29.4 million tonnes were cultivated (in freshwater, brackish, and salt water). Chile, China, and Norway were the leading producers of wild species while China, Indonesia, the Republic of Korea, and the Philippines led in cultivated species.   Today, the global seaweed industry is worth over USD 6 billion per annum. In 2018, 32.4 million tonnes of aquatic algae were wild-collected and cultivated with a majority of macroalgal farming taking place in coastal waters.^[1]FAO. 2018. The global status of seaweed production, trade and utilization. Globefish Research Programme Volume 124. Rome. 120 pp. License: CC BY-NC-SA 3.0 IGO. Macroalgae is used in a vast array of food and non-food items globally but has yet to be farmed specifically for CDR purposes. While historically dominated by a small number of countries, algae farming is becoming increasing popular in Europe and North America, with a particular interest in climate change mitigation and contribution to the blue economy.   There are wide knowledge and technology disparities across the globe with Asian countries, the European Union, and the United States leading the way with the most advanced farming practices including, integrated multi-trophic aquaculture ^[2]Thierry Chopin & Albert G. J. Tacon (2020): Importance of Seaweeds and Extractive Species in Global Aquaculture Production, Reviews in Fisheries Science & Aquaculture, DOI: 10.1080/23308249.2020.1810626 ,^[3]García-Poza, Sara, Adriana Leandro, Carla Cotas, João Cotas, João C. Marques, Leonel Pereira, and Ana M. M. Gonçalves. “The Evolution Road of Seaweed Aquaculture: Cultivation Technologies and the Industry 4.0.” International Journal of Environmental Research and Public Health 17, no. 18 (September 8, 2020): 6528. https://doi.org/10.3390/ijerph17186528. . History shows successful long-term cultivation and farming of seaweeds, and a revived interest in this aquatic organism promises great potential for innovation and climate change mitigation solutions^[4]Duarte CM, Wu J, Xiao X, Bruhn A and Krause-Jensen D (2017) Can Seaweed Farming Play a Role in Climate Change Mitigation and Adaptation? Front. Mar. Sci. 4:100. doi: 10.3389/fmars.2017.00100 .

The cultivation of seaweed has been practiced for centuries. Asia continues to dominate seaweed mariculture, producing 99.5% of global yields, with the vast majority coming from just four countries: China, Indonesia, Republic of Korea, and the Philippines. In 2015 the global production of seaweed totaled 30.4 million tonnes, of which over 1.1 million tonnes were wild harvest and 29.4 million tonnes were cultivated (in freshwater, brackish, and salt water). Chile, China, and Norway were the leading producers of wild species while China, Indonesia, the Republic of Korea, and the Philippines led in cultivated species. Today, the global seaweed industry is worth over USD 6 billion per annum. In 2018, 32.4 million tonnes of aquatic algae were wild-collected and cultivated with a majority of macroalgal farming taking place in coastal waters.^[1]FAO. 2018. The global status of seaweed production, trade and utilization. Globefish Research Programme Volume 124. Rome. 120 pp. License: CC BY-NC-SA 3.0 IGO. Macroalgae is used in a vast array of food and non-food items globally but has yet to be farmed specifically for CDR purposes. While historically dominated by a small number of countries, algae farming is becoming increasing popular in Europe and North America, with a particular interest in climate change mitigation and contribution to the blue economy. There are wide knowledge and technology disparities across the globe with Asian countries, the European Union, and the United States leading the way with the most advanced farming practices including, integrated multi-trophic aquaculture ^[2]Thierry Chopin & Albert G. J. Tacon (2020): Importance of Seaweeds and Extractive Species in Global Aquaculture Production, Reviews in Fisheries Science & Aquaculture, DOI: 10.1080/23308249.2020.1810626 ,^[3]García-Poza, Sara, Adriana Leandro, Carla Cotas, João Cotas, João C. Marques, Leonel Pereira, and Ana M. M. Gonçalves. “The Evolution Road of Seaweed Aquaculture: Cultivation Technologies and the Industry 4.0.” International Journal of Environmental Research and Public Health 17, no. 18 (September 8, 2020): 6528. https://doi.org/10.3390/ijerph17186528. . History shows successful long-term cultivation and farming of seaweeds, and a revived interest in this aquatic organism promises great potential for innovation and climate change mitigation solutions^[4]Duarte CM, Wu J, Xiao X, Bruhn A and Krause-Jensen D (2017) Can Seaweed Farming Play a Role in Climate Change Mitigation and Adaptation? Front. Mar. Sci. 4:100. doi: 10.3389/fmars.2017.00100 .

1. Carbon Capture - The ultimate CDR potential of large-scale macroalgal cultivation and sequestration is as yet difficult to determine. Recent consensus reports cite the following as possible carbon dioxide removal potential from macroalgae cultivation and sequestration:
   • 0.1 – 1.0 Gt CO₂/year (NASEM 2022: A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration)
   • 0.1 – 0.6 Gt CO₂/ year (NOAA 2022: Carbon Dioxide Removal Research)

Theoretically, macroalgal cultivation and sequestration  could be scaled to between one and greater than ten gigatons of CDR per year, but realized CDR may be much lower due to a number of factors, including:

• • Competition for space with existing ocean stakeholders – commercial shipping,  commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc. 
  • Challenges with expanding to offshore environments – engineering of moorings  and farm structures, challenges with operations and maintenance in distant environments,- and the likely associated cost increases (at least at first)  associated with offshore operations
  • Most open ocean environments require nutrient additions to sustain macroalgal yields. If these nutrients come from the deep ocean, they will be accompanied by high CO₂ water that can be released and will offset macroalgal carbon sequestration to some degree (which needs to be quantified).,
    • If macroalgal farms are located closer to the coastal zone, excess anthropogenic nitrogen inputs could support ~3.5 million km2 of macroalgal cultivation, corresponding with an estimated CDR range of 5- 10 Gt CDR annually, alleviating competition for “natural” nutrient supplies with phytoplankton and alleviating the need for an upwelled supply of nutrients (and the associated risks of releasing CO₂ from upwelled deep ocean waters).
  • It is hypothesized that macroalgae-based pathways could supply CDR at moderate cost ($25-125 per ton of CO₂ removed),,, but these cost estimates are preliminary and subject to change as macroalgae-based pathways advance in technological readiness.
    • Researchers at UC Irvine are developing a biophysical-economic model to better understand the geophysical, biological, and economic sensitivities of macroalgae-based CDR pathways

 

2. Sequestration Permanence -For cultivated macroalgae to contribute to CDR, the carbon captured in the macroalgal tissue needs to be sequestered to prevent its decomposition and return to the atmosphere. Sequestration pathways include:

• • Intentionally sinking the macroalgae (pre- or post-processing) into the deep ocean (>1000 meters depth) where it can be sequestered from contact with the atmosphere for hundreds to a few thousand years (if remineralized in the deep ocean) or for thousands-to-millions of years (if buried in marine sediments).
    • There are natural analogues to this process already. In macroalgal systems, already ~50% of natural macroalgal production breaks off and becomes dissolved or particulate organic matter. Some of the particulate organic matter is transported to the deep ocean, where the carbon is sequestered.
  • Relying on sedimentary burial below or adjacent to the farm (typically more applicable for farms not located over deep water).
    • Oceans2050 is working to determine the permanence of burial under coastal farms
  • Harvesting the macroalgae for:
    • Bioenergy
      • Combining combustion pathways with carbon capture and storage (thousands-to-millions of years if stored in a geologic reservoir)
      • Pyrolysis, resulting in biochar, (hundreds-to-thousands of years)
    • Production of long-lived bioproducts, such as bioplastics, that are capable of permanent (>100 years) sequestration{{18}

3. Differentiating from Avoided Emissions - Macroalgae can provide a number of other services that either reduce or replace greenhouse gas emissions, but do not remove legacy carbon dioxide pollution from the atmosphere. One example of reducing emissions: red algal supplements added to cattle feed may play a key role in reducing enteric methane emissions from cattle by over 50%,. Given methane’s potency as a greenhouse gas, such emissions reductions may play an outsized role in slowing planetary temperature increases through the reduction of greenhouse gases. Macroalgae biomass may also be converted into short-lived, high-value bioproducts, such as food and nutritional supplements. This process may contribute to avoided emissions if it substitutes for carbon-intensive feedstocks, but it does not represent CDR because of the short timescale over which these products sequester carbon before it returns to the atmosphere. Although the focus of this roadmap is on macroalgae pathways for CDR, many of the obstacles faced, development needs, and near-term opportunities for advancement are relevant to these associated pathways for avoided greenhouse gas emissions.

1. Carbon Capture - The ultimate CDR potential of large-scale macroalgal cultivation and sequestration is as yet difficult to determine. Theoretically, macroalgal cultivation and sequestration  could be scaled to between one and greater than ten gigatons of CDR per year,  but realized CDR may be much lower due to a number of factors, including:
   • Competition for space with existing ocean stakeholders – commercial shipping,  commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc. 
   • Challenges with expanding to offshore environments – engineering of moorings  and farm structures, challenges with operations and maintenance in distant environments,- and the likely associated cost increases (at least at first)  associated with offshore operations
   • Most open ocean environments require nutrient additions to sustain macroalgal yields. If these nutrients come from the deep ocean, they will be accompanied by high CO₂ water that can be released and will offset macroalgal carbon sequestration to some degree (which needs to be quantified).,
     • If macroalgal farms are located closer to the coastal zone, excess anthropogenic nitrogen inputs could support ~3.5 million km2 of macroalgal cultivation, corresponding with an estimated CDR range of 5- 10 Gt CDR annually, alleviating competition for “natural” nutrient supplies with phytoplankton and alleviating the need for an upwelled supply of nutrients (and the associated risks of releasing CO₂ from upwelled deep ocean waters).
   • It is hypothesized that macroalgae-based pathways could supply CDR at moderate cost ($25-125 per ton of CO₂ removed),, but these cost estimates are preliminary and subject to change as macroalgae-based pathways advance in technological readiness.
     • Researchers at UC Irvine are developing a biophysical-economic model to better understand the geophysical, biological, and economic sensitivities of macroalgae-based CDR pathways
    
2. Sequestration Permanence -For cultivated macroalgae to contribute to CDR, the carbon captured in the macroalgal tissue needs to be sequestered to prevent its decomposition and return to the atmosphere. Sequestration pathways include:
   • Intentionally sinking the macroalgae (pre- or post-processing) into the deep ocean (>1000 meters depth) where it can be sequestered from contact with the atmosphere for hundreds to a few thousand years (if remineralized in the deep ocean) or for thousands-to-millions of years (if buried in marine sediments).
     • There are natural analogues to this process already. In macroalgal systems, already ~50% of natural macroalgal production breaks off and becomes dissolved or particulate organic matter. Some of the particulate organic matter is transported to the deep ocean, where the carbon is sequestered.
   • Relying on sedimentary burial below or adjacent to the farm (typically more applicable for farms not located over deep water).
     • Oceans2050 is working to determine the permanence of burial under coastal farms
   • Harvesting the macroalgae for:
     • Bioenergy
       • Combining combustion pathways with carbon capture and storage (thousands-to-millions of years if stored in a geologic reservoir)
       • Pyrolysis, resulting in biochar, (hundreds-to-thousands of years)
     • Production of long-lived bioproducts, such as bioplastics, that are capable of permanent (>100 years) sequestration
3. Differentiating from Avoided Emissions - Macroalgae can provide a number of other services that either reduce or replace greenhouse gas emissions, but do not remove legacy carbon dioxide pollution from the atmosphere. One example of reducing emissions: red algal supplements added to cattle feed may play a key role in reducing enteric methane emissions from cattle by over 50%,. Given methane’s potency as a greenhouse gas, such emissions reductions may play an outsized role in slowing planetary temperature increases through the reduction of greenhouse gases. Macroalgae biomass may also be converted into short-lived, high-value bioproducts, such as food and nutritional supplements. This process may contribute to avoided emissions if it substitutes for carbon-intensive feedstocks, but it does not represent CDR because of the short timescale over which these products sequester carbon before it returns to the atmosphere.Although the focus of this roadmap is on macroalgae pathways for CDR, many of the obstacles faced, development needs, and near-term opportunities for advancement are relevant to these associated pathways for avoided greenhouse gas emissions.

1. Carbon Capture - The ultimate CDR potential of large-scale macroalgal cultivation and sequestration is as yet difficult to determine. Theoretically, macroalgal cultivation and sequestration  could be scaled to between one and greater than ten gigatons of CDR per year,  but realized CDR may be much lower due to a number of factors, including:
   • Competition for space with existing ocean stakeholders – commercial shipping,  commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc. 
   • Challenges with expanding to offshore environments – engineering of moorings  and farm structures, challenges with operations and maintenance in distant environments,- and the likely associated cost increases (at least at first)  associated with offshore operations
   • Most open ocean environments require nutrient additions to sustain macroalgal yields. If these nutrients come from the deep ocean, they will be accompanied by high CO₂ water that can be released and will offset macroalgal carbon sequestration to some degree (which needs to be quantified).,
     • If macroalgal farms are located closer to the coastal zone, excess anthropogenic nitrogen inputs could support ~3.5 million km2 of macroalgal cultivation, corresponding with an estimated CDR range of 5- 10 Gt CDR annually, alleviating competition for “natural” nutrient supplies with phytoplankton and alleviating the need for an upwelled supply of nutrients (and the associated risks of releasing CO₂ from upwelled deep ocean waters).
   • It is hypothesized that macroalgae-based pathways could supply CDR at moderate cost ($25-125 per ton of CO₂ removed),, but these cost estimates are preliminary and subject to change as macroalgae-based pathways advance in technological readiness.
     • Researchers at UC Irvine are developing a biophysical-economic model to better understand the geophysical, biological, and economic sensitivities of macroalgae-based CDR pathways
    
2. Sequestration Permanence -For cultivated macroalgae to contribute to CDR, the carbon captured in the macroalgal tissue needs to be sequestered to prevent its decomposition and return to the atmosphere. Sequestration pathways include:
   • Intentionally sinking the macroalgae (pre- or post-processing) into the deep ocean (>1000 meters depth) where it can be sequestered from contact with the atmosphere for hundreds to a few thousand years (if remineralized in the deep ocean) or for thousands-to-millions of years (if buried in marine sediments).
     • There are natural analogues to this process already. In macroalgal systems, already ~50% of natural macroalgal production breaks off and becomes dissolved or particulate organic matter. Some of the particulate organic matter is transported to the deep ocean, where the carbon is sequestered.
   • Relying on sedimentary burial below or adjacent to the farm (typically more applicable for farms not located over deep water).
     • Oceans2050 is working to determine the permanence of burial under coastal farms
   • Harvesting the macroalgae for:
     • Bioenergy
       • Combining combustion pathways with carbon capture and storage (thousands-to-millions of years if stored in a geologic reservoir)
       • Pyrolysis, resulting in biochar, (hundreds-to-thousands of years)
     • Production of long-lived bioproducts, such as bioplastics, that are capable of permanent (>100 years) sequestration
3. Differentiating from Avoided Emissions - Macroalgae can provide a number of other services that either reduce or replace greenhouse gas emissions, but do not remove legacy carbon dioxide pollution from the atmosphere. One example of reducing emissions: red algal supplements added to cattle feed may play a key role in reducing enteric methane emissions from cattle by over 50%,. Given methane’s potency as a greenhouse gas, such emissions reductions may play an outsized role in slowing planetary temperature increases through the reduction of greenhouse gases. Macroalgae biomass may also be converted into short-lived, high-value bioproducts, such as food and nutritional supplements. This process may contribute to avoided emissions if it substitutes for carbon-intensive feedstocks, but it does not represent CDR because of the short timescale over which these products sequester carbon before it returns to the atmosphere.Although the focus of this roadmap is on macroalgae pathways for CDR, many of the obstacles faced, development needs, and near-term opportunities for advancement are relevant to these associated pathways for avoided greenhouse gas emissions.

1. Carbon Capture - The ultimate CDR potential of large-scale macroalgal cultivation and sequestration is as yet difficult to determine. Theoretically, macroalgal cultivation and sequestration  could be scaled to between one and greater than ten gigatons of CDR per year,  but realized CDR may be much lower due to a number of factors, including:
   • Competition for space with existing ocean stakeholders – commercial shipping,  commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc. 
   • Challenges with expanding to offshore environments – engineering of moorings  and farm structures, challenges with operations and maintenance in distant environments,- and the likely associated cost increases (at least at first)  associated with offshore operations
   • Most open ocean environments require nutrient additions to sustain macroalgal yields. If these nutrients come from the deep ocean, they may be accompanied by high CO₂ water that can be released and will offset macroalgal carbon sequestration to some degree (which needs to be quantified).,
     • If macroalgal farms are located closer to the coastal zone, excess anthropogenic nitrogen inputs could support ~3.5 million km2 of macroalgal cultivation, corresponding with an estimated CDR range of 5- 10 Gt CDR annually, alleviating competition for “natural” nutrient supplies with phytoplankton and alleviating the need for an upwelled supply of nutrients (and the associated risks of releasing CO₂ from upwelled deep ocean waters).
   • It is hypothesized that macroalgae-based pathways could supply CDR at moderate cost ($25-125 per ton of CO₂ removed),, but these cost estimates are preliminary and subject to change as macroalgae-based pathways advance in technological readiness.
     • Researchers at UC Irvine are developing a biophysical-economic model to better understand the geophysical, biological, and economic sensitivities of macroalgae-based CDR pathways
    
2. Sequestration Permanence -For cultivated macroalgae to contribute to CDR, the carbon captured in the macroalgal tissue needs to be sequestered to prevent its decomposition and return to the atmosphere. Sequestration pathways include:
   • Intentionally sinking the macroalgae (pre- or post-processing) into the deep ocean (>1000 meters depth) where it can be sequestered from contact with the atmosphere for hundreds to a few thousand years (if remineralized in the deep ocean) or for thousands-to-millions of years (if buried in marine sediments).
     • There are natural analogues to this process already. In macroalgal systems, already ~50% of natural macroalgal production breaks off and becomes dissolved or particulate organic matter. Some of the particulate organic matter is transported to the deep ocean, where the carbon is sequestered.
   • Relying on sedimentary burial below or adjacent to the farm (typically more applicable for farms not located over deep water).
     • Oceans2050 is working to determine the permanence of burial under coastal farms
   • Harvesting the macroalgae for:
     • Bioenergy
       • Combining combustion pathways with carbon capture and storage (thousands-to-millions of years if stored in a geologic reservoir)
       • Pyrolysis, resulting in biochar, (hundreds-to-thousands of years)
     • Production of long-lived bioproducts, such as bioplastics, that are capable of permanent (>100 years) sequestration
3. Differentiating from Avoided Emissions - Macroalgae can provide a number of other services that either reduce or replace greenhouse gas emissions, but do not remove legacy carbon dioxide pollution from the atmosphere. One example of reducing emissions: red algal supplements added to cattle feed may play a key role in reducing enteric methane emissions from cattle by over 50%,. Given methane’s potency as a greenhouse gas, such emissions reductions may play an outsized role in slowing planetary temperature increases through the reduction of greenhouse gases. Macroalgae biomass may also be converted into short-lived, high-value bioproducts, such as food and nutritional supplements. This process may contribute to avoided emissions if it substitutes for carbon-intensive feedstocks, but it does not represent CDR because of the short timescale over which these products sequester carbon before it returns to the atmosphere.Although the focus of this roadmap is on macroalgae pathways for CDR, many of the obstacles faced, development needs, and near-term opportunities for advancement are relevant to these associated pathways for avoided greenhouse gas emissions.

1. Carbon Capture - The ultimate CDR potential of large-scale macroalgal cultivation and sequestration is as yet difficult to determine. Theoretically, macroalgal cultivation and sequestration  could be scaled to between one and greater than ten gigatons of CDR per year^[1]Energy Futures Initiative. “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. ,  but realized CDR may be much lower due to a number of factors, including:
   • Competition for space with existing ocean stakeholders – commercial shipping,  commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc. 
   • Challenges with expanding to offshore environments – engineering of moorings  and farm structures, challenges with operations and maintenance in distant environments^[2]Bak, Urd Grandorf, Gregersen, Ólavur and Infante, Javier. "Technical challenges for offshore cultivation of kelp species: lessons learned and future directions" Botanica Marina, vol. 63, no. 4, 2020, pp. 341- 353. https://doi-org.oca.ucsc.edu/10.1515/bot-2019-0005 ,^[3]Lovatelli, A., Aguilar-Manjarrez, J. and Soto, D. (2013). Expanding mariculture farther offshore: technical, environmental, spatial and governance challenges. In: FAO Technical Workshop, 22–25 March 2010, Orbetello, Italy. FAO Fisheries and Aquaculture Proceedings No. 24. FAO, Rome, p. 73. - and the likely associated cost increases (at least at first)  associated with offshore operations
   • Most open ocean environments require nutrient additions to sustain macroalgal yields. If these nutrients come from the deep ocean, they may be accompanied by high CO₂ water that can be released and will offset macroalgal carbon sequestration to some degree (which needs to be quantified).^[4]Pan, Y., Fan, W., Zhang, D. et al. Research progress in artificial upwelling and its potential environmental effects. Sci. China Earth Sci. 59, 236–248 (2016). https://doi.org/10.1007/s11430-015-5195-2 ,^[5]Oschlies, A., Pahlow, M., Yool, A., and Matear, R. J. (2010), Climate engineering by artificial ocean upwelling: Channelling the sorcerer's apprentice, Geophys. Res. Lett., 37, L04701, doi:10.1029/2009GL041961.
     • If macroalgal farms are located closer to the coastal zone, excess anthropogenic nitrogen inputs could support ~3.5 million km2 of macroalgal cultivation, corresponding with an estimated CDR range of 5- 10 Gt CDR annually^[6]Comments from C. Duarte, ‘A Workshop on Ocean-based CDR Opportunities and Challenges Part 3: Ecosystem Recovery & Seaweed Cultivation’. A Research Strategy for Ocean Carbon Dioxide Removal and Sequestration: Workshop Series, Part 3. 2nd February 2021. Accessible at: https://www.nationalacademies.org/event/02-02-2021/a-research-strategy-for-ocean-carbon-dioxide removal-and-sequestration-workshop-series-part-3 , alleviating competition for “natural” nutrient supplies with phytoplankton and alleviating the need for an upwelled supply of nutrients (and the associated risks of releasing CO₂ from upwelled deep ocean waters).
   • It is hypothesized that macroalgae-based pathways could supply CDR at moderate cost ($25-125 per ton of CO₂ removed)^[7]EFI Report. “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. ,^[8]Capron ME, Stewart JR, de Ramon N’Yeurt A, Chambers MD, Kim JK, Yarish C, Jones AT, Blaylock RB, James SC, Fuhrman R, Sherman MT, Piper D, Harris G, Hasan MA. Restoring Pre-Industrial CO₂ Levels While Achieving Sustainable Development Goals. Energies. 2020; 13(18):4972. https://doi.org/10.3390/en13184972 , but these cost estimates are preliminary and subject to change as macroalgae-based pathways advance in technological readiness^[9]TRL definitions: https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_accordion1.htm .
     • Researchers at UC Irvine are developing a biophysical-economic model to better understand the geophysical, biological, and economic sensitivities of macroalgae-based CDR pathways^[10]Sustainable Seaweed Solutions, https://www.ess.uci.edu/~sjdavis/seaweed.html
    
2. Sequestration Permanence -For cultivated macroalgae to contribute to CDR, the carbon captured in the macroalgal tissue needs to be sequestered to prevent its decomposition and return to the atmosphere. Sequestration pathways include:
   • Intentionally sinking the macroalgae (pre- or post-processing) into the deep ocean (>1000 meters depth) where it can be sequestered from contact with the atmosphere for hundreds to a few thousand years^[11]“Running Tide”. https://www.runningtide.com/removing (if remineralized in the deep ocean) or for thousands-to-millions of years (if buried in marine sediments).^[12]GESAMP (2019). “High level review of a wide range of proposed marine geoengineering techniques”. (Boyd, P.W. and Vivian, C.M.G., eds.). (IMO/FAO/UNESCO-IOC/UNIDO/WMO/IAEA/UN/UN Environment/ UNDP/ISA Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). Rep. Stud. GESAMP No. 98, 144 p.
     • There are natural analogues to this process already. In macroalgal systems, already ~50% of natural macroalgal production breaks off and becomes dissolved or particulate organic matter. Some of the particulate organic matter is transported to the deep ocean, where the carbon is sequestered.^[13]Krause-Jensen, D., Duarte, C. Substantial role of macroalgae in marine carbon sequestration. Nature Geosci 9, 737–742 (2016). https://doi.org/10.1038/ngeo2790
   • Relying on sedimentary burial below or adjacent to the farm (typically more applicable for farms not located over deep water).
     • Oceans2050 is working to determine the permanence of burial under coastal farms
   • Harvesting the macroalgae for:
     • Bioenergy^[14]Hughes, Adam D., et al. “Does Seaweed Offer a Solution for Bioenergy with Biological Carbon Capture and Storage?” Greenhouse Gases: Science and Technology, vol. 2, no. 6, 2012, pp. 402–407., doi:10.1002/ghg.1319.
       • Combining combustion pathways with carbon capture and storage^[15]Moreira, D., Pires, J.C.M., 2016. Atmospheric CO₂ capture by algae: Negative carbon dioxide emission path. Bioresource Technology 215, 371–379. https://doi.org/10.1016/j.biortech.2016.03.060 (thousands-to-millions of years if stored in a geologic reservoir)
       • Pyrolysis, resulting in biochar^[16]Zhang, C., Zhang, L., Gao, J., Zhang, S., Liu, Q., Duan, P., Hu, X., 2020. Evolution of the functional groups/structures of biochar and heteroatoms during the pyrolysis of seaweed. Algal Research 48, 101900. https://doi.org/10.1016/j.algal.2020.101900 ,^[17]Roberts, D., Paul, N., Dworjanyn, S. et al. Biochar from commercially cultivated seaweed for soil amelioration. Sci Rep 5, 9665 (2015). https://doi.org/10.1038/srep09665 (hundreds-to-thousands of years)
     • Production of long-lived bioproducts, such as bioplastics, that are capable of permanent (>100 years) sequestration^[18]Chia, Wen Yi, et al. “Nature’s Fight against Plastic Pollution: Algae for Plastic Biodegradation and Bioplastics Production.” Environmental Science and Ecotechnology, vol. 4, 2020, p. 100065., doi:10.1016/j.ese.2020.100065.
3. Differentiating from Avoided Emissions - Macroalgae can provide a number of other services that either reduce or replace greenhouse gas emissions, but do not remove legacy carbon dioxide pollution from the atmosphere. One example of reducing emissions: red algal supplements added to cattle feed may play a key role in reducing enteric methane emissions from cattle by over 50%^[19]Roque, B.M., Salwen, J.K., Kinley, R., Kebreab, E., 2019. Inclusion of Asparagopsis armata in lactating dairy cows’ diet reduces enteric methane emission by over 50 percent. Journal of Cleaner Production 234, 132–138. https://doi.org/10.1016/j.jclepro.2019.06.193 ,^[20]Vijn S. et al.,2020. Key Considerations for the Use of Seaweed to Reduce Enteric Methane Emissions From Cattle. Front. Vet. Sci., 23 December 2020. https://doi.org/10.3389/fvets.2020.597430 https://www.frontiersin.org/articles/10.3389/fvets.2020.597430/full . Given methane’s potency as a greenhouse gas, such emissions reductions may play an outsized role in slowing planetary temperature increases through the reduction of greenhouse gases.Macroalgae biomass may also be converted into short-lived, high-value bioproducts, such as food and nutritional supplements^[21]Biris-Dorhoi ES, Michiu D, Pop CR, Rotar AM, Tofana M, Pop OL, Socaci SA, Farcas AC. Macroalgae A Sustainable Source of Chemical Compounds with Biological Activities. Nutrients. 2020 Oct 11;12(10):3085. doi: 10.3390/nu12103085. PMID: 33050561; PMCID: PMC7601163. . This process may contribute to avoided emissions if it substitutes for carbon-intensive feedstocks, but it does not represent CDR because of the short timescale over which these products sequester carbon before it returns to the atmosphere.Although the focus of this roadmap is on macroalgae pathways for CDR, many of the obstacles faced, development needs, and near-term opportunities for advancement are relevant to these associated pathways for avoided greenhouse gas emissions.

1. Carbon Capture - The ultimate CDR potential of large-scale macroalgal cultivation and sequestration is as yet difficult to determine. Theoretically, macroalgal cultivation and sequestration  could be scaled to between one and greater than ten gigatons of CDR per year^[1]Energy Futures Initiative. “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. ,  but realized CDR may be much lower due to a number of factors, including:
   • Competition for space with existing ocean stakeholders – commercial shipping,  commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc. 
   • Challenges with expanding to offshore environments – engineering of moorings  and farm structures, challenges with operations and maintenance in distant environments^[2]Bak, Urd Grandorf, Gregersen, Ólavur and Infante, Javier. "Technical challenges for offshore cultivation of kelp species: lessons learned and future directions" Botanica Marina, vol. 63, no. 4, 2020, pp. 341- 353. https://doi-org.oca.ucsc.edu/10.1515/bot-2019-0005 ,^[3]Lovatelli, A., Aguilar-Manjarrez, J. and Soto, D. (2013). Expanding mariculture farther offshore: technical, environmental, spatial and governance challenges. In: FAO Technical Workshop, 22–25 March 2010, Orbetello, Italy. FAO Fisheries and Aquaculture Proceedings No. 24. FAO, Rome, p. 73. - and the likely associated cost increases (at least at first)  associated with offshore operations
   • Most open ocean environments require nutrient additions to sustain macroalgal yields. If these nutrients come from the deep ocean, they may be accompanied by high CO₂ water that can be released and will offset macroalgal carbon sequestration to some degree (which needs to be quantified).^[4]Pan, Y., Fan, W., Zhang, D. et al. Research progress in artificial upwelling and its potential environmental effects. Sci. China Earth Sci. 59, 236–248 (2016). https://doi.org/10.1007/s11430-015-5195-2 ,^[5]Oschlies, A., Pahlow, M., Yool, A., and Matear, R. J. (2010), Climate engineering by artificial ocean upwelling: Channelling the sorcerer's apprentice, Geophys. Res. Lett., 37, L04701, doi:10.1029/2009GL041961.
     • If macroalgal farms are located closer to the coastal zone, excess anthropogenic nitrogen inputs could support ~3.5 million km2 of macroalgal cultivation, corresponding with an estimated CDR range of 5- 10 Gt CDR annually^[6]Comments from C. Duarte, ‘A Workshop on Ocean-based CDR Opportunities and Challenges Part 3: Ecosystem Recovery & Seaweed Cultivation’. A Research Strategy for Ocean Carbon Dioxide Removal and Sequestration: Workshop Series, Part 3. 2nd February 2021. Accessible at: https://www.nationalacademies.org/event/02-02-2021/a-research-strategy-for-ocean-carbon-dioxide removal-and-sequestration-workshop-series-part-3 , alleviating competition for “natural” nutrient supplies with phytoplankton and alleviating the need for an upwelled supply of nutrients (and the associated risks of releasing CO₂ from upwelled deep ocean waters).
   • It is hypothesized that macroalgae-based pathways could supply CDR at moderate cost ($25-125 per ton of CO₂ removed)^[7]EFI Report. “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. ,^[8]Capron ME, Stewart JR, de Ramon N’Yeurt A, Chambers MD, Kim JK, Yarish C, Jones AT, Blaylock RB, James SC, Fuhrman R, Sherman MT, Piper D, Harris G, Hasan MA. Restoring Pre-Industrial CO₂ Levels While Achieving Sustainable Development Goals. Energies. 2020; 13(18):4972. https://doi.org/10.3390/en13184972 , but these cost estimates are preliminary and subject to change as macroalgae-based pathways advance in technological readiness^[9]TRL definitions: https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_accordion1.htm .
     • Researchers at UC Irvine are developing a biophysical-economic model to better understand the geophysical, biological, and economic sensitivities of macroalgae-based CDR pathways^[10]Sustainable Seaweed Solutions, https://www.ess.uci.edu/~sjdavis/seaweed.html
    
2. Sequestration Permanence -For cultivated macroalgae to contribute to CDR, the carbon captured in the macroalgal tissue needs to be sequestered to prevent its decomposition and return to the atmosphere. Sequestration pathways include:
   • Intentionally sinking the macroalgae (pre- or post-processing) into the deep ocean (>1000 meters depth) where it can be sequestered from contact with the atmosphere for hundreds to a few thousand years^[11]“Running Tide”. https://www.runningtide.com/removing (if remineralized in the deep ocean) or for thousands-to-millions of years (if buried in marine sediments).^[12]GESAMP (2019). “High level review of a wide range of proposed marine geoengineering techniques”. (Boyd, P.W. and Vivian, C.M.G., eds.). (IMO/FAO/UNESCO-IOC/UNIDO/WMO/IAEA/UN/UN Environment/ UNDP/ISA Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). Rep. Stud. GESAMP No. 98, 144 p.
     • There are natural analogues to this process already. In macroalgal systems, already ~50% of natural macroalgal production breaks off and becomes dissolved or particulate organic matter. Some of the particulate organic matter is transported to the deep ocean, where the carbon is sequestered.^[13]Krause-Jensen, D., Duarte, C. Substantial role of macroalgae in marine carbon sequestration. Nature Geosci 9, 737–742 (2016). https://doi.org/10.1038/ngeo2790
   • Relying on sedimentary burial below or adjacent to the farm (typically more applicable for farms not located over deep water).
     • Oceans2050 is working to determine the permanence of burial under coastal farms
   • Harvesting the macroalgae for:
     • Bioenergy^[14]Hughes, Adam D., et al. “Does Seaweed Offer a Solution for Bioenergy with Biological Carbon Capture and Storage?” Greenhouse Gases: Science and Technology, vol. 2, no. 6, 2012, pp. 402–407., doi:10.1002/ghg.1319.
       • Combining combustion pathways with carbon capture and storage^[15]Moreira, D., Pires, J.C.M., 2016. Atmospheric CO₂ capture by algae: Negative carbon dioxide emission path. Bioresource Technology 215, 371–379. https://doi.org/10.1016/j.biortech.2016.03.060 (thousands-to-millions of years if stored in a geologic reservoir)
       • Pyrolysis, resulting in biochar^[16]Zhang, C., Zhang, L., Gao, J., Zhang, S., Liu, Q., Duan, P., Hu, X., 2020. Evolution of the functional groups/structures of biochar and heteroatoms during the pyrolysis of seaweed. Algal Research 48, 101900. https://doi.org/10.1016/j.algal.2020.101900 ,^[17]Roberts, D., Paul, N., Dworjanyn, S. et al. Biochar from commercially cultivated seaweed for soil amelioration. Sci Rep 5, 9665 (2015). https://doi.org/10.1038/srep09665 (hundreds-to-thousands of years)
     • Production of long-lived bioproducts, such as bioplastics, that are capable of permanent (>100 years) sequestration^[18]Chia, Wen Yi, et al. “Nature’s Fight against Plastic Pollution: Algae for Plastic Biodegradation and Bioplastics Production.” Environmental Science and Ecotechnology, vol. 4, 2020, p. 100065., doi:10.1016/j.ese.2020.100065.
3. Differentiating from Avoided Emissions - Macroalgae can provide a number of other services that either reduce or replace greenhouse gas emissions, but do not remove legacy carbon dioxide pollution from the atmosphere. One example of reducing emissions: red algal supplements added to cattle feed may play a key role in reducing enteric methane emissions from cattle by over 50%^[19]Roque, B.M., Salwen, J.K., Kinley, R., Kebreab, E., 2019. Inclusion of Asparagopsis armata in lactating dairy cows’ diet reduces enteric methane emission by over 50 percent. Journal of Cleaner Production 234, 132–138. https://doi.org/10.1016/j.jclepro.2019.06.193 . Given methane’s potency as a greenhouse gas, such emissions reductions may play an outsized role in slowing planetary temperature increases through the reduction of greenhouse gases.Macroalgae biomass may also be converted into short-lived, high-value bioproducts, such as food and nutritional supplements^[20]Biris-Dorhoi ES, Michiu D, Pop CR, Rotar AM, Tofana M, Pop OL, Socaci SA, Farcas AC. Macroalgae A Sustainable Source of Chemical Compounds with Biological Activities. Nutrients. 2020 Oct 11;12(10):3085. doi: 10.3390/nu12103085. PMID: 33050561; PMCID: PMC7601163. . This process may contribute to avoided emissions if it substitutes for carbon-intensive feedstocks, but it does not represent CDR because of the short timescale over which these products sequester carbon before it returns to the atmosphere.Although the focus of this roadmap is on macroalgae pathways for CDR, many of the obstacles faced, development needs, and near-term opportunities for advancement are relevant to these associated pathways for avoided greenhouse gas emissions.

1. Carbon Capture - The ultimate CDR potential of large-scale macroalgal cultivation and sequestration is as yet difficult to determine. Theoretically, macroalgal cultivation and sequestration  could be scaled to between one and greater than ten gigatons of CDR per year^[1]Energy Futures Initiative. “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. ,  but realized CDR may be much lower due to a number of factors, including:
   • Competition for space with existing ocean stakeholders – commercial shipping,  commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc. 
   • Challenges with expanding to offshore environments – engineering of moorings  and farm structures, challenges with operations and maintenance in distant environments^[2]Bak, Urd Grandorf, Gregersen, Ólavur and Infante, Javier. "Technical challenges for offshore cultivation of kelp species: lessons learned and future directions" Botanica Marina, vol. 63, no. 4, 2020, pp. 341- 353. https://doi-org.oca.ucsc.edu/10.1515/bot-2019-0005 ,^[3]Lovatelli, A., Aguilar-Manjarrez, J. and Soto, D. (2013). Expanding mariculture farther offshore: technical, environmental, spatial and governance challenges. In: FAO Technical Workshop, 22–25 March 2010, Orbetello, Italy. FAO Fisheries and Aquaculture Proceedings No. 24. FAO, Rome, p. 73. - and the likely associated cost increases (at least at first)  associated with offshore operations
   • Most open ocean environments require nutrient additions to sustain macroalgal yields. If these nutrients come from the deep ocean, they may be accompanied by high CO₂ water that can be released and will offset macroalgal carbon sequestration to some degree (which needs to be quantified).^[4]Pan, Y., Fan, W., Zhang, D. et al. Research progress in artificial upwelling and its potential environmental effects. Sci. China Earth Sci. 59, 236–248 (2016). https://doi.org/10.1007/s11430-015- 5195-2 ,^[5]Oschlies, A., Pahlow, M., Yool, A., and Matear, R. J. (2010), Climate engineering by artificial ocean upwelling: Channelling the sorcerer's apprentice, Geophys. Res. Lett., 37, L04701, doi:10.1029/2009GL041961.
     • If macroalgal farms are located closer to the coastal zone, excess anthropogenic nitrogen inputs could support ~3.5 million km2 of macroalgal cultivation, corresponding with an estimated CDR range of 5- 10 Gt CDR annually^[6]Comments from C. Duarte, ‘A Workshop on Ocean-based CDR Opportunities and Challenges Part 3: Ecosystem Recovery & Seaweed Cultivation’. A Research Strategy for Ocean Carbon Dioxide Removal and Sequestration: Workshop Series, Part 3. 2nd February 2021. Accessible at: https://www.nationalacademies.org/event/02-02-2021/a-research-strategy-for-ocean-carbon-dioxide removal-and-sequestration-workshop-series-part-3 , alleviating competition for “natural” nutrient supplies with phytoplankton and alleviating the need for an upwelled supply of nutrients (and the associated risks of releasing CO₂ from upwelled deep ocean waters).
   • It is hypothesized that macroalgae-based pathways could supply CDR at moderate cost ($25-125 per ton of CO₂ removed)^[7]EFI Report. “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. ,^[8]Capron ME, Stewart JR, de Ramon N’Yeurt A, Chambers MD, Kim JK, Yarish C, Jones AT, Blaylock RB, James SC, Fuhrman R, Sherman MT, Piper D, Harris G, Hasan MA. Restoring Pre-Industrial CO₂ Levels While Achieving Sustainable Development Goals. Energies. 2020; 13(18):4972. https://doi.org/10.3390/en13184972 , but these cost estimates are preliminary and subject to change as macroalgae-based pathways advance in technological readiness^[9]TRL definitions: https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_accordion1.htm .
     • Researchers at UC Irvine are developing a biophysical-economic model to better understand the geophysical, biological, and economic sensitivities of macroalgae-based CDR pathways^[10]Sustainable Seaweed Solutions, www.ess.uci.edu/~sjdavis/seaweed.html
    
2. Sequestration Permanence -For cultivated macroalgae to contribute to CDR, the carbon captured in the macroalgal tissue needs to be sequestered to prevent its decomposition and return to the atmosphere. Sequestration pathways include:
   • Intentionally sinking the macroalgae (pre- or post-processing) into the deep ocean (>1000 meters depth) where it can be sequestered from contact with the atmosphere for hundreds to a few thousand years^[11]“Running Tide”. https://www.runningtide.com/removing (if remineralized in the deep ocean) or for thousands-to-millions of years (if buried in marine sediments).^[12]GESAMP (2019). “High level review of a wide range of proposed marine geoengineering techniques”. (Boyd, P.W. and Vivian, C.M.G., eds.). (IMO/FAO/UNESCO-IOC/UNIDO/WMO/IAEA/UN/UN Environment/ UNDP/ISA Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). Rep. Stud. GESAMP No. 98, 144 p.
     • There are natural analogues to this process already. In macroalgal systems, already ~50% of natural macroalgal production breaks off and becomes dissolved or particulate organic matter. Some of the particulate organic matter is transported to the deep ocean, where the carbon is sequestered.^[13]Krause-Jensen, D., Duarte, C. Substantial role of macroalgae in marine carbon sequestration. Nature Geosci 9, 737–742 (2016). https://doi.org/10.1038/ngeo2790
   • Relying on sedimentary burial below or adjacent to the farm (typically more applicable for farms not located over deep water).
     • Oceans2050 is working to determine the permanence of burial under coastal farms
   • Harvesting the macroalgae for:
     • Bioenergy^[14]Hughes, Adam D., et al. “Does Seaweed Offer a Solution for Bioenergy with Biological Carbon Capture and Storage?” Greenhouse Gases: Science and Technology, vol. 2, no. 6, 2012, pp. 402–407., doi:10.1002/ghg.1319.
       • Combining combustion pathways with carbon capture and storage^[15]Moreira, D., Pires, J.C.M., 2016. Atmospheric CO₂ capture by algae: Negative carbon dioxide emission path. Bioresource Technology 215, 371–379. https://doi.org/10.1016/j.biortech.2016.03.060 (thousands-to-millions of years if stored in a geologic reservoir)
       • Pyrolysis, resulting in biochar^[16]Zhang, C., Zhang, L., Gao, J., Zhang, S., Liu, Q., Duan, P., Hu, X., 2020. Evolution of the functional groups/structures of biochar and heteroatoms during the pyrolysis of seaweed. Algal Research 48, 101900. https://doi.org/10.1016/j.algal.2020.101900 ,^[17]Roberts, D., Paul, N., Dworjanyn, S. et al. Biochar from commercially cultivated seaweed for soil amelioration. Sci Rep 5, 9665 (2015). https://doi.org/10.1038/srep09665 (hundreds-to-thousands of years)
     • Production of long-lived bioproducts, such as bioplastics, that are capable of permanent (>100 years) sequestration^[18]Chia, Wen Yi, et al. “Nature’s Fight against Plastic Pollution: Algae for Plastic Biodegradation and Bioplastics Production.” Environmental Science and Ecotechnology, vol. 4, 2020, p. 100065., doi:10.1016/j.ese.2020.100065.
3. Differentiating from Avoided Emissions - Macroalgae can provide a number of other services that either reduce or replace greenhouse gas emissions, but do not remove legacy carbon dioxide pollution from the atmosphere. One example of reducing emissions: red algal supplements added to cattle feed may play a key role in reducing enteric methane emissions from cattle by over 50%^[19]Roque, B.M., Salwen, J.K., Kinley, R., Kebreab, E., 2019. Inclusion of Asparagopsis armata in lactating dairy cows’ diet reduces enteric methane emission by over 50 percent. Journal of Cleaner Production 234, 132–138. https://doi.org/10.1016/j.jclepro.2019.06.193 . Given methane’s potency as a greenhouse gas, such emissions reductions may play an outsized role in slowing planetary temperature increases through the reduction of greenhouse gases.Macroalgae biomass may also be converted into short-lived, high-value bioproducts, such as food and nutritional supplements^[20]Biris-Dorhoi ES, Michiu D, Pop CR, Rotar AM, Tofana M, Pop OL, Socaci SA, Farcas AC. Macroalgae A Sustainable Source of Chemical Compounds with Biological Activities. Nutrients. 2020 Oct 11;12(10):3085. doi: 10.3390/nu12103085. PMID: 33050561; PMCID: PMC7601163. . This process may contribute to avoided emissions if it substitutes for carbon-intensive feedstocks, but it does not represent CDR because of the short timescale over which these products sequester carbon before it returns to the atmosphere. Although the focus of this roadmap is on macroalgae pathways for CDR, many of the obstacles faced, development needs, and near-term opportunities for advancement are relevant to these associated pathways for avoided greenhouse gas emissions.

1. Carbon Capture - The ultimate CDR potential of large-scale macroalgal cultivation and sequestration is as yet difficult to determine. Theoretically, macroalgal cultivation and sequestration  could be scaled to between one and greater than ten gigatons of CDR per year,  but realized CDR may be much lower due to a number of factors, including:
   • Competition for space with existing ocean stakeholders – commercial shipping,  commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc. 
   • Challenges with expanding to offshore environments – engineering of moorings  and farm structures, challenges with operations and maintenance in distant environments,- and the likely associated cost increases (at least at first)  associated with offshore operations
   • Most open ocean environments require nutrient additions to sustain macroalgal yields. If these nutrients come from the deep ocean, they may be accompanied by high CO₂ water that can be released and will offset macroalgal carbon sequestration to some degree (which needs to be quantified).,
     • If macroalgal farms are located closer to the coastal zone, excess anthropogenic nitrogen inputs could support ~3.5 million km2 of macroalgal cultivation, corresponding with an estimated CDR range of 5- 10 Gt CDR annually, alleviating competition for “natural” nutrient supplies with phytoplankton and alleviating the need for an upwelled supply of nutrients (and the associated risks of releasing CO₂ from upwelled deep ocean waters).
   • It is hypothesized that macroalgae-based pathways could supply CDR at moderate cost ($25-125 per ton of CO₂ removed),, but these cost estimates are preliminary and subject to change as macroalgae-based pathways advance in technological readiness.
     • Researchers at UC Irvine are developing a biophysical-economic model to better understand the geophysical, biological, and economic sensitivities of macroalgae-based CDR pathways
    
2. Sequestration Permanence -For cultivated macroalgae to contribute to CDR, the carbon captured in the macroalgal tissue needs to be sequestered to prevent its decomposition and return to the atmosphere. Sequestration pathways include:
   • Intentionally sinking the macroalgae (pre- or post-processing) into the deep ocean (>1000 meters depth) where it can be sequestered from contact with the atmosphere for hundreds to a few thousand years (if remineralized in the deep ocean) or for thousands-to-millions of years (if buried in marine sediments).
     • There are natural analogues to this process already. In macroalgal systems, already ~50% of natural macroalgal production breaks off and becomes dissolved or particulate organic matter. Some of the particulate organic matter is transported to the deep ocean, where the carbon is sequestered.
   • Relying on sedimentary burial below or adjacent to the farm (typically more applicable for farms not located over deep water).
     • Oceans2050 is working to determine the permanence of burial under coastal farms
   • Harvesting the macroalgae for:
     • Bioenergy
       • Combining combustion pathways with carbon capture and storage (thousands-to-millions of years if stored in a geologic reservoir)
       • Pyrolysis, resulting in biochar, (hundreds-to-thousands of years)
     • Production of long-lived bioproducts, such as bioplastics, that are capable of permanent (>100 years) sequestration
3. Differentiating from Avoided Emissions

Macroalgae can provide a number of other services that either reduce or replace greenhouse gas emissions, but do not remove legacy carbon dioxide pollution from the atmosphere. One example of reducing emissions: red algal supplements added to cattle feed may play a key role in reducing enteric methane emissions from cattle by over 50%. Given methane’s potency as a greenhouse gas, such emissions reductions may play an outsized role in slowing planetary temperature increases through the reduction of greenhouse gases. Macroalgae biomass may also be converted into short-lived, high-value bioproducts, such as food and nutritional supplements. This process may contribute to avoided emissions if it substitutes for carbon-intensive feedstocks, but it does not represent CDR because of the short timescale over which these products sequester carbon before it returns to the atmosphere. Although the focus of this roadmap is on macroalgae pathways for CDR, many of the obstacles faced, development needs, and near-term opportunities for advancement are relevant to these associated pathways for avoided greenhouse gas emissions.

Chemical 

• Localized buffering/reductions in ocean acidification due to CO₂ uptake. ,,,,

Biological

• Nutrient remediation and metal uptake in eutrophied, polluted coastal waters
• Building macroalgae cultivation facilities near shellfish or fish aquaculture facilities may alleviate negative impacts from such activities (e.g., deoxygenation, eutrophication)

Physical

• Macroalgae farms may attenuate wave energy
• Creation of habitat with resulting nurseries for fish and other marine life

Other

• Carbon capture at sea may reduce the demand for terrestrial-based CDR alternatives that compete for land and/or freshwater

• Localized buffering/reductions in ocean acidification due to CO₂ uptake. ^[1]Koweek, D. A., Nickols, K. J., Leary, P. R., Litvin, S. Y., Bell, T. W., Luthin, T., Lummis, S., Mucciarone, D. A., and Dunbar, R. B.: A year in the life of a central California kelp forest: physical and biological insights into biogeochemical variability, Biogeosciences, 14, 31–44, https://doi.org/10.5194/bg-14-31- 2017, 2017. ,^[2]Hirsh, H. K., Nickols, K. J., Takeshita, Y., Traiger, S. B., Mucciarone, D. A., Monismith, S., et al. (2020). Drivers of biogeochemical variability in a central California kelp forest: Implications for local amelioration of ocean acidification. Journal of Geophysical Research: Oceans, 125, e2020JC016320. https://doi.org/10.1029/2020JC016320 ,^[3]Kapsenberg, L, Cyronak, T. Ocean acidification refugia in variable environments. Glob Change Biol. 2019; 25: 3201– 3214. https://doi.org/10.1111/gcb.14730 ,^[4]Pamela A. Fernández, Pablo P. Leal & Luis A. Henríquez (2019) Co-culture in marine farms: macroalgae can act as chemical refuge for shell-forming molluscs under an ocean acidification scenario, Phycologia, 58:5, 542-551, DOI: 10.1080/00318884.2019.1628576 ,^[5] Xiao, X., Agustí, S., Yu, Y., Huang, Y., Chen, W., Hu, J., Li, C., Li, K., Wei, F., Lu, Y. and Xu, C., 2021. Seaweed farms provide refugia from ocean acidification. Science of The Total Environment, 776, p.145192, https://doi.org/10.1016/j.scitotenv.2021.145192
• Nutrient remediation and metal uptake in eutrophied, polluted coastal waters ^[6]Neveux, N., Bolton, J., Bruhn, A., Roberts, D., Ras, M., 2017. The Bioremediation Potential of Seaweeds: Recycling Nitrogen, Phosphorus, and Other Waste Products. https://doi.org/10.1002/9783527801718.ch7
• Carbon capture at sea may reduce the demand for terrestrial-based CDR alternatives that compete for land and/or fresh water
• Macroalgae farms may attenuate wave energy ^[7]Mork, M. (1996). “Wave attenuation due to bottom vegetation,” in Waves and Nonlinear Processes in Hydrodynamics, eds J. Grue, B. Gjevik, and J. E. Weber (Oslo: Kluwer Academic Publishing), 371–382.
• Creation of habitat with resulting nurseries for fish and other marine life ^[8]Smale, D.A., Burrows, M.T., Moore, P., O’Connor, N., Hawkins, S.J., 2013. Threats and knowledge gaps for ecosystem services provided by kelp forests: a northeast Atlantic perspective. Ecology and Evolution 3, 4016–4038. https://doi.org/10.1002/ece3.774

Chemical

• Macroalgae are known to release bromoform and other halomethanes,, and thus, large-scale macroalgae cultivation seems likely to increase the release of these substances. This needs additional research as the natural marine sources of these gases are currently estimated to be responsible for around 9% of stratospheric ozone loss, including depletion due to anthropogenic causes.
• Production of methane, nitrous oxide, and other potentially hazardous gases by the macroalgae
• The potential for CO₂ outgassing from pumping deep water to the surface  (artificial upwelling) to supply needed nutrients for the macroalgae.
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific

  Biological

• Impacts to biodiversity and ecosystem function from large-scale cultivation operations, including,:
  • Enhanced disease and parasite risk
  • Alteration of population genetics
  • Introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO₂ release
• Reduced phytoplankton production in and around large macroalgae farms due to competition for nutrients and light
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific

  Physical

• Changes in light and nutrient availability (including possible changes in ocean albedo)
• In near-coastal zones, changes in circulation patterns due to drag by the seaweed could change residence time in nearshore environments
  • Changes in coastal residence time may affect the prevalence and intensity of harmful algal blooms (HABs)
• Entanglement of marine megafauna (e.g., whales)

  Other Addition of noise pollution due to vessel traffic and machinery

• Impacts to biodiversity and ecosystem function from large-scale cultivation operations, including: ,
  • Enhanced disease and parasite risk
  • Alteration of population genetics
  • Introduction of non-native species into new environments
  • Macroalgae are known to release bromoform and other halomethanes and thus, large-scale macroalgae cultivation seems likely to increase the release of these substances. This needs additional research as the natural marine sources of these gases are currently estimated to be responsible for around 9% of stratospheric ozone loss, including depletion due to anthropogenic causes.
  • Production of methane, nitrous oxide, and other potentially hazardous gases by the macroalgae
  • Changes in light and nutrient availability (including possible changes in ocean albedo)
  • Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO₂ release
  • Addition of noise pollution due to vessel traffic and machinery
  • Reduced phytoplankton production in and around large macroalgae farms due to competition for nutrients and light
• In near-coastal zones, changes in circulation patterns due to drag by the seaweed could change residence time in nearshore environments
  • Changes in coastal residence time may affect the prevalence and intensity of harmful algal blooms (HABs)
• Entanglement of marine megafauna (e.g., whales)
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific
• The potential for CO₂ outgassing from pumping deep water to the surface  (artificial upwelling) to supply needed nutrients for the macroalgae.

• Impacts to biodiversity and ecosystem function from large-scale cultivation operations, including: ,
  • Enhanced disease and parasite risk
  • Alteration of population genetics
  • Introduction of non-native species into new environments
  • Macroalgae are known to release bromoform and other halomethanes and thus, large-scale macroalgae cultivation seems likely to increase the release of these substances. This needs additional research as the natural marine sources of these gases are currently estimated to be responsible for around 9% of stratospheric ozone loss, including depletion due to anthropogenic causes.
  • Changes in light and nutrient availability (including possible change in ocean albedo)
  • Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO₂ release
  • Addition of noise pollution due to vessel traffic and machinery
  • Reduced phytoplankton production in and around large macroalgae farms due to competition for nutrients and light
• In near-coastal zones, changes in circulation patterns due to drag by the seaweed could change residence time in nearshore environments
  • Changes in coastal residence time may affect the prevalence and intensity of harmful algal blooms (HABs)
• Entanglement of marine megafauna (e.g., whales)
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific
• The potential for CO₂ outgassing from pumping deep water to the surface  (artificial upwelling) to supply needed nutrients for the macroalgae.

• Impacts to biodiversity and ecosystem function from large-scale cultivation operations, including: ^[1]Campbell, Iona, et al. “The Environmental Risks Associated With the Development of Seaweed Farming in Europe - Prioritizing Key Knowledge Gaps.” Frontiers in Marine Science, vol. 6, 2019, doi:10.3389/fmars.2019.00107. ,^[2]Campbell I, Macleod A, Sahlmann C, Neves L, Funderud J, Øverland M, Hughes AD and Stanley M (2019) The Environmental Risks Associated With the Development of Seaweed Farming in Europe - Prioritizing Key Knowledge Gaps. Front. Mar. Sci. 6:107. doi: 10.3389/fmars.2019.00107
  • Enhanced disease and parasite risk
  • Alteration of population genetics
  • Introduction of non-native species into new environments
  • Macroalgae are known to release bromoform and other halomethanes ^[4]Carpenter, L. J., et al. “Air-Sea Fluxes of Biogenic Bromine from the Tropical and North Atlantic Ocean.” Atmospheric Chemistry and Physics, vol. 9, no. 5, 2009, pp. 1805–1816., https://doi.org/10.5194/acp-9-1805-2009. ^[5]Mehlmann, Melina, et al. “Natural and Anthropogenic Sources of Bromoform and Dibromomethane in the Oceanographic and Biogeochemical Regime of the Subtropical North East Atlantic.” Environmental Science: Processes & Impacts, vol. 22, no. 3, 2020, pp. 679–707., https://doi.org/10.1039/c9em00599d. and thus, large-scale macroalgae cultivation seems likely to increase the release of these substances. This needs additional research as the natural marine sources of these gases are currently estimated to be responsible for around 9% of stratospheric ozone loss, including depletion due to anthropogenic causes^[6]Tegtmeier, S., Ziska, F., Pisso, I., Quack, B., Velders, G. J. M., Yang, X., and Krüger, K.: Oceanic bromoform emissions weighted by their ozone depletion potential, Atmos. Chem. Phys., 15, 13647–13663, https://doi.org/10.5194/acp-15-13647-2015, 2015. .
  • Changes in light and nutrient availability (including possible change in ocean albedo)
  • Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO₂ release
  • Addition of noise pollution due to vessel traffic and machinery
• In near-coastal zones, changes in circulation patterns due to drag by the seaweed could change residence time in nearshore environments
  • Changes in coastal residence time may affect the prevalence and intensity of harmful algal blooms (HABs)
• Entanglement of marine megafauna (e.g., whales)
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific
• The potential for CO₂ outgassing from pumping deep water to the surface  (artificial upwelling) to supply needed nutrients for the macroalgae. ^[3]Pan, Y., Fan, W., Zhang, D. et al. Research progress in artificial upwelling and its potential environmental effects. Sci. China Earth Sci. 59, 236–248 (2016). https://doi.org/10.1007/s11430-015-5195-2

• Impacts to biodiversity and ecosystem function from large-scale cultivation operations, including: ^[1]Campbell, Iona, et al. “The Environmental Risks Associated With the Development of Seaweed Farming in Europe - Prioritizing Key Knowledge Gaps.” Frontiers in Marine Science, vol. 6, 2019, doi:10.3389/fmars.2019.00107. ,^[2]Campbell I, Macleod A, Sahlmann C, Neves L, Funderud J, Øverland M, Hughes AD and Stanley M (2019) The Environmental Risks Associated With the Development of Seaweed Farming in Europe - Prioritizing Key Knowledge Gaps. Front. Mar. Sci. 6:107. doi: 10.3389/fmars.2019.00107
  • Enhanced disease and parasite risk
  • Alteration of population genetics
  • Introduction of non-native species into new environments
  • Macroalgae are known to release bromoform and other halomethanes ^[4]Carpenter, L. J., et al. “Air-Sea Fluxes of Biogenic Bromine from the Tropical and North Atlantic Ocean.” Atmospheric Chemistry and Physics, vol. 9, no. 5, 2009, pp. 1805–1816., https://doi.org/10.5194/acp-9-1805-2009. ^[5]Mehlmann, Melina, et al. “Natural and Anthropogenic Sources of Bromoform and Dibromomethane in the Oceanographic and Biogeochemical Regime of the Subtropical North East Atlantic.” Environmental Science: Processes & Impacts, vol. 22, no. 3, 2020, pp. 679–707., https://doi.org/10.1039/c9em00599d. and thus, large-scale macroalgae cultivation seems likely to increase the release of these substances. This needs additional research as the natural marine sources of these gases are currently estimated to be responsible for around 9% of stratospheric ozone loss, including depletion due to anthropogenic causes^[6]Tegtmeier, S., Ziska, F., Pisso, I., Quack, B., Velders, G. J. M., Yang, X., and Krüger, K.: Oceanic bromoform emissions weighted by their ozone depletion potential, Atmos. Chem. Phys., 15, 13647–13663, https://doi.org/10.5194/acp-15-13647-2015, 2015. .
  • Changes in light and nutrient availability
  • Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO₂ release
  • Addition of noise pollution due to vessel traffic and machinery
• In near-coastal zones, changes in circulation patterns due to drag by the seaweed could change residence time in nearshore environments
  • Changes in coastal residence time may affect the prevalence and intensity of harmful algal blooms (HABs)
• Entanglement of marine megafauna (e.g., whales)
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific
• The potential for CO₂ outgassing from pumping deep water to the surface  (artificial upwelling) to supply needed nutrients for the macroalgae. ^[3]Pan, Y., Fan, W., Zhang, D. et al. Research progress in artificial upwelling and its potential environmental effects. Sci. China Earth Sci. 59, 236–248 (2016). https://doi.org/10.1007/s11430-015-5195-2

• Impacts to biodiversity and ecosystem function from large-scale cultivation operations, including: ^[1]Campbell, Iona, et al. “The Environmental Risks Associated With the Development of Seaweed Farming in Europe - Prioritizing Key Knowledge Gaps.” Frontiers in Marine Science, vol. 6, 2019, doi:10.3389/fmars.2019.00107. ,^[2]Campbell I, Macleod A, Sahlmann C, Neves L, Funderud J, Øverland M, Hughes AD and Stanley M (2019) The Environmental Risks Associated With the Development of Seaweed Farming in Europe - Prioritizing Key Knowledge Gaps. Front. Mar. Sci. 6:107. doi: 10.3389/fmars.2019.00107
  • Enhanced disease and parasite risk
  • Alteration of population genetics
  • Introduction of non-native species into new environments
  • Release of large quantities of halocarbons and/or other trace gases
  • Changes in light and nutrient availability
  • Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO₂ release
  • Addition of noise pollution due to vessel traffic and machinery
• In near-coastal zones, changes in circulation patterns due to drag by the seaweed could change residence time in nearshore environments
  • Changes in coastal residence time may affect the prevalence and intensity of harmful algal blooms (HABs)
• Entanglement of marine megafauna (e.g., whales)
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific
• The potential for CO₂ outgassing from pumping deep water to the surface  (artificial upwelling) to supply needed nutrients for the macroalgae. ^[3]Pan, Y., Fan, W., Zhang, D. et al. Research progress in artificial upwelling and its potential environmental effects. Sci. China Earth Sci. 59, 236–248 (2016). https://doi.org/10.1007/s11430-015-5195-2

• Impacts to biodiversity and ecosystem function from large-scale cultivation operations, including: ^[1]Campbell, Iona, et al. “The Environmental Risks Associated With the Development of Seaweed Farming in Europe - Prioritizing Key Knowledge Gaps.” Frontiers in Marine Science, vol. 6, 2019, doi:10.3389/fmars.2019.00107. ,^[2]Campbell I, Macleod A, Sahlmann C, Neves L, Funderud J, Øverland M, Hughes AD and Stanley M (2019) The Environmental Risks Associated With the Development of Seaweed Farming in Europe - Prioritizing Key Knowledge Gaps. Front. Mar. Sci. 6:107. doi: 10.3389/fmars.2019.00107
  • Enhanced disease and parasite risk
  • Alteration of population genetics
  • Introduction of non-native species into new environments
  • Release of large quantities of halocarbons and/or other trace gases
  • Changes in light and nutrient availability
  • Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification
  • Addition of noise pollution due to vessel traffic and machinery
• In near-coastal zones, changes in circulation patterns due to drag by the seaweed could change residence time in nearshore environments
  • Changes in coastal residence time may affect the prevalence and intensity of harmful algal blooms (HABs)
• Entanglement of marine megafauna (e.g., whales)
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific
• The potential for CO₂ outgassing from pumping deep water to the surface  (artificial upwelling) to supply needed nutrients for the macroalgae. ^[3]Pan, Y., Fan, W., Zhang, D. et al. Research progress in artificial upwelling and its potential environmental effects. Sci. China Earth Sci. 59, 236–248 (2016). https://doi.org/10.1007/s11430-015-5195-2

• Impacts to biodiversity and ecosystem function from large-scale cultivation operations, including: ^[1]Campbell, Iona, et al. “The Environmental Risks Associated With the Development of Seaweed Farming in Europe - Prioritizing Key Knowledge Gaps.” Frontiers in Marine Science, vol. 6, 2019, doi:10.3389/fmars.2019.00107. ,^[2]Campbell I, Macleod A, Sahlmann C, Neves L, Funderud J, Øverland M, Hughes AD and Stanley M (2019) The Environmental Risks Associated With the Development of Seaweed Farming in Europe - Prioritizing Key Knowledge Gaps. Front. Mar. Sci. 6:107. doi: 10.3389/fmars.2019.00107
  • Enhanced disease and parasite risk
  • Alteration of population genetics
  • Introduction of non-native species into new environments
  • Release of large quantities of halocarbons and/or other trace gases
  • Changes in light and nutrient availability
  • Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification
  • Addition of noise pollution due to vessel traffic and machinery
• In near-coastal zones, changes in circulation patterns due to drag by the seaweed could change residence time in nearshore environments
  • Changes in coastal residence time may affect the prevalence and intensity of harmful algal blooms (HABs)
• Entanglement of marine megafauna (e.g., whales)
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific
• The potential for CO₂ outgassing from pumping deep water to the surface  (artificial upwelling) to supply needed nutrients for the macroalgae. ^[3]Pan, Y., Fan, W., Zhang, D. et al. Research progress in artificial upwelling and its potential environmental effects. Sci. China Earth Sci. 59, 236–248 (2016). https://doi.org/10.1007/s11430-015- 5195-2

Enhance the Blue Economy: There is increased interest in the potential for seaweed aquaculture and its potential to increase the economic opportunities for seaweed farmers if practices are modified to increase climate benefits and if these benefits are sufficiently monetized.

• Job Creation: Seaweed farming could also present new job opportunities for fishers whose work is threatened by climate change.
• Value-Added Products: High-value bioproducts (pigments, lipids, proteins, etc) can replace more carbon-intensive alternatives in feed, food, fuel, and other commodities.

 

There is increased interest in the potential for seaweed aquaculture and its potential to increase the economic opportunities for seaweed farmers if practices are modified to increase climate benefits and if these benefits are sufficiently monetized ^[1]Duarte CM, Wu J, Xiao X, Bruhn A and Krause-Jensen D (2017) Can Seaweed Farming Play a Role in Climate Change Mitigation and Adaptation? Front. Mar. Sci. 4:100. doi: 10.3389/fmars.2017.00100  

1. Competition for Space: Competition for space with existing ocean stakeholders – commercial shipping, commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc.
   • Potential for entanglement of macroalgae cultivation equipment with shipping, commercial fishing gear, aquaculture farms etc, particularly if the macroalgae farm is free floating
2. Financing: Financing any CDR approach brings with it the risk of creating inequity and decreasing social welfare. With macroalgae cultivation, this may be especially applicable to coastal dwelling communities who rely on the ocean for their livelihoods, food, and cultural meaning.

1. Competition for space with existing ocean stakeholders – commercial shipping, commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc.
   • Potential for entanglement of macroalgae cultivation equipment with shipping, commercial fishing gear, aquaculture farms etc, particularly if the macroalgae farm is free floating