What’s happening? UK-based start-up Seafields plans to establish farms in the Atlantic Ocean, growing sargassum seaweed that can be used to capture and sequester CO2. The algae will absorb CO2 during its growth, after which the plant can be harvested, formed into bales and sunk to the bottom of the sea where a portion of the CO2 will be retained. Sargassum’s vigorous growth rate has led to its invasion of large areas of the Caribbean coast. To address this, Seafields aims to build dedicated farms in areas where it would not ordinarily survive to prevent it from becoming an invasive species elsewhere.

Why does this matter? Interest in exploring ocean-based carbon sequestration is growing as an alternative to land-based offsetting projects – such as reforestation – which have historically been more widely recognised in the context of nature-based carbon solutions. The role of the ocean as a natural carbon sink is immense, given it covers around 70% of the Earth’s surface. Some ocean ecosystems have also been proven to be more efficient in carbon sequestration compared to terrestrial forests, with seagrass, salt marshes and coastal mangrove habitats able to store up to 10 timesmore CO2, while large marine organisms also represent as yet unquantified carbon stores. There is also a huge carbon accounting gap on the broader seabed itself.

The High-Level Panel for a Sustainable Ocean Economy states using ocean-based solutions for climate mitigation could reduce the emissions gap – the difference between current emissions pledges and those consistent with the 1.5C temperature trajectory – by up to 21%. Alongside private sector interest, ocean-based climate action is also gaining recognition and high-level action from governments and becoming a feature of international agreements.

Utilising seaweed to sequester carbon could have significant potential in long-term carbon storage. Sargassum requires minimal nutrients, is resilient once established and is an unattractive food source for marine life. Adding seaweed to the ocean can offer other benefits, such as improving fish populations and encouraging marine biodiversity. Elsewhere, seaweed is being explored for other applications including mixed-use offshore systems and cultivation for use in low-emission livestock feed.

Seafields is aiming to sequester 1 gigatonne of CO2 from the atmosphere each year by 2025. The aquafarms will be irrigated using underwater pipes to upwell nutrients, taken from deeper waters to sustain and control production. The firm’s process of shredding and baling the seaweed is designed to reduce the likelihood of remineralisation on the seabed, enabling it to sequester carbon for longer periods of time. Once established, Seafields plans to sell carbon credits in the voluntary carbon market to fund its work. However there are multiple uncertainties to be resolved, including the impacts of baled sargassum on marine ecosystems and habitats when it sinks to the ocean floor. Additionally, the company will have to quantify and monitor how much carbon is sequestered in the sargassum in order to verify its blue carbon-capture technology.

Utilising seaweed for carbon sequestration is being explored by other companies, including Carbon Kapture, which plans to establish a network of aquaculture farms to cultivate kelp across south Wales, Ireland and the UK’s south coast which will sequester CO2 30 times faster in comparison to land-based reforestation.

The agglomeration of sargassum is invasive and has been exacerbated by warmer ocean temperatures linked to climate change, creating problems for the environment and local communities such as Mexico and the Caribbean coast. Seafields’ farms will be located in areas of the south Atlantic, where sargassum does not naturally survive due to inhospitable conditions including water temperature and depth, reducing the risk of it becoming an invasive problem. Additionally, researchers are exploring commercial ways to mitigate the presence of sargassum in areas struggling with excessive natural build-ups, such as its potential as an energy source, including producing biogas, bioethanol and biodiesel.

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