Research suggests that a popular method for marine carbon dioxide removal could backfire
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Globally, we produce 36.5 million tonnes of seaweed – and other edible algae – every year. The vast majority of this – at least 95 per cent – is grown in the Asia-Pacific, where seaweed has long been a staple of local cuisine. But the fastest-growing markets are in Europe and North America, where seaweed is also valued as a source of plastic alternatives and for organic fertilisers – and, increasingly, for its potential in carbon dioxide removal.
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The ocean stores an estimated 40,000 billion tonnes of carbon – more than 50 times as much as the atmosphere – making it the planet’s largest carbon sink. This natural reserve could be the ideal place to store the excess carbon accumulating in our atmosphere, a theory that has prompted significant research into marine carbon-removal strategies, with seaweed cultivation emerging as one of the most popular approaches. Carbon-credit schemes – in which companies pay to offset their emissions by funding projects that pull CO2 from the air – are already banking on seaweed as a carbon removal tool.
One prominent strategy relies on sinking massive quantities of seaweed to the deep seafloor – a process designed to lock away carbon for hundreds of years, at least in theory. But according to new research, seaweed cultivation may not only be less promising than it seems; it could actually be counterproductive.
Manon Berger, a marine biogeochemist at the University of Bern, has been investigating the limitations of seaweed farming as a tool for carbon removal. In a new study, published with colleagues at IPSL and the University of Hawaii at Manoa, she highlights a critical but previously overlooked factor: the availability of iron, an essential micronutrient for almost all life.
‘We’ve generally assumed that nitrogen is the main constraint, and since most modelling studies are based on nearshore aquaculture farms – where iron levels are naturally high – iron limitation hasn’t really been on the radar,’ says Berger. ‘That changed when new research revealed that, even in natural, iron-rich coastal ecosystems, the amount of bioavailable iron – iron that marine life can actually use – is limited. We now realise that in the open ocean – where iron is even more scarce – cultivation might simply not be sustainable.’
Because iron is scarce in the open sea, seaweed ends up in direct competition with phytoplankton – the microscopic organisms that already pull CO2 from the atmosphere on an enormous scale. Berger’s research is the first to show that, when this iron limitation is factored in, seaweed production can drop threefold. In some regions, competition for nutrients is so intense that phytoplankton production is cut by up to 80 per cent, meaning seaweed cultivation actually reduces the amount of carbon the ocean naturally absorbs.
Monitoring all these changes in real time isn’t possible – the cost of deploying and maintaining enough sensors across the vast open ocean would be enormous. Instead, scientists use sophisticated computer simulations to model these complex interactions.
This type of modelling is currently being used to evaluate several proposed CO2-removal methods, including iron fertilisation – adding the mineral directly to the ocean to boost the growth of carbon-absorbing organisms. In theory, Berger says, iron fertilisation could help seaweed overcome these nutrient limits, potentially closing the gap in carbon-removal efficiency. ‘But that’s adding a whole additional layer of complexity,’ she cautions, ‘with cascading impacts on the phytoplankton that we’d also need to assess.’
There are, of course, lots of good reasons why we might want to cultivate seaweed – it’s a low-carbon food source, and it acts as a natural habitat for marine life, supporting biodiversity and filtering pollutants from the water. ‘I find it a bit odd to cultivate huge amounts of seaweed only to dump it in the deep ocean, discarding a nutritious, multi-purpose resource that could be used for food, feed, or other sustainability goals. We could disrupt an ecosystem we don’t fully understand. Based on our results, I’m not convinced it can scale to gigaton levels.’
