Brown algae absorb large amounts of carbon dioxide from the air and store it in the form of organic compounds. A study now shows that one of these compounds, called fucoidan, is particularly stable and can serve as a long-term carbon sink. It is estimated that brown algae may be able to remove up to 0.55 gigatonnes of carbon dioxide from the atmosphere in the form of fucoidan per year worldwide. In doing so, they make a hitherto underestimated contribution to climate protection.
One of the most important climate protection measures is to reduce emissions of the greenhouse gas carbon dioxide. In addition, researchers are exploring various options for removing CO2 from the atmosphere. In addition to technical possibilities, the natural photosynthesis of plants in particular can help. With the help of sunlight, plants convert carbon dioxide into sugar, from which they in turn produce numerous other organic compounds. After the plant dies, however, the stored carbon is released back into the atmosphere over time through decomposition or combustion of the organic material. In some cases, however, this process can be significantly delayed - for example, if the organic material sinks to the seabed or is buried deep in the ground.
Algae slime in focus
A team led by Hagen Buck-Wiese from the Max Planck Institute for Marine Microbiology in Bremen has now investigated the extent to which brown algae contribute to the global reduction in CO2 levels in the atmosphere. "Brown algae are known to absorb more CO2 than terrestrial forests, and brown algal biomass is considered a potential carbon sink," the researchers explain. The algae use part of the carbon they absorb for their growth, and they release another part back into the seawater, for example in the form of sugary excretions. Depending on how these excretions are structured, they are either quickly used by other organisms so that the bound carbon is released again, or they sink towards the sea floor.
"The excretions of the brown algae are very complex and therefore incredibly difficult to measure," says Buck-Wiese. "But we managed to develop a method to analyze them in detail." The researchers focused on the compound fucoidan. The algae form a layer of slime from this carbon compound, which they use to protect their surface against external influences. Previous studies have shown that fucoidan has an antibacterial effect, among other things, and also increases in response to increased salinity in the surrounding water. For climate research, however, it is interesting for another reason: "The fucoidan is so complex that it is difficult for other organisms to use it. Nobody seems to like it," explains Buck-Wiese. So instead of getting back into the atmosphere after a short time, the carbon stored in the fucoidan can sink to the sea floor. "The brown algae are therefore particularly good helpers in removing carbon dioxide from the atmosphere in the long term - for hundreds to thousands of years," says Buck-Wiese.
excretions analyzed
However, how much fucoidan the algae produce and how large this contribution to climate protection is, was previously unknown. "The high solubility of fucoidan and its location at the interface between algae and water suggest that fucoidan must be constantly regenerated," say the researchers. But how much does it actually matter? To answer this question, Buck-Wiese and his team collected brown algae in the form of bladderwrack (Fucus vesiculosus) in the Baltic Sea in south-west Finland and examined their excretions using various analysis methods.
The result: "We found out that the brown alga Fucus vesiculosus releases 0.3 percent of its biomass per day as fucoidan," report Buck-Wiese and his team. "According to our analysis, fucoidan accounts for up to 50 percent of the total dissolved organic carbon excreted in bladderwrack." Carbon sequestration occurs independently of algal growth.”
Great potential for climate protection
According to the researchers, brown algae are thus able to remove large amounts of CO2 from the global cycle in the long term and thus counteract global warming. Combining the new calculations with estimates of how much carbon brown algae absorb each year, it turns out that they sequester 0.55 gigatonnes of CO2 in the form of fucoidan each year. For comparison: According to estimates by the Federal Environment Agency, Germany's total CO2 emissions in 2020 were around 0.75 gigatonnes.
"Extrapolated over a year, the algae bind more carbon in the secreted fucoidan than in their biomass," the researchers write. "The global benefit of fucoidan secretion is an alternative route for brown algae to remove carbon dioxide without the need to harvest or bury the algal biomass." However, since the current study only included one brown algal species at one site, it is still unclear whether the results can actually be generalized. "Next, we want to see how it looks with other brown algae species and at other locations," says Buck-Wiese. "The great potential of brown algae for climate protection must be further researched and used."
Source: Hagen Buck-Wiese (Max Planck Institute for Marine Microbiology, Bremen) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2210561119