Brown seaweeds shown to sequester huge volumes of carbon

These species take up large amounts of carbon dioxide from the air and release parts of the carbon contained therein back into the environment as mucous that is hard to break down for other ocean inhabitants, thus the carbon is removed from the atmosphere for a long time, the researchers claim.

They reveal that the algal mucous called fucoidan is particularly responsible for this carbon removal and estimate that brown algae could remove up to 550 million tonnes of carbon dioxide from the air every year – almost the amount of Germany's entire annual greenhouse gas emissions.

Brown algae absorb more carbon dioxide from the air than terrestrial forests do, but what happens to the carbon dioxide after the algae have absorbed it has been the subject of debate. But the new report explains that the brown algae can remove large amounts of carbon dioxide from the global cycle in the long-term and thus can counteract global warming.

Algae take up carbon dioxide from the atmosphere and use the carbon to grow. They release up to a third of the carbon they absorb back into the seawater, for example in the form of sugary excretions. Depending on the structure of these excretions, they are either quickly used by other organisms or sink towards the seafloor.

“The excretions of brown algae are very complex and therefore incredibly complicated to measure,” said lead author, Hagen Buck-Wiese from the Max Planck Institute for Marine Microbiology, in a press release. “However, we have managed to develop a method to analyse them in detail.”

With this method, the researchers scrutinised a large number of different substances. The fucoidan turned out to be particularly exciting.

“Fucoidan made up about half of the excretions of the brown algae species we studied, the so-called bladderwrack,” said Buck-Wiese. “The fucoidan is so complex that it is very hard for other organisms to use it. No one seems to like it.”

As a result, the carbon from the fucoidan does not return to the atmosphere quickly.

“This makes the brown algae particularly good helpers in removing carbon dioxide from the atmosphere in the long term – for hundreds to thousands of years," he added.

It is estimated that brown algae absorb about 1 gigatonne (one billion tonnes) of carbon per year from the air. Using the results of the present study, this would mean that up to 0.15 gigatonnes of carbon, equivalent to 0.55 gigatonnes of carbon dioxide, are sequestered by these species each year in the long term. For comparison: Germany's annual greenhouse gas emissions currently amount to about 0.74 gigatonnes of carbon dioxide, according to Germany's Federal Environment Agency.

“And even better: The fucoidan does not contain any nutrients such as nitrogen,” Buck-Wiese added. Thus, the growth of the brown algae is not affected by the carbon losses.

For the current study, Buck-Wiese and his colleagues from the MARUM MPG Bridge Group Marine Glycobiology, which is based at both the Bremen Max Planck Institute and MARUM - Centre for Marine and Environmental Sciences at the University of Bremen, conducted their experiments at the Tvärminne Zoological Station in southern Finland.

“Next we want to look into other brown algae species and other locations,” said Buck-Wiese. “The great potential of brown algae for climate protection definitely needs to be further researched and utilised.”