Brown algae absorb large amounts of carbon dioxide and remove it from the global carbon cycle in the long term. They not only use it for growth, but also use the greenhouse gas to form a sugary slime that is difficult to break down and can remain in sediments for centuries.
This is what scientists from the Max Planck Institute for Marine Microbiology in Bremen report in the specialist magazine “PNAS‘ according to studies with bladderwrack. This hitherto neglected contribution of brown algae to removing carbon dioxide from the atmosphere could counteract climate change.
Brown algae are multicellular algae that are mostly found in the sea. They include seaweed, knotted kelp or the bladderwrack examined in the study Fucus vesiculosus, which is widespread in the North and Baltic Seas and in the North Atlantic. Brown algae fixed more carbon per unit area than forests on land, writes the team led by Hagen Buck-Wiese in its specialist article. The biomass formed during growth removes carbon dioxide from the atmosphere, making it a carbon sink. For this reason, the targeted establishment of algae farms, where the algae are cultivated, harvested and used, is also being discussed as a measure against climate change.
A slime as a carbon sink
But not only the growth of algae binds carbon, as the current study now shows. The algae secrete significant amounts of fucoidan – a carbonaceous, sugary slime. Experimental measurements on bladderwrack in the Finnish Baltic Sea showed that the algae release 0.3 percent of their biomass as fucoidan slime every day. The fucoidan formed is released to the outside via special mucus channels and forms an antimicrobial protective layer around the algae.
“Fucoidan made up about half of the excretions of the brown algae we studied called bladderwrack,” says Buck-Wiese. According to the researchers, the slime is very resistant to degradation. “Fucoidan is so complex that it is difficult for other organisms to use it. Nobody seems to like it.”
The result: the fucoidan can sink towards the seabed and be permanently stored there in the sediments, the carbon in the fucoidan does not get back into the atmosphere as quickly. “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.
It is particularly advantageous that the slime consists exclusively of carbon, oxygen, hydrogen and sulfur. No nutrients such as nitrogen would be used for its formation. This means that the slime production can take place independently of the growth of the algae. It probably also takes place in nutrient-poor environments as long as light and carbon dioxide are present.
When it comes to the value of algae as carbon sinks, only the amount bound during growth is usually considered, the scientists write. The release of carbon-binding algal slime is believed to be a previously overlooked contribution of brown algae to carbon dioxide removal.
A major goal of the study was the development of measurement techniques for fucoidan. “The excretions of the brown algae are very complex and therefore incredibly difficult to measure,” says Buck-Wiese. The researchers point out that the measurements were only carried out at one location and at one time. Further investigations into the role of fucoidan in the carbon cycle are therefore needed.
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