Brown algae absorb large amounts of carbon dioxide and remove it from the global carbon cycle over the long term. Not only do they use it for growth, but they also use the greenhouse gas to form a sugary slime that is hard to break down and can remain in sediment for centuries.
This is what the scientists of the Max Planck Institute for Marine Microbiology in Bremen report in the specialized journal”PNASaccording to studies with bladderwrack. This hitherto overlooked contribution of brown algae to removing carbon dioxide from the atmosphere could counteract climate change.
Brown algae are multicellular algae found mainly in the sea. They include kelp, knotweed, or the bladder wrasse investigated in the study Vesicular ink, widespread in the North and Baltic Seas and in the North Atlantic. Brown algae fix more carbon per unit area than terrestrial forests, writes the team led by Hagen Buck-Wiese in their 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 algae are grown, harvested and used, is also being discussed as a measure against climate change.
A slime as a carbon reservoir
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 fucus in the Finnish Baltic Sea showed that the algae release 0.3% of their biomass in the form of fucoidan slime every day. The fucoidan formed is released to the outside through special mucus channels and forms a protective antimicrobial layer around the algae.
“Fucoidan made up about half of the excretions of the brown algae we studied called fucus,” says Buck-Wiese. According to the researchers, the slime is very resistant to decay. “Fucoidan is so complex that it is difficult for other organisms to use it. Nobody seems to like it.”
The result: Fucoidan can sink to the bottom of the sea and be stored there permanently in sediments, the carbon in fucoidan does not return to the atmosphere as quickly. “Brown algae are therefore particularly effective at removing carbon dioxide from the atmosphere over the long term, over hundreds to thousands of years,” says Buck-Wiese.
It is particularly advantageous that the slime consists exclusively of carbon, oxygen, hydrogen and sulfur. No nutrient like nitrogen would be used for its formation. This means that slime production can occur independently of algae growth. 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, usually only the amount bound during growth is considered, the scientists write. The release of carbon-binding algal slime is thought to be a previously overlooked contribution of brown algae to carbon dioxide removal.
One of the main objectives of the study was to develop measurement techniques for fucoidan. “The excretions of brown algae are very complex and therefore incredibly difficult to measure,” says Buck-Wiese. The researchers point out that the measurements were only taken in one place and at one time. Further investigations into the role of fucoidan in the carbon cycle are therefore needed.
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