Decomposition of brown algae in the ocean by microbiota: biological insights for recycling blue carbon

Brown algae are one of the most abundant biomasses on Earth. To recycle them as blue carbon sources, an effective decomposition system is necessary. This study focused on microorganisms present in seawater that decompose brown algae which contain laminarin and alginate. Where Undaria and Sargassum spp. were present, genera Psychromonas, Psychrobacter, and Pseudoalteromonas were predominant in seawater, while genera Arcobacter and Fusobacterium increased in abundance during the process of decomposition. The inoculation of Undaria samples into laminarin-minimal media led to a predominance of Pseudoalteromonas species. A Pseudoalteromonas isolate, identified as Pseudoalteromonas distincta, possesses genes encoding a putative laminarinase, polysaccharide lyase family 6 (PL6) alginate lyases, and a PL7 alginate lyase. The culture media of P. distincta contained no monosaccharides, suggesting the rapid conversion of polysaccharides to metabolites. These findings indicated that Pseudoalteromonas species play a major role in the decomposition of brown algae and affect the microbiota associated with them. Graphical Abstract Blue carbon recycling through decomposition of laminarin- and alginate-enriched brown algae by marine microbiota.