Utilization of alpha- and beta-mannan by marine Flavobacteriia

About one half of the global, biogenic carbon dioxide fixation into organic matter is driven by microscopic algae in the surface oceans. These microalgal activities generate, among other molecules, polysaccharides that are food for and recycled by bacteria with polysaccharide utilization loci (PULs). These genetic clusters of co-evolved genes, which work together in recognition, depolymerizing and uptake of one type of polysaccharide. However, we rarely know the substrates of PULs present in marine bacteria. Here we investigated the proteomic and physiological response of mannan PULs from marine Flavobacteriia isolated in the North Sea. The genomic clusters of these marine Bacteroidetes are related to PULs of human gut Bacteroides strains, which are known to digest α- and β-mannans from yeasts and plants respectively. Proteomics and defined growth experiments with these types of mannans as sole carbon source confirmed the functional prediction. Our data suggest that biochemical principles established for gut or terrestrial microbes apply to marine bacteria even though the PULs are evolutionary distant. Moreover, our data support discoveries from the 60th reporting mannans in microalgae suggesting that these polysaccharides play an important role in the marine carbon cycle.