Marine bacteroidetes use a conserved enzymatic cascade to digest diatom β-mannan

The polysaccharide β-mannan, which is common in terrestrial plants but unknown in microalgae, was recently detected during diatom blooms. We identified a β-mannan polysaccharide utilization locus (PUL) in the genome of the marine flavobacterium Muricauda sp. MAR_2010_75. Proteomics showed β-mannan induced translation of 22 proteins encoded within the PUL. Biochemical and structural analyses deduced the enzymatic cascade for β-mannan utilization. A conserved GH26 β-mannanase with endo-activity depolymerized the β-mannan. Consistent with the biochemistry, X-ray crystallography showed the typical TIM-barrel fold of related enzymes found in terrestrial β-mannan degraders. Structural and biochemical analyses of a second GH26 allowed the prediction of an exo-activity on shorter manno-gluco oligosaccharides. Further analysis demonstrated exo-α-1,6-galactosidase- and endo-β-1,4-glucanase activity of the PUL-encoded GH27 and GH5_26, respectively, indicating the target substrate is a galactoglucomannan. Epitope deletion assays with mannanases as analytic tools indicate the presence of β-mannan in the diatoms Coscinodiscus wailesii and Chaetoceros affinis. Mannanases from the PUL were active on diatom β-mannan and polysaccharide extracts sampled during a microalgal bloom at the North Sea. Together these results demonstrate that marine microorganisms use a conserved enzymatic cascade to degrade β-mannans of marine and terrestrial origin and that this metabolic pathway plays a role in marine carbon cycling

Alpha-glucans from bacterial necromass indicate an intra-population loop within the marine carbon cycle

Abstract Phytoplankton blooms provoke bacterioplankton blooms, from which bacterial biomass (necromass) is released via increased zooplankton grazing and viral lysis. While bacterial consumption of algal biomass during blooms is well-studied, little is known about the concurrent recycling of these substantial amounts of bacterial necromass. We demonstrate that bacterial biomass, such as bacterial alpha-glucan storage polysaccharides, generated from the consumption of algal organic matter, is reused and thus itself a major bacterial carbon source in vitro and during a diatom-dominated bloom. We highlight conserved enzymes and binding proteins of dominant bloom-responder clades that are presumably involved in the recycling of bacterial alpha-glucan by members of the bacterial community. We furthermore demonstrate that the corresponding protein machineries can be specifically induced by extracted alpha-glucan-rich bacterial polysaccharide extracts. This recycling of bacterial necromass likely constitutes a large-scale intra-population energy conservation mechanism that keeps substantial amounts of carbon in a dedicated part of the microbial loop