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

Abstract

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.

Deep-sea Bacteroidetes from the Mariana Trench specialize in hemicellulose and pectin degradation typically associated with terrestrial systems

“…Most of these genes were affiliated to Flavobacteriaceae. Whilst the importance of Bacteroidetes- mediated polysaccharide cycling has been well documented in surface waters [ 21 , 23 , 38 , 39 ], we observed that the relative abundance of Bacteroidetes CAZyme genes generally increased with depth, except for a large decrease in near-bottom waters (Fig. 2 A).…”

“…Recently, these polysaccharides were detected in surface waters during a diatom bloom [ 47 ] and associated Bacteroidetes spp. were shown to possess PULs for their degradation [ 21 , 23 , 39 ]. Since algal cell walls evolved to be highly resistant to a wide range of biotic stresses like enzymatic hydrolysis, their pectic and hemicellulosic glycan components may persist longer than internal storage glycans like laminarin [ 47 ] and, thus, be more likely to sink to deeper waters.…”

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

“…This utilization of dissolved algal glycans represents an essential part of the marine microbial loop 6 and thus the global carbon cycle. Particularly prominent in this process are marine members of the phylum Bacteroidota 7 – 9 . These bacteria target algal polysaccharides by specific sets of enzymes and transporters encoded in genomic islands, termed polysaccharide utilization loci (sg.…”

Marine Bacteroidetes enzymatically digest xylans from terrestrial plants

“…Members of the phylum Bacteroidetes are selected in such glycan‐rich environments (McKee et al, 2021). Recent studies showed the utilization of a diverse range of algal glycans by members of this phylum (e.g., Barbeyron et al, 2016; Beidler et al, 2023; Ficko‐Blean et al, 2017; Hehemann et al, 2012; Reisky et al, 2019; Robb et al, 2022; Unfried et al, 2018). Their carbohydrate‐active enzymes, or CAZymes, are often co‐located with transporters and regulatory proteins in polysaccharide utilization loci (PULs) within bacterial genomes (Martens et al, 2009).…”