Reaching out in anticipation: bacterial membrane extensions represent a permanent investment in polysaccharide sensing and utilization.

Outer membrane extensions are common in many marine bacteria. However, the function of these surface enlargements or extracellular compartments is poorly understood. Using a combined approach of microscopy and subproteome analyses, we therefore examined Pseudoalteromonas distincta ANT/505, an Antarctic polysaccharide degrading gamma-proteobacterium. P. distincta produced outer membrane vesicles (MV) and vesicle chains (VC) on polysaccharide and non-polysaccharide carbon sources during the exponential and stationary growth phase. Surface structures of carbohydrate-grown cells were equipped with increased levels of highly substrate-specific proteins. At the same time, proteins encoded in all other polysaccharide degradation-related genomic regions were also detected in MV and VC samples under all growth conditions, indicating a basal expression. In addition, two alkaline phosphatases were highly abundant under non-limiting phosphate conditions. Surface structures may thus allow rapid sensing and fast responses in nutritionally deprived environments. It may also facilitate efficient carbohydrate processing and reduce loss of substrates and enzymes by diffusion as important adaptions to the aquatic ecosystem.

A marine bacterial enzymatic cascade degrades the algal polysaccharide ulvan.

Marine seaweeds increasingly grow into extensive algal blooms, which are detrimental to coastal ecosystems, tourism and aquaculture. However, algal biomass is also emerging as a sustainable raw material for the bioeconomy. The potential exploitation of algae is hindered by our limited knowledge of the microbial pathways-and hence the distinct biochemical functions of the enzymes involved-that convert algal polysaccharides into oligo- and monosaccharides. Understanding these processes would be essential, however, for applications such as the fermentation of algal biomass into bioethanol or other value-added compounds. Here, we describe the metabolic pathway that enables the marine flavobacterium Formosa agariphila to degrade ulvan, the main cell wall polysaccharide of bloom-forming Ulva species. The pathway involves 12 biochemically characterized carbohydrate-active enzymes, including two polysaccharide lyases, three sulfatases and seven glycoside hydrolases that sequentially break down ulvan into fermentable monosaccharides. This way, the enzymes turn a previously unexploited renewable into a valuable and ecologically sustainable bioresource.

Exploiting fine-scale genetic and physiological variation of closely related microbes to reveal unknown enzyme functions.

Polysaccharide degradation by marine microbes represents one of the largest and most rapid heterotrophic transformations of organic matter in the environment. Microbes employ systems of complementary carbohydrate-specific enzymes to deconstruct algal or plant polysaccharides (glycans) into monosaccharides. Because of the high diversity of glycan substrates, the functions of these enzymes are often difficult to establish. One solution to this problem may lie within naturally occurring microdiversity; varying numbers of enzymes, due to gene loss, duplication, or transfer, among closely related environmental microbes create metabolic differences akin to those generated by knock-out strains engineered in the laboratory used to establish the functions of unknown genes. Inspired by this natural fine-scale microbial diversity, we show here that it can be used to develop hypotheses guiding biochemical experiments for establishing the role of these enzymes in nature. In this work, we investigated alginate degradation among closely related strains of the marine bacterium Vibrio splendidus One strain, V. splendidus 13B01, exhibited high extracellular alginate lyase activity compared with other V. splendidus strains. To identify the enzymes responsible for this high extracellular activity, we compared V. splendidus 13B01 with the previously characterized V. splendidus 12B01, which has low extracellular activity and lacks two alginate lyase genes present in V. splendidus 13B01. Using a combination of genomics, proteomics, biochemical, and functional screening, we identified a polysaccharide lyase family 7 enzyme that is unique to V. splendidus 13B01, secreted, and responsible for the rapid digestion of extracellular alginate. These results demonstrate the value of querying the enzymatic repertoires of closely related microbes to rapidly pinpoint key proteins with beneficial functions.

Alpha- and beta-mannan utilization by marine Bacteroidetes.

Marine microscopic algae carry out about half of the global carbon dioxide fixation into organic matter. They provide organic substrates for marine microbes such as members of the Bacteroidetes that degrade algal polysaccharides using carbohydrate-active enzymes (CAZymes). In Bacteroidetes genomes CAZyme encoding genes are mostly grouped in distinct regions termed polysaccharide utilization loci (PULs). While some studies have shown involvement of PULs in the degradation of algal polysaccharides, the specific substrates are for the most part still unknown. We investigated four marine Bacteroidetes isolated from the southern North Sea that harbour putative mannan-specific PULs. These PULs are similarly organized as PULs in human gut Bacteroides that digest α- and ß-mannans from yeasts and plants respectively. Using proteomics and defined growth experiments with polysaccharides as sole carbon sources we could show that the investigated marine Bacteroidetes express the predicted functional proteins required for α- and ß-mannan degradation. Our data suggest that algal mannans play an as yet unknown important role in the marine carbon cycle, and that biochemical principles established for gut or terrestrial microbes also apply to marine bacteria, even though their PULs are evolutionarily distant.

Aquatic adaptation of a laterally acquired pectin degradation pathway in marine gammaproteobacteria.

Mobile genomic islands distribute functional traits between microbes and habitats, yet it remains unclear how their proteins adapt to new environments. Here we used a comparative phylogenomic and proteomic approach to show that the marine bacterium Pseudoalteromonas haloplanktis ANT/505 acquired a genomic island with a functional pathway for pectin catabolism. Bioinformatics and biochemical experiments revealed that this pathway encodes a series of carbohydrate-active enzymes including two multi-modular pectate lyases, PelA and PelB. PelA is a large enzyme with a polysaccharide lyase family 1 (PL1) domain and a carbohydrate esterase family 8 domain, and PelB contains a PL1 domain and two carbohydrate-binding domains of family 13. Comparative phylogenomic analyses indicate that the pathway was most likely acquired from terrestrial microbes, yet we observed multi-modular orthologues only in marine bacteria. Proteomic experiments showed that P. haloplanktis ANT/505 secretes both pectate lyases into the environment in the presence of pectin. These multi-modular enzymes may therefore represent a marine innovation that enhances physical interaction with pectins to reduce loss of substrate and enzymes by diffusion. Our results revealed that marine bacteria can catabolize pectin, and highlight enzyme fusion as a potential adaptation that may facilitate microbial consumption of polymeric substrates in aquatic environments.

Diverse events have transferred genes for edible seaweed digestion from marine to human gut bacteria.

Humans harbor numerous species of colonic bacteria that digest fiber polysaccharides in commonly consumed terrestrial plants. More recently in history, regional populations have consumed edible macroalgae seaweeds containing unique polysaccharides. It remains unclear how extensively gut bacteria have adapted to digest these nutrients. Here, we show that the ability of gut bacteria to digest seaweed polysaccharides is more pervasive than previously appreciated. Enrichment-cultured Bacteroides harbor previously discovered genes for seaweed degradation, which have mobilized into several members of this genus. Additionally, other examples of marine bacteria-derived genes, and their mobile DNA elements, are involved in gut microbial degradation of seaweed polysaccharides, including genes in gut-resident Firmicutes. Collectively, these results uncover multiple separate events that have mobilized the genes encoding seaweed-degrading-enzymes into gut bacteria. This work further underscores the metabolic plasticity of the human gut microbiome and global exchange of genes in the context of dietary selective pressures.

Verrucomicrobia use hundreds of enzymes to digest the algal polysaccharide fucoidan.

Brown algae are important players in the global carbon cycle by fixing carbon dioxide into 1 Gt of biomass annually, yet the fate of fucoidan-their major cell wall polysaccharide-remains poorly understood. Microbial degradation of fucoidans is slower than that of other polysaccharides, suggesting that fucoidans are more recalcitrant and may sequester carbon in the ocean. This may be due to the complex, branched and highly sulfated structure of fucoidans, which also varies among species of brown algae. Here, we show that 'Lentimonas' sp. CC4, belonging to the Verrucomicrobia, acquired a remarkably complex machinery for the degradation of six different fucoidans. The strain accumulated 284 putative fucoidanases, including glycoside hydrolases, sulfatases and carbohydrate esterases, which are primarily located on a 0.89-megabase pair plasmid. Proteomics reveals that these enzymes assemble into substrate-specific pathways requiring about 100 enzymes per fucoidan from different species of brown algae. These enzymes depolymerize fucoidan into fucose, which is metabolized in a proteome-costly bacterial microcompartment that spatially constrains the metabolism of the toxic intermediate lactaldehyde. Marine metagenomes and microbial genomes show that Verrucomicrobia including 'Lentimonas' are abundant and highly specialized degraders of fucoidans and other complex polysaccharides. Overall, the complexity of the pathways underscores why fucoidans are probably recalcitrant and more slowly degraded, since only highly specialized organisms can effectively degrade them in the ocean.

Polysaccharide utilization loci of North Sea Flavobacteriia as basis for using SusC/D-protein expression for predicting major phytoplankton glycans.

Marine algae convert a substantial fraction of fixed carbon dioxide into various polysaccharides. Flavobacteriia that are specialized on algal polysaccharide degradation feature genomic clusters termed polysaccharide utilization loci (PULs). As knowledge on extant PUL diversity is sparse, we sequenced the genomes of 53 North Sea Flavobacteriia and obtained 400 PULs. Bioinformatic PUL annotations suggest usage of a large array of polysaccharides, including laminarin, α-glucans, and alginate as well as mannose-, fucose-, and xylose-rich substrates. Many of the PULs exhibit new genetic architectures and suggest substrates rarely described for marine environments. The isolates' PUL repertoires often differed considerably within genera, corroborating ecological niche-associated glycan partitioning. Polysaccharide uptake in Flavobacteriia is mediated by SusCD-like transporter complexes. Respective protein trees revealed clustering according to polysaccharide specificities predicted by PUL annotations. Using the trees, we analyzed expression of SusC/D homologs in multiyear phytoplankton bloom-associated metaproteomes and found indications for profound changes in microbial utilization of laminarin, α-glucans, ß-mannan, and sulfated xylan. We hence suggest the suitability of SusC/D-like transporter protein expression within heterotrophic bacteria as a proxy for the temporal utilization of discrete polysaccharides.

Adaptive mechanisms that provide competitive advantages to marine bacteroidetes during microalgal blooms.

Polysaccharide degradation by heterotrophic microbes is a key process within Earth's carbon cycle. Here, we use environmental proteomics and metagenomics in combination with cultivation experiments and biochemical characterizations to investigate the molecular details of in situ polysaccharide degradation mechanisms during microalgal blooms. For this, we use laminarin as a model polysaccharide. Laminarin is a ubiquitous marine storage polymer of marine microalgae and is particularly abundant during phytoplankton blooms. In this study, we show that highly specialized bacterial strains of the Bacteroidetes phylum repeatedly reached high abundances during North Sea algal blooms and dominated laminarin turnover. These genomically streamlined bacteria of the genus Formosa have an expanded set of laminarin hydrolases and transporters that belonged to the most abundant proteins in the environmental samples. In vitro experiments with cultured isolates allowed us to determine the functions of in situ expressed key enzymes and to confirm their role in laminarin utilization. It is shown that laminarin consumption of Formosa spp. is paralleled by enhanced uptake of diatom-derived peptides. This study reveals that genome reduction, enzyme fusions, transporters, and enzyme expansion as well as a tight coupling of carbon and nitrogen metabolism provide the tools, which make Formosa spp. so competitive during microalgal blooms.

Niches of two polysaccharide-degrading Polaribacter isolates from the North Sea during a spring diatom bloom.

Members of the flavobacterial genus Polaribacter thrive in response to North Sea spring phytoplankton blooms. We analyzed two respective Polaribacter species by whole genome sequencing, comparative genomics, substrate tests and proteomics. Both can degrade algal polysaccharides but occupy distinct niches. The liquid culture isolate Polaribacter sp. strain Hel1_33_49 has a 3.0-Mbp genome with an overall peptidase:CAZyme ratio of 1.37, four putative polysaccharide utilization loci (PULs) and features proteorhodopsin, whereas the agar plate isolate Polaribacter sp. strain Hel1_85 has a 3.9-Mbp genome with an even peptidase:CAZyme ratio, eight PULs, a mannitol dehydrogenase for decomposing algal mannitol-capped polysaccharides but no proteorhodopsin. Unlike other sequenced Polaribacter species, both isolates have larger sulfatase-rich PULs, supporting earlier assumptions that Polaribacter take part in the decomposition of sulfated polysaccharides. Both strains grow on algal laminarin and the sulfated polysaccharide chondroitin sulfate. For strain Hel1_33_49, we identified by proteomics (i) a laminarin-induced PUL, (ii) chondroitin sulfate-induced CAZymes and (iii) a chondroitin-induced operon that likely enables chondroitin sulfate recognition. These and other data suggest that strain Hel1_33_49 is a planktonic flavobacterium feeding on proteins and a small subset of algal polysaccharides, while the more versatile strain Hel1_85 can decompose a broader spectrum of polysaccharides and likely associates with algae.