Polysaccharide utilisation loci ofBacteroidetesfrom two contrasting open ocean sites in the North Atlantic

Temporal succession of marine microbes drives plastisphere community convergence in subtropical coastal waters

Marine plastic pollution is a pervasive environmental issue, with microplastics serving as novel substrates for microbial colonization in aquatic ecosystems. This study investigates the succession of plastisphere communities on four common plastic types (polyethylene, polypropylene, polyethylene terephthalate, and polystyrene) in subtropical coastal waters of Hong Kong SAR. Over a 42-day period, we analysed the temporal development of microbial communities using a three-domain universal metabarcoding method. Our results reveal that temporal succession is a stronger driver of community structure than plastic type, with prokaryotic communities converging across different plastics as biofilms mature. Despite this convergence, plastisphere communities remain distinct from planktonic communities throughout the experiment, suggesting that plastics create unique ecological niches in marine environments. We observed differences in diversity patterns and community composition among prokaryotic, eukaryotic, and chloroplastic communities, highlighting the importance of multi-domain analyses in plastisphere research. Functional predictions suggest potential roles of prokaryotic communities in biogeochemical cycling and possible pathogenicity, highlighting the ecological and public health implications of plastisphere formation. This study provides valuable insights into the dynamics of microbial colonization across domains on marine plastics and enhances our understanding of how these anthropogenic substrates influence microbial ecology in marine ecosystems.

Characterization of a novel multifunctional glycoside hydrolase family in the metagenome-assembled genomes of horse gut

The gut microbiota is a treasure trove of carbohydrate-active enzymes (CAZymes). To explore novel and efficient CAZymes, we analyzed the 4,142 metagenome-assembled genomes (MAGs) of the horse gut microbiota and found the MAG117.bin13 genome (Bacteroides fragilis) contains the highest number of polysaccharide utilisation loci sites (PULs), indicating its high capability for carbohydrate degradation. Bioinformatics analysis indicate that the PULs region of the MAG117.bin13 genome encodes many hypothetical proteins, which are important sources for exploring novel CAZymes. Interestingly, we discovered a hypothetical protein (595 amino acids). This protein exhibits potential CAZymes activity and has a lower similarity to CAZymes, we named it BfLac2275. We purified the protein using prokaryotic expression technology and studied its enzymatic function. The hydrolysis experiment of the polysaccharide substrate showed that the BfLac2275 protein has the ability to degrade α-lactose (156.94 U/mg), maltose (92.59 U/mg), raffinose (86.81 U/mg), and hyaluronic acid (5.71 U/mg). The enzyme activity is optimal at pH 5.0 and 30 ℃, indicating that the hypothetical protein BfLac2275 is a novel and multifunctional CAZymes in the glycoside hydrolases (GHs). These properties indicate that BfLac2275 has broad application prospects in many fields such as plant polysaccharide decomposition, food industry, animal feed additives and enzyme preparations. This study not only serves as a reference for exploring novel CAZymes encoded by gut microbiota but also provides an example for further studying the functional annotation of hypothetical genes in metagenomic assembly genomes.

Specialized Bacteroidetes dominate the Arctic Ocean during marine spring blooms

A metagenomic time series from Arctic seawater was obtained from Dease Strait, to analyse the changes in bacterioplankton caused by the summer phytoplankton bloom. Bacterial clades specialized in the metabolism of polysaccharides, such as Bacteroidetes, became dominant along the bloom. These specialized taxa quickly displaced the microbial clades that dominate nutrient-poor waters during early spring, such as Archaea, Alpha-and Gammaproteobacteria. At the functional level, phyla Bacteroidetes, Planctomycetes, and Verrucomicrobia showed higher contents of polysaccharide-degradation functions. The Bacteroidetes community shifted toward species with higher polysaccharide-degrading capabilities, targeting algal polysaccharides in summer. Regarding transporters, Bacteroidetes dominated SusC-TonB transporters and had an exclusive family of glycoside-binding proteins (SusD). These proteins were used to identify polysaccharide-utilization loci that clustered transporters and polysaccharide-active enzymes, showing a higher level of specialization toward polysaccharide use. Altogether, these genomic features point to the genetic adaptations that promote the dominance of Bacteroidetes during phytoplankton blooms.

Temporal variations of bacterial and eukaryotic community in coastal waters-implications for aquaculture

Despite increased attention to the aquaculture environment, there is still a lack of understanding regarding the significance of water quality. To address this knowledge gap, this study utilized high-throughput sequencing of 16S rRNA and 18S rRNA to examine microbial communities (bacteria and eukaryotes) in coastal water over different months through long-term observations. The goal was to explore interaction patterns in the microbial community and identify potential pathogenic bacteria and red tide organisms. The results revealed significant differences in composition, diversity, and richness of bacterial and eukaryotic operational taxonomic units (OTUs) across various months. Principal coordinate analysis (PCoA) demonstrated distinct temporal variations in bacterial and eukaryotic communities, with significant differences (P = 0.001) among four groups: F (January-April), M (May), S (June-September), and T (October-December). Moreover, a strong association was observed between microbial communities and months, with most OTUs showing a distinct temporal preference. The Kruskal-Wallis test (P < 0.05) indicated significant differences in dominant bacterial and eukaryotic taxa among months, with each group exhibiting unique dominant taxa, including potential pathogenic bacteria and red tide organisms. These findings emphasize the importance of monitoring changes in potentially harmful microorganisms in aquaculture. Network analysis highlighted positive correlations between bacteria and eukaryotes, with bacteria playing a key role in network interactions. The key bacterial genera associated with other microorganisms varied significantly (P < 0.05) across different groups. In summary, this study deepens the understanding of aquaculture water quality and offers valuable insights for maintaining healthy aquaculture practices. KEY POINTS: • Bacterial and eukaryotic communities displayed distinct temporal variations. • Different months exhibited unique potential pathogenic bacteria and red tide organisms. • Bacteria are key taxonomic taxa involved in microbial network interactions.

Responses of attached bacterial communities to blooms of the swimming shelled pteropod Creseis acicula in Daya Bay, southern China

The shelled pteropod Creseis acicula is a marine pelagic shellfish widely distributed from temperate to tropical seas around the world. From June to July 2020, a C. acicula bloom first happened in the Daya Bay, southern China, and its density reached the highest value (5600 ind. m-3) ever recorded around the world. However, few studies have investigated the responses of bacterial communities to the C. acicula bloom. In the present study, we examined the community profiles of three communities of bacteria including the free-living and particle-attached bacteria in the blooming and reference waters, and bacteria attached to the whole body and shell of C. acicula using a high-throughput sequencing method. The results indicated that the C. acicula bloom had a greater impact on particle-attached bacteria than free-living bacteria. Among the bloom-sensitive particle-attached bacteria, the predominant bacterial phyla were Pseudomonadota, Bacteroidota and Verrucomicrobiota in the blooming areas, whereas they were Actinomycetota and Planctomycetota in the reference areas. Specifically, fecal bacteria Haloferula and Halioglobus spp. were significantly enriched in the blooming waters and accumulated on C. acicula shells. Conversely, the significantly lower relative abundance of Nocardioides sp. in the blooming area and accumulated on the whole body of C. acicula indicated their attachment to particles consumed by C. acicula. Overall, our results suggested that the C. acicula bloom influenced marine bacteria, particularly particle-attached bacteria, by increasing (e.g. providing shells and feces) or decreasing (e.g. filter-feeding the suspended particles) the abundance of available substances.

Ecological function and interaction of different bacterial groups during alginate processing in coastal seawater community

The degradation of high molecular weight organic matter (HMWOM) is a core process of oceanic carbon cycle, which is determined by the activity of microbial communities harboring hundreds of different species. Illustrating the active microbes and their interactions during HMWOM processing can provide key information for revealing the relationship between community composition and its ecological functions. In this study, the genomic and transcriptional responses of microbial communities to the availability of alginate, an abundant HMWOM in coastal ecosystem, were elucidated. The main degraders transcribing alginate lyase (Aly) genes came from genera Alteromonas, Psychrosphaera and Colwellia. Meanwhile, some strains, mainly from the Rhodobacteraceae family, did not transcribe Aly gene but could utilize monosaccharides to grow. The co-culture experiment showed that the activity of Aly-producing strain could promote the growth of Aly-non-producing strain when alginate was the sole carbon source. Interestingly, this interaction did not reduce the alginate degradation rate, possibly due to the easily degradable nature of alginate. This study can improve our understanding of the relationship between microbial community activity and alginate metabolism function as well as further manipulation of microbial community structure for alginate processing.

Candidatus Nemesobacterales is a sponge-specific clade of the candidate phylum Desulfobacterota adapted to a symbiotic lifestyle

Members of the candidate phylum Dadabacteria, recently reassigned to the phylum Candidatus Desulfobacterota, are cosmopolitan in the marine environment found both free-living and associated with hosts that are mainly marine sponges. Yet, these microorganisms are poorly characterized, with no cultured representatives and an ambiguous phylogenetic position in the tree of life. Here, we performed genome-centric metagenomics to elucidate their phylogenomic placement and predict the metabolism of the sponge-associated members of this lineage. Rank-based phylogenomics revealed several new species and a novel family (Candidatus Spongomicrobiaceae) within a sponge-specific order, named here Candidatus Nemesobacterales. Metabolic reconstruction suggests that Ca. Nemesobacterales are aerobic heterotrophs, capable of synthesizing most amino acids, vitamins and cofactors and degrading complex carbohydrates. We also report functional divergence between sponge- and seawater-associated metagenome-assembled genomes. Niche-specific adaptations to the sponge holobiont were evident from significantly enriched genes involved in defense mechanisms against foreign DNA and environmental stressors, host-symbiont interactions and secondary metabolite production. Fluorescence in situ hybridization gave a first glimpse of the morphology and lifestyle of a member of Ca. Desulfobacterota. Candidatus Nemesobacterales spp. were found both inside sponge cells centred around sponge nuclei and in the mesohyl of the sponge Geodia barretti. This study sheds light on the enigmatic group Ca. Nemesobacterales and their functional characteristics that reflect a symbiotic lifestyle.

Importance of Bacteroidetes in host-microbe interactions and ecosystem functioning

Bacteroidetes are prevalent in soil ecosystems and are associated with various eukaryotic hosts, including plants, animals, and humans. The ubiquity and diversity of Bacteroidetes exemplify their impressive versatility in niche adaptation and genomic plasticity. Over the past decade, a wealth of knowledge has been obtained on the metabolic functions of clinically relevant Bacteroidetes, but much less attention has been given to Bacteroidetes living in close association with plants. To improve our understanding of the functional roles of Bacteroidetes for plants and other hosts, we review the current knowledge of their taxonomy and ecology, in particular their roles in nutrient cycling and host fitness. We highlight their environmental distribution, stress resilience, genomic diversity, and functional importance in diverse ecosystems, including, but not limited to, plant-associated microbiomes.

β-glucans: a potential source for maintaining gut microbiota and the immune system

The human gastrointestinal (GI) tract holds a complex and dynamic population of microbial communities, which exerts a marked influence on the host physiology during homeostasis and disease conditions. Diet is considered one of the main factors in structuring the gut microbiota across a lifespan. Intestinal microbial communities play a vital role in sustaining immune and metabolic homeostasis as well as protecting against pathogens. The negatively altered gut bacterial composition has related to many inflammatory diseases and infections. β-glucans are a heterogeneous assemblage of glucose polymers with a typical structure comprising a leading chain of β-(1,4) and/or β-(1,3)-glucopyranosyl units with various branches and lengths as a side chain. β-glucans bind to specific receptors on immune cells and initiate immune responses. However, β-glucans from different sources differ in their structures, conformation, physical properties, and binding affinity to receptors. How these properties modulate biological functions in terms of molecular mechanisms is not known in many examples. This review provides a critical understanding of the structures of β-glucans and their functions for modulating the gut microbiota and immune system.

Prokaryotic maintenance respiration and growth efficiency field patterns reproduced by temperature and nutrient control at mesocosm scale

The distribution of prokaryotic metabolism between maintenance and growth activities has a profound impact on the transformation of carbon substrates to either biomass or CO₂ . Knowledge of key factors influencing prokaryotic maintenance respiration is, however, highly limited. This mesocosm study validated the significance of prokaryotic maintenance respiration by mimicking temperature and nutrients within levels representative of winter and summer conditions. A global range of growth efficiencies (0.05-0.57) and specific growth rates (0.06-2.7 d^-1 ) were obtained. The field pattern of cell-specific respiration versus specific growth rate and the global relationship between growth efficiency and growth rate were reproduced. Maintenance respiration accounted for 75% and 15% of prokaryotic respiration corresponding to winter and summer conditions, respectively. Temperature and nutrients showed independent positive effects for all prokaryotic variables except abundance and cell-specific respiration. All treatments resulted in different taxonomic diversity, with specific populations of amplicon sequence variants associated with either maintenance or growth conditions. These results validate a significant relationship between specific growth and respiration rate under productive conditions and show that elevated prokaryotic maintenance respiration can occur under cold and oligotrophic conditions. The experimental design provides a tool for further study of prokaryotic energy metabolism under realistic conditions at the mesocosm scale.

Gramella sediminis sp. nov., isolated from a tidal flat of the Yellow Sea

A novel species of the genus Gramella , designated ASW11-100T, was isolated from a tidal flat sediment in the Yellow Sea, PR China. Phylogenetic analysis based on 16S rRNA gene sequences and single-copy orthologous clusters revealed that strain ASW11-100T belonged to the genus Gramella , and exhibited 16S rRNA gene sequence similarities of 98.9, 98.8 and 98.7 % to Gramella sabulilitoris HSMS-1T, Gramella sediminilitoris GHTF-27T and Gramella forsetii KT0803T, respectively. The genome of strain ASW11-100T harbours 2950 protein-coding genes and 105 carbohydrate-active enzymes including 38 glycoside hydrolases. Seventeen of the glycoside hydrolases are organized in five distinct polysaccharide utilization loci, which are predicted to involve in the degradation of starch, glucans, arabinoxylans, arabinomannan, arabinans and arabinogalactans. The genomic DNA G+C content was 37.3 mol%. The digital DNA–DNA hybridization and average nucleotide identity values between strain ASW11-100T and its closely related relatives were in ranges of 19.8–23.9% and 76.6–80.9 %, respectively. Cells of the isolate were Gram-negative, aerobic, non-flagellated and short rod-shaped. Carotenoid pigments were produced, but flexirubin-type pigments were absent. The major fatty acids (>10 %) were iso-C15 : 0, iso-C17 : 0 3-OH and summed feature 3 (C16 : 1  ω6c and/or C16 : 1  ω7c). The sole respiratory quinone was menaquinone-6 and the major polar lipid was phosphatidylethanolamine. Based on the above polyphasic evidence, strain ASW11-100T should be considered to represent a novel Gramella species, for which the name Gramella sediminis sp. nov. is proposed. The type strain is ASW11-100T (=KCTC 82502T=MCCC 1K05580T).

Red, Gold and Green: Microbial Contribution of Rhodophyta and Other Algae to Green Turtle (Chelonia mydas) Gut Microbiome

The fitness of the endangered green sea turtle (Chelonia mydas) may be strongly affected by its gut microbiome, as microbes play important roles in host nutrition and health. This study aimed at establishing environmental microbial baselines that can be used to assess turtle health under altered future conditions. We characterized the microbiome associated with the gastrointestinal tract of green turtles from Guinea Bissau in different life stages and associated with their food items, using 16S rRNA metabarcoding. We found that the most abundant (% relative abundance) bacterial phyla across the gastrointestinal sections were Proteobacteria (68.1 ± 13.9% “amplicon sequence variants”, ASVs), Bacteroidetes (15.1 ± 10.1%) and Firmicutes (14.7 ± 21.7%). Additionally, we found the presence of two red algae bacterial indicator ASVs (the Alphaproteobacteria Brucella pinnipedialis with 75 ± 0% and a Gammaproteobacteria identified as methanotrophic endosymbiont of Bathymodiolus, with <1%) in cloacal compartments, along with six bacterial ASVs shared only between cloacal and local environmental red algae samples. We corroborate previous results demonstrating that green turtles fed on red algae (but, to a lower extent, also seagrass and brown algae), thus, acquiring microbial components that potentially aid them digest these food items. This study is a foundation for better understanding the microbial composition of sea turtle digestive tracts.

Glycoside hydrolase from the GH76 family indicates that marine Salegentibacter sp. Hel_I_6 consumes alpha-mannan from fungi

Microbial glycan degradation is essential to global carbon cycling. The marine bacterium Salegentibacter sp. Hel_I_6 (Bacteroidota) isolated from seawater off Helgoland island (North Sea) contains an α-mannan inducible gene cluster with a GH76 family endo-α-1,6-mannanase (ShGH76). This cluster is related to genetic loci employed by human gut bacteria to digest fungal α-mannan. Metagenomes from the Hel_I_6 isolation site revealed increasing GH76 gene frequencies in free-living bacteria during microalgae blooms, suggesting degradation of α-1,6-mannans from fungi. Recombinant ShGH76 protein activity assays with yeast α-mannan and synthetic oligomannans showed endo-α-1,6-mannanase activity. Resolved structures of apo-ShGH76 (2.0 Å) and of mutants co-crystalized with fungal mannan-mimicking α-1,6-mannotetrose (1.90 Å) and α-1,6-mannotriose (1.47 Å) retained the canonical (α/α)6 fold, despite low identities with sequences of known GH76 structures (GH76s from gut bacteria: <27%). The apo-form active site differed from those known from gut bacteria, and co-crystallizations revealed a kinked oligomannan conformation. Co-crystallizations also revealed precise molecular-scale interactions of ShGH76 with fungal mannan-mimicking oligomannans, indicating adaptation to this particular type of substrate. Our data hence suggest presence of yet unknown fungal α-1,6-mannans in marine ecosystems, in particular during microalgal blooms.

Description of Aureibaculum luteum sp. nov. and Aureibaculum flavum sp. nov. isolated from Antarctic intertidal sediments

Two Gram-stain-negative, aerobic, non-motile, and rod-shaped bacterial strains, designated SM1352^T and A20^T, were isolated from intertidal sediments collected from King George Island, Antarctic. They shared 99.8% 16S rRNA gene sequence similarity with each other and had the highest sequence similarity of 98.1% to type strain of Aureibaculum marinum but < 93.4% sequence similarity to those of other known bacterial species. The genomes of strains SM1352^T and A20^T consisted of 5,108,092 bp and 4,772,071 bp, respectively, with the G + C contents both being 32.0%. They respectively encoded 4360 (including 37 tRNAs and 6 rRNAs) and 4032 (including 36 tRNAs and 5 rRNAs) genes. In the phylogenetic trees based on 16S rRNA gene and single-copy orthologous clusters (OCs), both strains clustered with Aureibaculum marinum and together formed a separate branch within the family Flavobacteriaceae. The ANI and DDH values between the two strains and Aureibaculum marinum BH-SD17^T were all below the thresholds for species delineation. The major cellular fatty acids (> 10%) of the two strains included iso-C_15:0, iso-C_15:1 G, iso-C_17:0 3-OH. Their polar lipids predominantly included phosphatidylethanolamine, one unidentified aminophospholipid, one unidentified aminolipid, and two unidentified lipids. Genomic comparison revealed that both strains possessed much more glycoside hydrolases and sulfatase-rich polysaccharide utilization loci (PULs) than Aureibaculum marinum BH-SD17^T. Based on the above polyphasic evidences, strains SM1352^T and A20^T represent two novel species within the genus Aureibaculum, for which the names Aureibaculum luteum sp. nov. and Aureibaculum flavum sp. nov. are proposed. The type strains are SM1352^T (= CCTCC AB 2014243 ^T = JCM 30335 ^T) and A20^T (= CCTCC AB 2020370 ^T = KCTC 82503 ^T), respectively.

Taxon-Specific Shifts in Bacterial and Archaeal Transcription of Dissolved Organic Matter Cycling Genes in a Stratified Fjord

A considerable fraction of organic matter derived from photosynthesis in the euphotic zone settles into the ocean’s interior and, as it progresses, is degraded by diverse microbial consortia that utilize a suite of extracellular enzymes and membrane transporters. Still, the molecular details that regulate carbon cycling across depths remain little explored. As stratification in fjords has made them attractive models to explore patterns in biological oceanography, we here analyzed bacterial and archaeal transcription in samples from five depth layers in the Gullmar Fjord, Sweden. Transcriptional variation over depth correlated with gradients in chlorophyll a and nutrient concentrations. Differences in transcription between sampling dates (summer and early autumn) were strongly correlated with ammonium concentrations, which potentially was linked with a stronger influence of (micro-)zooplankton grazing in summer. Transcriptional investment in carbohydrate-active enzymes (CAZymes) decreased with depth and shifted toward peptidases, partly a result of elevated CAZyme transcription by Flavobacteriales, Cellvibrionales, and Synechococcales at 2 to 25 m and a dominance of peptidase transcription by Alteromonadales and Rhodobacterales from 50 m down. In particular, CAZymes for chitin, laminarin, and glycogen were important. High levels of transcription of ammonium transporter genes by Thaumarchaeota at depth (up to 18% of total transcription), along with the genes for ammonia oxidation and CO₂ fixation, indicated that chemolithoautotrophy contributed to the carbon flux in the fjord. The taxon-specific expression of functional genes for processing of the marine pool of dissolved organic matter and inorganic nutrients across depths emphasizes the importance of different microbial foraging mechanisms over spatiotemporal scales for shaping biogeochemical cycles.

IMPORTANCE It is generally recognized that stratification in the ocean strongly influences both the community composition and the distribution of ecological functions of microbial communities, which in turn are expected to shape the biogeochemical cycling of essential elements over depth. Here, we used metatranscriptomics analysis to infer molecular detail on the distribution of gene systems central to the utilization of organic matter in a stratified marine system. We thereby uncovered that pronounced shifts in the transcription of genes encoding CAZymes, peptidases, and membrane transporters occurred over depth among key prokaryotic orders. This implies that sequential utilization and transformation of organic matter through the water column is a key feature that ultimately influences the efficiency of the biological carbon pump.

Cultivable Winogradskyella species are genomically distinct from the sympatric abundant candidate species

Winogradskyella is a genus within the phylum Bacteroidetes with a clear marine origin. Most members of this genus have been found associated with marine animals and algae, but also with inorganic surfaces such as sand. In this study, we analyzed genomes of eleven species recently isolated from surface seawater samples from the North Sea during a single spring algae bloom. Corresponding metagenomes yielded a single Candidatus species for this genus. All species in culture, with the exception of W. ursingii, affiliated with a Winogradskyella lineage characterized by large genomes (~4.3 ± 0.4 Mb), with high complexity in their carbohydrate and protein degradation genes. Specifically, the polysaccharide utilization loci (PULs) were diverse within each individual strain, indicating large substrate versatility. Although present in the North Sea, the abundances of these strains were at, or below, the detection limit of the metagenomes. In contrast, the single species, classified as Candidatus W. atlantica, to which all North Sea MAGs belonged, affiliated with a lineage in which the cultivated representatives showed small genomes of ~3.0-3.5 Mb, with the MAGs having ~2.3 Mb. In Ca. W. atlantica, genome streamlining has apparently resulted in the loss of biosynthesis pathways for several amino acids including arginine, methionine, leucine and valine, and the PUL loci were reduced to a single one for utilizing laminarin. This as-yet uncultivated species seems to capitalize on sporadically abundant substrates that are released by algae blooms, mainly laminarin. We also suggest that this streamlined genome might be responsible for the lack of growth on plates for this Candidatus species, in contrast to growth of the less abundant but coexisting members of the genus.

High-resolution 3D mapping of rhizosphere glycan patterning using molecular probes in a transparent soil system

Rhizospheres are microecological zones at the interface of roots and soils. Interactions between bacteria and roots are critical for maintaining plant and soil health but are difficult to study because of constraints inherent in working with underground systems. We have developed an in-situ rhizosphere imaging system based on transparent soils and molecular probes that can be imaged using confocal microscopy. We observed spatial patterning of polysaccharides along roots and on cells deposited into the rhizosphere and also co-localised fluorescently tagged soil bacteria. These studies provide insight into the complex glycan landscape of rhizospheres and suggest a means by which root / rhizobacteria interactions can be non-disruptively studied.

Cultivation of Dominant Freshwater Bacterioplankton Lineages Using a High-Throughput Dilution-to-Extinction Culturing Approach Over a 1-Year Period

Although many culture-independent molecular analyses have elucidated a great diversity of freshwater bacterioplankton, the ecophysiological characteristics of several abundant freshwater bacterial groups are largely unknown due to the scarcity of cultured representatives. Therefore, a high-throughput dilution-to-extinction culturing (HTC) approach was implemented herein to enable the culture of these bacterioplankton lineages using water samples collected at various seasons and depths from Lake Soyang, an oligotrophic reservoir located in South Korea. Some predominant freshwater bacteria have been isolated from Lake Soyang via HTC (e.g., the acI lineage); however, large-scale HTC studies encompassing different seasons and water depths have not been documented yet. In this HTC approach, bacterial growth was detected in 14% of 5,376 inoculated wells. Further, phylogenetic analyses of 16S rRNA genes from a total of 605 putatively axenic bacterial cultures indicated that the HTC isolates were largely composed of Actinobacteria, Bacteroidetes, Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Verrucomicrobia. Importantly, the isolates were distributed across diverse taxa including phylogenetic lineages that are widely known cosmopolitan and representative freshwater bacterial groups such as the acI, acIV, LD28, FukuN57, MNG9, and TRA3-20 lineages. However, some abundant bacterial groups including the LD12 lineage, Chloroflexi, and Acidobacteria could not be domesticated. Among the 71 taxonomic groups in the HTC isolates, representative strains of 47 groups could either form colonies on agar plates or be revived from frozen glycerol stocks. Additionally, season and water depth significantly affected bacterial community structure, as demonstrated by 16S rRNA gene amplicon sequencing analyses. Therefore, our study successfully implemented a dilution-to-extinction cultivation strategy to cultivate previously uncultured or underrepresented freshwater bacterial groups, thus expanding the basis for future multi-omic studies.

Maribellus comscasis sp. nov., a novel deep-sea Bacteroidetes bacterium, possessing a prominent capability of degrading cellulose

Bacteroidetes are thought to be specialized for the degradation of algae-derived ocean polysaccharides. Here, we show that Bacteroidetes are the predominant phylum in deep-sea sediments and possess more genes associated with polysaccharides degradation than other bacteria. We have isolated a novel Bacteroidetes species from the deep-sea sediments by using a special polysaccharide containing medium, Maribellus comscasis WC007, which possesses 82 putative polysaccharide utilization loci (PULs) containing 374 glycoside hydrolases and 82 SusC/D pairs (Sus indicates starch utilization system; SusC represents the actual TonB-dependent transporter, and SusD is an associated substrate-binding outer membrane lipoprotein) together with 58 sigma/antisigma factors. Through an in-depth analysis of these PULs, strain WC007 can efficiently degrade numerous different polysaccharides including cellulose, pectin, fucoidan, mannan, xylan and starch, which are verified by growth assays. Notably, we find that cellulose has the most significant growth-promoting effect on M. comscasis WC007. And based on scanning electron microscope observation, transcriptomics and metabolomics, we further report on the underlying mechanisms of cellulose degradation and utilization, as well as potential contributions to the carbon cycle. Overall, our results suggest that Bacteroidetes may play key roles in the carbon cycle, likely due to their high abundance and prominent polysaccharide degradation capabilities.

A comprehensive review on the impact of β-glucan metabolism by Bacteroides and Bifidobacterium species as members of the gut microbiota

β-glucans are polysaccharides which can be obtained from different sources, and which have been described as potential prebiotics. The beneficial effects associated with β-glucan intake are that they reduce energy intake, lower cholesterol levels and support the immune system. Nevertheless, the mechanism(s) of action underpinning these health effects related to β-glucans are still unclear, and the precise impact of β-glucans on the gut microbiota has been subject to debate and revision. In this review, we summarize the most recent advances involving structurally different types of β-glucans as fermentable substrates for Bacteroidetes (mainly Bacteroides) and Bifidobacterium species as glycan degraders. Bacteroides is one of the most abundant bacterial components of the human gut microbiota, while bifidobacteria are widely employed as a probiotic ingredient. Both are generalist glycan degraders capable of using a wide range of substrates: Bacteroides spp. are specialized as primary degraders in the metabolism of complex carbohydrates, whereas Bifidobacterium spp. more commonly metabolize smaller glycans, in particular oligosaccharides, sometimes through syntrophic interactions with Bacteroides spp., in which they act as secondary degraders.

In-depth Spatiotemporal Characterization of Planktonic Archaeal and Bacterial Communities in North and South San Francisco Bay

Despite being the largest estuary on the west coast of North America, no in-depth survey of microbial communities in San Francisco Bay (SFB) waters currently exists. In this study, we analyze bacterioplankton and archaeoplankton communities at several taxonomic levels and spatial extents (i.e., North versus South Bay) to reveal patterns in alpha and beta diversity. We assess communities using high-throughput sequencing of the 16S rRNA gene in 177 water column samples collected along a 150-km transect over a 2-year monthly time-series. In North Bay, the microbial community is strongly structured by spatial salinity changes while in South Bay seasonal variations dominate community dynamics. Along the steep salinity gradient in North Bay, we find that operational taxonomic units (OTUs; 97% identity) have higher site specificity than at coarser taxonomic levels and turnover ("species" replacement) is high, revealing a distinct brackish community (in oligo-, meso-, and polyhaline samples) from fresh and marine end-members. At coarser taxonomic levels (e.g., phylum, class), taxa are broadly distributed across salinity zones (i.e., present/abundant in a large number of samples) and brackish communities appear to be a mix of fresh and marine communities. We also observe variations in brackish communities between samples with similar salinities, likely related to differences in water residence times between North and South Bay. Throughout SFB, suspended particulate matter is positively correlated with richness and influences changes in beta diversity. Within several abundant groups, including the SAR11 clade (comprising up to 30% of reads in a sample), OTUs appear to be specialized to a specific salinity range. Some other organisms also showed pronounced seasonal abundance, including Synechococcus, Ca. Actinomarina, and Nitrosopumilus-like OTUs. Overall, this study represents the first in-depth spatiotemporal survey of SFB microbial communities and provides insight into how planktonic microorganisms have specialized to different niches along the salinity gradient.

The root endophytic bacterial community of Ricinus communis L. resembles the seeds community more than the rhizosphere bacteria independent of soil water content

Rhizosphere and root endophytic bacteria are crucial for plant development, but the question remains if their composition is similar and how environmental conditions, such as water content, affect their resemblance. Ricinus communis L., a highly drought resistant plant, was used to study how varying soil water content affected the bacterial community in uncultivated, non-rhizosphere and rhizosphere soil, and in its roots. Additionally, the bacterial community structure was determined in the seeds of R. communis at the onset of the experiment. Plants were cultivated in soil at three different watering regimes, i.e. 50% water holding capacity (WHC) or adjusted to 50% WHC every two weeks or every month. Reducing the soil water content strongly reduced plant and root dry biomass and plant development, but had little effect on the bacterial community structure. The bacterial community structure was affected significantly by cultivation of R. communis and showed large variations over time. After 6 months, the root endophytic bacterial community resembled that in the seeds more than in the rhizosphere. It was found that water content had only a limited effect on the bacterial community structure and the different bacterial groups, but R. communis affected the bacterial community profoundly.

Labile Dissolved Organic Matter Compound Characteristics Select for Divergence in Marine Bacterial Activity and Transcription

Bacteria play a key role in the planetary carbon cycle partly because they rapidly assimilate labile dissolved organic matter (DOM) in the ocean. However, knowledge of the molecular mechanisms at work when bacterioplankton metabolize distinct components of the DOM pool is still limited. We, therefore, conducted seawater culture enrichment experiments with ecologically relevant DOM, combining both polymer and monomer model compounds for distinct compound classes. This included carbohydrates (polysaccharides vs. monosaccharides), proteins (polypeptides vs. amino acids), and nucleic acids (DNA vs. nucleotides). We noted pronounced changes in bacterial growth, activity, and transcription related to DOM characteristics. Transcriptional responses differed between compound classes, with distinct gene sets (“core genes”) distinguishing carbohydrates, proteins, and nucleic acids. Moreover, we found a strong divergence in functional transcription at the level of particular monomers and polymers (i.e., the condensation state), primarily in the carbohydrates and protein compound classes. These specific responses included a variety of cellular and metabolic processes that were mediated by distinct bacterial taxa, suggesting pronounced functional partitioning of organic matter. Collectively, our findings show that two important facets of DOM, compound class and condensation state, shape bacterial gene expression, and ultimately select for distinct bacterial (functional) groups. This emphasizes the interdependency of marine bacteria and labile carbon compounds for regulating the transformation of DOM in surface waters.

Genome characteristics of Kordia antarctica IMCC3317T and comparative genome analysis of the genus Kordia

The genus Kordia is one of many genera affiliated with the family Flavobacteriaceae of the phylum Bacteroidetes, well known for its degradation of high molecular weight organic matters. The genus Kordia currently comprises eight species, type strains of which have been isolated from a diverse range of marine environments. As of this report, four genome sequences have been submitted for cultured strains of Kordia, but none are complete nor have they been analyzed comprehensively. In this study, we report the complete genome of Kordia antarctica IMCC3317^T, isolated from coastal seawater off the Antarctic Peninsula. The complete genome of IMCC3317^T consists of a single circular chromosome with 5.5 Mbp and a 33.2 mol% of G+C DNA content. The IMCC3317^T genome showed features typical of chemoheterotrophic marine bacteria and similar to other Kordia genomes, such as complete gene sets for the Embden–Meyerhof–Parnas glycolysis pathway, tricarboxylic acid cycle and oxidative phosphorylation. The genome also encoded many carbohydrate-active enzymes, some of which were clustered into approximately seven polysaccharide utilization loci, thereby demonstrating the potential for polysaccharide utilization. Finally, a nosZ gene encoding nitrous oxide reductase, an enzyme that catalyzes the reduction of N₂O to N₂ gas, was also unique to the IMCC3317^T genome.

Insights into the Vertical Stratification of Microbial Ecological Roles across the Deepest Seawater Column on Earth

The Earth's oceans are a huge body of water with physicochemical properties and microbial community profiles that change with depth, which in turn influences their biogeochemical cycling potential. The differences between microbial communities and their functional potential in surface to hadopelagic water samples are only beginning to be explored. Here, we used metagenomics to investigate the microbial communities and their potential to drive biogeochemical cycling in seven different water layers down the vertical profile of the Challenger Deep (0-10,500 m) in the Mariana Trench, the deepest natural point in the Earth's oceans. We recovered 726 metagenome-assembled genomes (MAGs) affiliated to 27 phyla. Overall, biodiversity increased in line with increased depth. In addition, the genome size of MAGs at ≥4000 m layers was slightly larger compared to those at 0-2000 m. As expected, surface waters were the main source of primary production, predominantly from Cyanobacteria. Intriguingly, microbes conducting an unusual form of nitrogen metabolism were identified in the deepest waters (>10,000 m), as demonstrated by an enrichment of genes encoding proteins involved in dissimilatory nitrate to ammonia conversion (DNRA), nitrogen fixation and urea transport. These likely facilitate the survival of ammonia-oxidizing archaea α lineage, which are typically present in environments with a high ammonia concentration. In addition, the microbial potential for oxidative phosphorylation and the glyoxylate shunt was enhanced in >10,000 m waters. This study provides novel insights into how microbial communities and their genetic potential for biogeochemical cycling differs through the Challenger deep water column, and into the unique adaptive lifestyle of microbes in the Earth's deepest seawater.

Comparative genomic analysis of Flavobacteriaceae: insights into carbohydrate metabolism, gliding motility and secondary metabolite biosynthesis

BACKGROUND

Members of the bacterial family Flavobacteriaceae are widely distributed in the marine environment and often found associated with algae, fish, detritus or marine invertebrates. Yet, little is known about the characteristics that drive their ubiquity in diverse ecological niches. Here, we provide an overview of functional traits common to taxonomically diverse members of the family Flavobacteriaceae from different environmental sources, with a focus on the Marine clade. We include seven newly sequenced marine sponge-derived strains that were also tested for gliding motility and antimicrobial activity.

RESULTS

Comparative genomics revealed that genome similarities appeared to be correlated to 16S rRNA gene- and genome-based phylogeny, while differences were mostly associated with nutrient acquisition, such as carbohydrate metabolism and gliding motility. The high frequency and diversity of genes encoding polymer-degrading enzymes, often arranged in polysaccharide utilization loci (PULs), support the capacity of marine Flavobacteriaceae to utilize diverse carbon sources. Homologs of gliding proteins were widespread among all studied Flavobacteriaceae in contrast to members of other phyla, highlighting the particular presence of this feature within the Bacteroidetes. Notably, not all bacteria predicted to glide formed spreading colonies. Genome mining uncovered a diverse secondary metabolite biosynthesis arsenal of Flavobacteriaceae with high prevalence of gene clusters encoding pathways for the production of antimicrobial, antioxidant and cytotoxic compounds. Antimicrobial activity tests showed, however, that the phenotype differed from the genome-derived predictions for the seven tested strains.

CONCLUSIONS

Our study elucidates the functional repertoire of marine Flavobacteriaceae and highlights the need to combine genomic and experimental data while using the appropriate stimuli to unlock their uncharted metabolic potential.

Candidatus Abditibacter, a novel genus within the Cryomorphaceae, thriving in the North Sea

Coastal phytoplankton blooms are frequently followed by successive blooms of heterotrophic bacterial clades. The class Flavobacteriia within the Bacteroidetes has been shown to play an important role in the degradation of high molecular weight substrates that become available in the later stages of such blooms. One of the flavobacterial clades repeatedly observed over the course of several years during phytoplankton blooms off the coast of Helgoland, North Sea, is Vis6. This genus-level clade belongs to the family Cryomorphaceae and has been resistant to cultivation to date. Based on metagenome assembled genomes, comparative 16S rRNA gene sequence analyses and fluorescence in situ hybridization, we here propose a novel candidate genus Abditibacter, comprising three novel species Candidatus Abditibacter vernus, Candidatus Abditibacter forsetii and Candidatus Abditibacter autumni. While the small genomes of the three novel photoheterotrophic species encode highly similar gene repertoires, including genes for degradation of proteins and algal storage polysaccharides such as laminarin, two of them - Ca. A. vernus and Ca. A. forsetii - seem to have a preference for spring blooms, while Ca. A. autumni almost exclusively occurs in late summer and autumn.

Comparative Genomic Study of Polar Lichen-Associated Hymenobacter sp. PAMC 26554 and Hymenobacter sp. PAMC 26628 Reveals the Presence of Polysaccharide-Degrading Ability Based on Habitat

The genus Hymenobacter is classified in the family Hymenobacteraceae under the phylum Bacteroidetes. They have been isolated from diverse environments, such as air, soil, and lichen, along with extreme polar environments, including the Arctic and Antarctic regions. The polar regions have attracted intense research interest for the discovery of novel microorganisms and their functions. Analysis of the polysaccharide utilization-related carbohydrate-active enzyme among the two lichen-associated polar organisms Hymenobacter sp. PAMC 26554 and Hymenobacter sp. PAMC 26628 was performed, along with its comparison with the complete genome of the same genus available in the NCBI database. The study was conducted relying on the AZCL screening data for the two polar lichen-associated species. While comparing with eight other complete genomes, differences in polysaccharide preferences based on the isolation environment and biosample source were discovered. All the species showed almost similar percentage of cellulose synthesis and degradation genes. However, the polar lichen-associated microorganism was found to have a high percentage of hemicellulose degradation genes, and less starch and laminarin degradation. The Hymenobacter species with higher number of hemicellulose degradation genes was found to have a lower number of starch and laminarin degradation genes and vice versa, highlighting the differences in polysaccharide utilization among the species.

Ancestral niche separation and evolutionary rate differentiation between sister marine flavobacteria lineages

Marine flavobacteria are specialists for polysaccharide degradation. They dominate in habitats enriched with polysaccharides, but are also prevalent in pelagic environments where polysaccharides are less available. These niches are likely occupied by distinct lineages, but evolutionary processes underlying their niche differentiation remain elusive. Here, genomic analyses and physiological assays indicate that the sister flavobacteria lineages Leeuwenhoekiella and Nonlabens likely explore polysaccharide-rich macroalgae and polysaccharide-poor pelagic niches respectively. Phylogenomic analyses inferred that the niche separation likely occurred anciently and coincided with increased sequence evolutionary rate in Nonlabens compared with Leeuwenhoekiella. Further analyses ruled out the known mechanisms likely driving evolutionary rate acceleration, including reduced selection efficiency, decreased generation time and increased mutation rate. In particular, the mutation rates were determined using an unbiased experimental method, which measures the present-day populations and may not reflect ancestral populations. These data collectively lead to a new hypothesis that an ancestral and transient mutation rate increase resulted in evolutionary rate increase in Nonlabens. This hypothesis was supported by inferring that gains and losses of genes involved in SOS response, a mechanism known to drive transiently increased mutation rate, coincided with evolutionary rate acceleration. Our analyses highlight the evolutionary mechanisms underlying niche differentiation of flavobacteria lineages.

Polysaccharide niche partitioning of distinct Polaribacter clades during North Sea spring algal blooms

Massive releases of organic substrates during marine algal blooms trigger growth of many clades of heterotrophic bacteria. Algal polysaccharides represent the most diverse and structurally complex class of these substrates, yet their role in shaping the microbial community composition is poorly understood. We investigated, whether polysaccharide utilization capabilities contribute to niche differentiation of Polaribacter spp. (class Flavobacteriia; known to include relevant polysaccharide-degraders) that were abundant during 2009–2012 spring algal blooms in the southern North Sea. We identified six distinct Polaribacter clades using phylogenetic and phylogenomic analyses, quantified their abundances via fluorescence in situ hybridization, compared metagenome-assembled genomes, and assessed in situ gene expression using metaproteomics. Four clades with distinct polysaccharide niches were dominating. Polaribacter 2-a comprised typical first responders featuring small genomes with limited polysaccharide utilization capacities. Polaribacter 3-a were abundant only in 2010 and possessed a distinct sulfated α-glucoronomannan degradation potential. Polaribacter 3-b responded late in blooms and had the capacity to utilize sulfated xylan. Polaribacter 1-a featured high numbers of glycan degradation genes and were particularly abundant following Chattonella algae blooms. These results support the hypothesis that sympatric Polaribacter clades occupy distinct glycan niches during North Sea spring algal blooms.

Leeuwenhoekiella aestuarii sp. nov., isolated from salt-water sediment and first insights in the genomes of Leeuwenhoekiella species

Four Gram-negative, aerobic, rod-shaped and yellow-orange pigmented bacteria (R-46770, R-48165^T, R-50232 and R-50233) were isolated from intertidal sediment and water of the Westerschelde estuary between 2006 and 2012. Analysis of their 16S rRNA gene sequences revealed that the four strains form a separate cluster between validly described type strains of the genus Leeuwenhoekiella. DNA-DNA reassociation values of two representative strains (i.e. R-48165^T and R-50232) of the new group with type strains of Leeuwenhoekiella species ranged from 18.7 to 56.6 %. A comparative genome analysis of the two strains and the type strains confirmed average nucleotide identity values from 75.6 to 94.4 %. The G+C contents of the genomic DNA of strains R-48165^T and R-50232 were 37.80 and 37.83 mol%, respectively. The predominant cellular fatty acids of the four novel strains were summed feature 3 (i.e. C_16 : 1ω7c and/or iso-C_15 : 0 2-OH), iso-C_15 : 0, iso-C_15 : 1 G and iso-C_17 : 0 3-OH. The four new Leeuwenhoekiella-like strains grew with 0.5-12 % (w/v) NaCl, at pH 5.5-9.0 and displayed optimum growth between 20 and 30 °C. Based on the results of phenotypic, genomic, phylogenetic and chemotaxonomic analyses, the four new strains represent a novel species of the genus Leeuwenhoekiella for which the name Leeuwenhoekiella aestuarii sp. nov. is proposed. The type strain is LMG 30908^T (=R-48165^T=CECT 9775^T=DSM 107866^T). Genome analysis of type strains of the genus Leeuwenhoekiella revealed a large number of glycosyl hydrolases, peptidases and carboxyl esterases per Mb, whereas the number of transporters per Mb was low compared to other bacteria. This confirmed the environmental role of Leeuwenhoekiella species as (bio)polymer degraders, with a specialization on degrading proteins and high molecular weight compounds. Additionally, the presence of a large number of genes involved in gliding motility and surface adhesion, and large numbers of glycosyl transferases per Mb confirmed the importance of these features for Leeuwenhoekiella species.

In marine Bacteroidetes the bulk of glycan degradation during algae blooms is mediated by few clades using a restricted set of genes

We investigated Bacteroidetes during spring algae blooms in the southern North Sea in 2010–2012 using a time series of 38 deeply sequenced metagenomes. Initial partitioning yielded 6455 bins, from which we extracted 3101 metagenome-assembled genomes (MAGs) including 1286 Bacteroidetes MAGs covering ~120 mostly uncultivated species. We identified 13 dominant, recurrent Bacteroidetes clades carrying a restricted set of conserved polysaccharide utilization loci (PULs) that likely mediate the bulk of bacteroidetal algal polysaccharide degradation. The majority of PULs were predicted to target the diatom storage polysaccharide laminarin, alpha-glucans, alpha-mannose-rich substrates, and sulfated xylans. Metaproteomics at 14 selected points in time revealed expression of SusC-like proteins from PULs targeting all of these substrates. Analyses of abundant key players and their PUL repertoires over time furthermore suggested that fewer and simpler polysaccharides dominated early bloom stages, and that more complex polysaccharides became available as blooms progressed.

Delineation of ecologically distinct units of marine Bacteroidetes in the Northwestern Mediterranean Sea

Bacteroidetes is one of the dominant phyla of ocean bacterioplankton, yet its diversity and population structure is poorly understood. To advance in the delineation of ecologically meaningful units within this group, we constructed near full-length 16S rRNA gene clone libraries from contrasting marine environments in the NW Mediterranean. Based on phylogeny and the associated ecological variables (depth and season), 24 different Bacteroidetes clades were delineated. By considering their relative abundance (from iTag amplicon sequencing studies), we described the distribution patterns of each of these clades, delimiting them as Ecologically Significant Taxonomic Units (ESTUs). Spatially, there was almost no overlap among ESTUs at different depths. In deep waters there was predominance of Owenweeksia, Leeuwenhoekiella, Muricauda-related genera, and some depth-associated ESTUs within the NS5 and NS2b marine clades. Seasonally, multi-annual dynamics of recurring ESTUs were present with dominance of some ESTUs within the NS4, NS5 and NS2b marine clades along most of the year, but with variable relative frequencies between months. A drastic change towards the predominance of Formosa-related ESTUs and one ESTU from the NS5 marine clade was typically present after the spring bloom. Even though there are no isolates available for these ESTUs to determine their physiology, correlation analyses identified the environmental preference of some of them. Overall, our results suggest that there is a high degree of niche specialisation within these closely related clades. This work constitutes a step forward in disentangling the ecology of marine Bacteroidetes, which are essential players in organic matter processing in the oceans.

Biogeochemical Regimes in Shallow Aquifers Reflect the Metabolic Coupling of the Elements Nitrogen, Sulfur, and Carbon

Near-surface groundwaters are prone to receive (in)organic matter input from their recharge areas and are known to harbor autotrophic microbial communities linked to nitrogen and sulfur metabolism. Here, we use multi-omic profiling to gain holistic insights into the turnover of inorganic nitrogen compounds, carbon fixation processes, and organic matter processing in groundwater. We sampled microbial biomass from two superimposed aquifers via monitoring wells that follow groundwater flow from its recharge area through differences in hydrogeochemical settings and land use. Functional profiling revealed that groundwater microbiomes are mainly driven by nitrogen (nitrification, denitrification, and ammonium oxidation [anammox]) and to a lesser extent sulfur cycling (sulfur oxidation and sulfate reduction), depending on local hydrochemical differences. Surprisingly, the differentiation potential of the groundwater microbiome surpasses that of hydrochemistry for individual monitoring wells. Being dominated by a few phyla (Bacteroidetes, Proteobacteria, Planctomycetes, and Thaumarchaeota), the taxonomic profiling of groundwater metagenomes and metatranscriptomes revealed pronounced differences between merely present microbiome members and those actively participating in community gene expression and biogeochemical cycling. Unexpectedly, we observed a constitutive expression of carbohydrate-active enzymes encoded by different microbiome members, along with the groundwater flow path. The turnover of organic carbon apparently complements for lithoautotrophic carbon assimilation pathways mainly used by the groundwater microbiome depending on the availability of oxygen and inorganic electron donors, like ammonium.IMPORTANCE Groundwater is a key resource for drinking water production and irrigation. The interplay between geological setting, hydrochemistry, carbon storage, and groundwater microbiome ecosystem functioning is crucial for our understanding of these important ecosystem services. We targeted the encoded and expressed metabolic potential of groundwater microbiomes along an aquifer transect that diversifies in terms of hydrochemistry and land use. Our results showed that the groundwater microbiome has a higher spatial differentiation potential than does hydrochemistry.

Repeated evolutionary transitions of flavobacteria from marine to non-marine habitats

The taxonomy of marine and non-marine organisms rarely overlap, but the mechanisms underlying this distinction are often unknown. Here, we predicted three major ocean-to-land transitions in the evolutionary history of Flavobacteriaceae, a family known for polysaccharide and peptide degradation. These unidirectional transitions were associated with repeated losses of marine signature genes and repeated gains of non-marine adaptive genes. This included various Na⁺ -dependent transporters, osmolyte transporters and glycoside hydrolases (GH) for sulfated polysaccharide utilization in marine descendants, and in non-marine descendants genes for utilizing the land plant material pectin and genes facilitating terrestrial host interactions. The K⁺ scavenging ATPase was repeatedly gained whereas the corresponding low-affinity transporter repeatedly lost upon transitions, reflecting K⁺ ions are less available to non-marine bacteria. Strikingly, the central metabolism Na⁺ -translocating NADH: quinone dehydrogenase gene was repeatedly gained in marine descendants, whereas the H⁺ -translocating counterpart was repeatedly gained in non-marine lineages. Furthermore, GH genes were depleted in isolates colonizing animal hosts but abundant in bacteria inhabiting other non-marine niches; thus relative abundances of GH versus peptidase genes among Flavobacteriaceae lineages were inconsistent with the marine versus non-marine dichotomy. We suggest that phylogenomic analyses can cast novel light on mechanisms explaining the distribution and ecology of key microbiome components.

Biphasic cellular adaptations and ecological implications of Alteromonas macleodii degrading a mixture of algal polysaccharides

Algal polysaccharides are an important bacterial nutrient source and central component of marine food webs. However, cellular and ecological aspects concerning the bacterial degradation of polysaccharide mixtures, as presumably abundant in natural habitats, are poorly understood. Here, we contextualize marine polysaccharide mixtures and their bacterial utilization in several ways using the model bacterium Alteromonas macleodii 83-1, which can degrade multiple algal polysaccharides and contributes to polysaccharide degradation in the oceans. Transcriptomic, proteomic and exometabolomic profiling revealed cellular adaptations of A. macleodii 83-1 when degrading a mix of laminarin, alginate and pectin. Strain 83-1 exhibited substrate prioritization driven by catabolite repression, with initial laminarin utilization followed by simultaneous alginate/pectin utilization. This biphasic phenotype coincided with pronounced shifts in gene expression, protein abundance and metabolite secretion, mainly involving CAZymes/polysaccharide utilization loci but also other functional traits. Distinct temporal changes in exometabolome composition, including the alginate/pectin-specific secretion of pyrroloquinoline quinone, suggest that substrate-dependent adaptations influence chemical interactions within the community. The ecological relevance of cellular adaptations was underlined by molecular evidence that common marine macroalgae, in particular Saccharina and Fucus, release mixtures of alginate and pectin-like rhamnogalacturonan. Moreover, CAZyme microdiversity and the genomic predisposition towards polysaccharide mixtures among Alteromonas spp. suggest polysaccharide-related traits as an ecophysiological factor, potentially relating to distinct 'carbohydrate utilization types' with different ecological strategies. Considering the substantial primary productivity of algae on global scales, these insights contribute to the understanding of bacteria-algae interactions and the remineralization of chemically diverse polysaccharide pools, a key step in marine carbon cycling.

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 β-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 β-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.

Mini-review: Phytoplankton-derived polysaccharides in the marine environment and their interactions with heterotrophic bacteria

Within the wealth of molecules constituting marine dissolved organic matter, carbohydrates make up the largest coherent and quantifiable fraction. Their main sources are from primary producers, which release large amounts of photosynthetic products - mainly polysaccharides - directly into the surrounding water via passive and active exudation. The organic carbon and other nutrients derived from these photosynthates enrich the 'phycosphere' and attract heterotrophic bacteria. The rapid uptake and remineralization of dissolved free monosaccharides by heterotrophic bacteria account for the barely detectable levels of these compounds. By contrast, dissolved combined polysaccharides can reach high concentrations, especially during phytoplankton blooms. Polysaccharides are too large to be taken up directly by heterotrophic bacteria, instead requiring hydrolytic cleavage to smaller oligo- or monomers by bacteria with a suitable set of exoenzymes. The release of diverse polysaccharides by various phytoplankton taxa is generally interpreted as the deposition of excess organic material. However, these molecules likely also fulfil distinct, yet not fully understood functions, as inferred from their active modulation in terms of quality and quantity when phytoplankton becomes nutrient limited or is exposed to heterotrophic bacteria. This minireview summarizes current knowledge regarding the exudation and composition of phytoplankton-derived exopolysaccharides and acquisition of these compounds by heterotrophic bacteria.

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.

High-Quality Draft Single-Cell Genome Sequence of the NS5 Marine Group from the Coastal Red Sea

The uncultured NS5 marine group represents one of the most ubiquitous flavobacterial bacterioplankton associated with marine blooms in the pelagic ocean. Here, we present a single-cell genome sampled from coastal waters in the Red Sea that represents the first high-quality draft genome sequence within the NS5 lineage.

Sediment Depth-Dependent Spatial Variations of Bacterial Communities in Mud Deposits of the Eastern China Marginal Seas

The mud sediments of the eastern China marginal seas (ECMS) are deposited under different hydrodynamic conditions with different organic matter sources. These events have been demonstrated to exert significant influences on microbial communities and biogeochemical processes in surface sediments. However, the extent to which such effects occur in subsurface microbial communities remains unclear. In this study, both horizontal and vertical (five sites, each for eight layers) distributions of bacterial abundance and community composition in mud deposits of the South Yellow Sea (SYS) and East China Sea (ECS) were investigated by quantitative PCR and Illumina sequencing of the 16S rRNA gene. Both bacterial abundance and diversity were higher in the ECS than in the SYS, and tended to be higher in up than in deep layers. Proteobacteria (JTB255 marine benthic group), Acidobacteria and Bacteroidetes were dominant in the upper layers, whereas Lactococcus, Pseudomonas, and Dehalococcoidia were enriched in the deep layers. The bacterial communities in surface and subsurface sediments showed different inter-taxa relationships, indicating contrasting co-occurrence patterns. The bacterial communities in the upper layer samples clustered in accordance with mud zones, whereas those in the deep layer samples of all sites tended to cluster together. TOC δ¹³C and TON δ¹⁵N significantly affected the bacterial community composition, suggesting that the abundance and composition of organic matter played critical roles in shaping of sedimentary bacterial communities. This study provides novel insights into the distribution of subsurface bacterial communities in mud deposits of the ECMS, and provides clues for understanding the biogeochemical cycles in this area.

Polysaccharide Utilization Loci: Fueling Microbial Communities

The complex carbohydrates of terrestrial and marine biomass represent a rich nutrient source for free-living and mutualistic microbes alike. The enzymatic saccharification of these diverse substrates is of critical importance for fueling a variety of complex microbial communities, including marine, soil, ruminant, and monogastric microbiota. Consequently, highly specific carbohydrate-active enzymes, recognition proteins, and transporters are enriched in the genomes of certain species and are of critical importance in competitive environments. In Bacteroidetes bacteria, these systems are organized as polysaccharide utilization loci (PULs), which are strictly regulated, colocalized gene clusters that encode enzyme and protein ensembles required for the saccharification of complex carbohydrates. This review provides historical perspectives and summarizes key findings in the study of these systems, highlighting a critical shift from sequence-based PUL discovery to systems-based analyses combining reverse genetics, biochemistry, enzymology, and structural biology to precisely illuminate the molecular mechanisms underpinning PUL function. The ecological implications of dynamic PUL deployment by key species in the human gastrointestinal tract are explored, as well as the wider distribution of these systems in other gut, terrestrial, and marine environments.