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The activities of different microbes in the cow rumen have been shown to modulate the host’s ability to utilize plant biomass, while the host–rumen interface has received little attention. As datasets collected worldwide have pointed to Campylobacteraceae as particularly abundant members of the rumen epithelial microbiome, we targeted this group in a subset of seven cows with meta- and isolate genome analysis. We show that the dominant Campylobacteraceae lineage has recently speciated into two populations that were structured by genome-wide selective sweeps followed by population-specific gene import and recombination. These processes led to differences in gene expression and enzyme domain composition that correspond to the ability to utilize acetate, the main carbon source for the host, at the cost of inhibition by propionate. This trade-off in competitive ability further manifests itself in differential dynamics of the two populations in vivo. By exploring population-level adaptations that otherwise remain cryptic in culture-independent analyses, our results highlight how recent evolutionary dynamics can shape key functional roles in the rumen microbiome.

Microbial activity mediates the fluxes of greenhouse gases. However, in the global models of the marine and terrestrial biospheres used for climate change projections, typically only photosynthetic microbial activity is resolved mechanistically. To move forward, we argue that global biogeochemical models need a theoretically grounded framework with which to constrain parameterizations of diverse microbial metabolisms. Here, we explain how the key redox chemistry underlying metabolisms provides a path towards this goal. Using this first-principles approach, the presence or absence of metabolic functional types emerges dynamically from ecological interactions, expanding model applicability to unobserved environments. “Nothing is less real than realism. It is only by selection, by elimination, by emphasis, that we get at the real meaning of things.” –Georgia O’Keefe, Marine microbial activities fuel biogeochemical cycles that impact the climate, but global models do not account for the myriad physiological processes that microbes perform. Here the authors argue for a model framework that reinterprets the ocean as physics coupled to biologically-driven redox chemistry.

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. ‘Lentimonas’—a marine microorganism from the Verrucomicrobia phylum—has >200 different glycoside hydrolase and sulfatase enzymes enabling digestion of the algal polysaccharide fucoidan, which was thought to be a recalcitrant source of carbon in our oceans.

Although Clostridium difficile is widely considered an antibiotic- and hospital-associated pathogen, recent evidence indicates that this is an insufficient depiction of the risks and reservoirs. A common thread that links all major risk factors of infection is their association with gastrointestinal disturbances, but this relationship to C. difficile colonization has never been tested directly. Here, we show that disturbances caused by diarrhoeal events trigger susceptibility to C. difficile colonization. Using survey data of the human gut microbiome, we detected C. difficile colonization and blooms in people recovering from food poisoning and Vibrio cholerae infections. Carriers remained colonized for year-long time scales and experienced highly variable patterns of C. difficile abundance, where increased shedding over short periods of 1–2 d interrupted week-long periods in which C. difficile was undetectable. Given that short shedding events were often linked to gastrointestinal disturbances, our results help explain why C. difficile is frequently detected as a co-infecting pathogen in patients with diarrhoea. To directly test the impact of diarrhoea on susceptibility to colonization, we developed a mouse model of variable disturbance intensity, which allowed us to monitor colonization in the absence of disease. As mice exposed to avirulent C. difficile spores ingested increasing quantities of laxatives, more individuals experienced C. difficile blooms. Our results indicate that the likelihood of colonization is highest in the days immediately following acute disturbances, suggesting that this could be an important window during which transmission could be interrupted and the incidence of infection lowered. Gastrointestinal disturbances, such as non-antibiotic-induced diarrhoea, promote susceptibility to colonization by Clostridium difficile, providing additional insights into antibiotic-independent triggers for this infection.

Long-term nitrogen (N) fertilization imposes strong selection on nitrifying communities in agricultural soil, but how a progressively changing niche affects potentially active nitrifiers in the field remains poorly understood. Using a 44-year grassland fertilization experiment, we investigated community shifts of active nitrifiers by DNA-based stable isotope probing (SIP) of field soils that received no fertilization (CK), high levels of organic cattle manure (HC), and chemical N fertilization (CF). Incubation of DNA-SIP microcosms showed significant nitrification activities in CF and HC soils, whereas no activity occurred in CK soils. The 44 years of inorganic N fertilization selected only 13C-ammonia-oxidizing bacteria (AOB), whereas cattle slurry applications created a niche in which both ammonia-oxidizing archaea (AOA) and AOB could be actively 13C-labeled. Phylogenetic analysis indicated that Nitrosospira sp. 62-like AOB dominated inorganically fertilized CF soils, while Nitrosospira sp. 41-like AOB were abundant in organically fertilized HC soils. The 13C-AOA in HC soils were affiliated with the 29i4 lineage. The 13C-nitrite-oxidizing bacteria (NOB) were dominated by both Nitrospira- and Nitrobacter-like communities in CF soils, and the latter was overwhelmingly abundant in HC soils. The 13C-labeled nitrifying communities in SIP microcosms of CF and HC soils were largely similar to those predominant under field conditions. These results provide direct evidence for a strong selection of distinctly active nitrifiers after 44 years of different fertilization regimes in the field. Our findings imply that niche differentiation of nitrifying communities could be assessed as a net result of microbial adaption over 44 years to inorganic and organic N fertilization in the field, where distinct nitrifiers have been shaped by intensified anthropogenic N input.

International audience; Coevolution between bacteriophages (phages) and their bacterial hosts occurs through changes in resistance and counter-resistance mechanisms. To assess phage-host evolution in wild populations, we isolated 195 Vibrio crassostreae strains and 243 vibriophages during a 5-month time series from an oyster farm and combined these isolates with existing V. crassostreae and phage isolates. Cross-infection studies of 81,926 host-phage pairs delineated a modular network where phages are best at infecting cooccurring hosts, indicating local adaptation. Successful propagation of phage is restricted by the ability to adsorb to closely related bacteria and further constrained by strain-specific defence systems. These defences are highly diverse and predominantly located on mobile genetic elements, and multiple defences are active within a single genome. We further show that epigenetic and genomic modifications enable phage to adapt to bacterial defences and alter host range. Our findings reveal that the evolution of Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. bacterial defences and phage counter-defences is underpinned by frequent genetic exchanges with, and between, mobile genetic elements.

Background Microbial communities in both natural and applied settings reliably carry out myriads of functions, yet how stable these taxonomically diverse assemblages can be and what causes them to transition between states remains poorly understood. We studied monthly activated sludge (AS) samples collected over 9 years from a full-scale wastewater treatment plant to answer how complex AS communities evolve in the long term and how the community functions change when there is a disturbance in operational parameters. Results Here, we show that a microbial community in activated sludge (AS) system fluctuated around a stable average for 3 years but was then abruptly pushed into an alternative stable state by a simple transient disturbance (bleaching). While the taxonomic composition rapidly turned into a new state following the disturbance, the metabolic profile of the community and system performance remained remarkably stable. A total of 920 metagenome-assembled genomes (MAGs), representing approximately 70% of the community in the studied AS ecosystem, were recovered from the 97 monthly AS metagenomes. Comparative genomic analysis revealed an increased ability to aggregate in the cohorts of MAGs with correlated dynamics that are dominant after the bleaching event. Fine-scale analysis of dynamics also revealed cohorts that dominated during different periods and showed successional dynamics on seasonal and longer time scales due to temperature fluctuation and gradual changes in mean residence time in the reactor, respectively. Conclusions Our work highlights that communities can assume different stable states under highly similar environmental conditions and that a specific disturbance threshold may lead to a rapid shift in community composition.

As one of the largest biotechnological applications, activated sludge (AS) systems in wastewater treatment plants (WWTPs) harbor enormous viruses, with 10-1,000-fold higher concentrations than in natural environments. However, the compositional variation and host-connections of AS viruses remain poorly explored. Here, we report a catalogue of ~50,000 prokaryotic viruses from six WWTPs, increasing the number of described viral species of AS by 23-fold, and showing the very high viral diversity which is largely unknown (98.4-99.6% of total viral contigs). Most viral genera are represented in more than one AS system with 53 identified across all. Viral infection widely spans 8 archaeal and 58 bacterial phyla, linking viruses with aerobic/anaerobic heterotrophs, and other functional microorganisms controlling nitrogen/phosphorous removal. Notably, Mycobacterium, notorious for causing AS foaming, is associated with 402 viral genera. Our findings expand the current AS virus catalogue and provide reference for the phage treatment to control undesired microorganisms in WWTPs., Activated sludge (AS) systems in wastewater treatment plants (WWTPs) contain high concentration of viruses. Here, the authors apply a systematic metagenomic pipeline and retrieve a catalogue of around 50,000 prokaryotic viruses from samples of six WWTPs, revealing a large and uncharacterized viral diversity in AS communities.

Microbial communities are shaped by viral predators1. Yet, resolving which viruses (phages) and bacteria are interacting is a major challenge in the context of natural levels of microbial diversity2. Thus, fundamental features of how phage-bacteria interactions are structured and evolve in “the wild” remain poorly resolved3, 4. Here we use large-scale isolation of environmental marine Vibrio bacteria and their phages to obtain quantitative estimates of strain-level phage predator loads, and use all-by-all host range assays to discover how phage and host genomic diversity shape interactions. We show that killing in environmental interaction networks is sparse - with phage predator loads low for most bacterial strains and phages host-strain-specific in their killing. Paradoxically, we also find that although overlap in killing is generally rare between phages, recombination is common. Together, these results indicate that the number of hosts that phages infect is often larger than the number that they kill and suggest that recombination during cryptic co-infections is an important mode of phage evolution in microbial communities. In the development of phages for bioengineering and therapeutics it will be important to consider that nucleic acids of introduced phages may spread into local phage populations through recombination, and that the likelihood of transfer is not predictable based on killing host range.

We thank Liam Shaw for his letter (1), “Null models for gene enrichment in plasmids,” and appreciate the detailed simulation regarding the enrichment analysis of AMR genes on different types of plasmids, which confirms our original conclusions that conjugative plasmids are enriched in these genes (2). We agree that considering the plasmid size is a good point when using a null model for the enrichment analysis as it tests for a completely neutral accumulation of genes. However, using a completely neutral model for all genes on plasmids does not capture the reality of gene content constraints. Conjugative plasmids contain a much larger portion of … [↵][1]1To whom correspondence may be addressed. Email: zhangt{at}hku.hk. [1]: #xref-corresp-1-1

Coevolution between bacteriophage (or phage) and their bacterial host is thought to be key for the coexistence of these antagonists. Recent studies have revealed the major role of mobile genetic elements in the emergence of phage resistant hosts but how phage escape these defenses in the wild remained to be explored. Here we show a striking parallel in phage evolving counter defenses to host defenses in natural population. We established a large collection of phages and their bacterial hosts and we explored the genetic structure of their interaction. We find that clearly delineated genomic clusters of phage are specific for distinct clades within a bacterial species, Vibrio crassostreae, yet while all phages can adsorb, only a subset of hosts are killed due to intracellular defense mechanisms. Host genomes contain multiple mobile defense genes and susceptibility to phage is negatively correlated with genome size. Phages also display extensive gene content variation, but their genome size remains conserved. We show that this gene content variation in hosts and phage is due to rapid turnover of genes involved in defense and escape, and that by exchanging anti-defense genes, phages irreversibly switch host. This could be indicative of co-evolution following the matching-allele-model of specificity and the spatial and temporal variability of phage infectivity further suggests that negative-frequency dependent selection drives phage-vibrio coevolutionary dynamics. We propose a “pan-escape system” that can be shared among phages by homologous recombination within a population that infects a bacterial host.

Antibiotic subsistence in bacteria represents an alternative resistance machinery, while paradoxically, it is also a cure for environmental resistance. Antibiotic-subsisting bacteria can detoxify antibiotic-polluted environments and prevent the development of antibiotic resistance in environments. However, progress toward efficient in situ engineering of antibiotic-subsisting bacteria is hindered by the lack of mechanistic and predictive understanding of the assembly of the functioning microbiome. By top-down manipulation of wastewater microbiomes using sulfadiazine as the single limiting source, we monitored the ecological selection process that forces the wastewater microbiome to perform efficient sulfadiazine subsistence. We found that the community-level assembly selects for the same three families rising to prominence across different initial pools of microbiomes. We further analyzed the assembly patterns using a linear model. Detailed inspections of the sulfonamide metabolic gene clusters in individual genomes of isolates and assembled metagenomes reveal limited transfer potential beyond the boundaries of the Micrococcaceae lineage. Our results open up new possibilities for engineering specialist bacteria for environmental applications.

Although it is generally accepted that viruses (phages) drive bacterial evolution, how these dynamics play out in the wild remains poorly understood. Here we show that the arms race between phages and their hosts is mediated by large and highly diverse mobile genetic elements. These phage-defense elements display exceedingly fast evolutionary turnover, resulting in differential phage susceptibility among clonal bacterial strains while phage receptors remain invariant. Protection afforded by multiple elements is cumulative, and a single bacterial genome can harbor as many as 18 putative phage-defense elements. Overall, elements account for 90% of the flexible genome amongst closely related strains. The rapid turnover of these elements demonstrates that phage resistance is unlinked from other genomic features and that resistance to phage therapy might be as easily acquired as antibiotic resistance.

It is well established that plasmids play an important role in the dissemination of antimicrobial resistance (AMR) genes; however, little is known about the role of the underlying interactions between different plasmid categories and other mobile genetic elements (MGEs) in shaping the promiscuous spread of AMR genes. Here, we developed a tool designed for plasmid classification, AMR gene annotation, and plasmid visualization and found that most plasmid-borne AMR genes, including those localized on class 1 integrons, are enriched in conjugative plasmids. Notably, we report the discovery and characterization of a massive insertion sequence (IS)-associated AMR gene transfer network (245 combinations covering 59 AMR gene subtypes and 53 ISs) linking conjugative plasmids and phylogenetically distant pathogens, suggesting a general evolutionary mechanism for the horizontal transfer of AMR genes mediated by the interaction between conjugative plasmids and ISs. Moreover, our experimental results confirmed the importance of the observed interactions in aiding the horizontal transfer and expanding the genetic range of AMR genes within complex microbial communities.

© 2021 by the authors., Current warming in the Western Antarctic Peninsula (WAP) has multiple effects on the marine ecosystem, modifying the trophic web and the nutrient regime. In this study, the effect of decreased surface salinity on the marine microbial community as a consequence of freshening from nearby glaciers was investigated in Chile Bay, Greenwich Island, WAP. In the summer of 2016, samples were collected from glacier ice and transects along the bay for 16S rRNA gene sequencing, while in situ dilution experiments were conducted and analyzed using 16S rRNA gene sequencing and metatranscriptomic analysis. The results reveal that certain common seawater genera, such as Polaribacter, Pseudoalteromonas and HTCC2207, responded positively to decreased salinity in both the bay transect and experiments. The relative abundance of these bacteria slightly decreased, but their functional activity was maintained and increased the over time in the dilution experiments. However, while ice bacteria, such as Flavobacterium and Polaromonas, tolerated the increased salinity after mixing with seawater, their gene expression decreased considerably. We suggest that these bacterial taxa could be defined as sentinels of freshening events in the Antarctic coastal system. Furthermore, these results suggest that a significant portion of the microbial community is resilient and can adapt to disturbances, such as freshening due to the warming effect of climate change in Antarctica., This work was financially supported by the Agencia Nacional de Investigación y Desarrollo de Chile (ANID/FONDAP/15110009; DPI, grant number DPI20140044-ANID; ANID/INACH/ FONDECYT 3170807), Spanish Ministry of Economy and Competitivity grant CTM2016-80095-C2, and the Instituto Antártico Chileno (grant numbers INACH RT_15-10, INACH RG_09-17 and INACH RT_04-19).

Microbial communities are shaped by viral predators. Yet, resolving which viruses (phages) and bacteria are interacting is a major challenge in the context of natural levels of microbial diversity. Thus, fundamental features of how phage-bacteria interactions are structured and evolve in the wild remain poorly resolved. Here we use large-scale isolation of environmental marine Vibrio bacteria and their phages to obtain estimates of strain-level phage predator loads, and use all-by-all host range assays to discover how phage and host genomic diversity shape interactions. We show that lytic interactions in environmental interaction networks (as observed in agar overlay) are sparse—with phage predator loads being low for most bacterial strains, and phages being host-strain-specific. Paradoxically, we also find that although overlap in killing is generally rare between tailed phages, recombination is common. Together, these results suggest that recombination during cryptic co-infections is an important mode of phage evolution in microbial communities. In the development of phages for bioengineering and therapeutics it is important to consider that nucleic acids of introduced phages may spread into local phage populations through recombination, and that the likelihood of transfer is not predictable based on lytic host range., Understanding the interactions between bacteria and their viruses (phages) in natural communities is a major challenge. Here, the authors isolate and study large numbers of marine Vibrio bacteria and their phages, and find that lytic interactions are sparse and many phages are host-strain-specific, but nevertheless recombination between some phages is common.

There is an increasing recognition of the importance of strain differences in the ecology of a symbiotic bacterial species and, in particular, how these differences underlie crucial interactions with their host. Nevertheless, little is known about the genetic bases for these differences, how they manifest themselves in specific behaviors, and their distribution among symbionts of different host species. In this study, we sequenced the genomes of Vibrio fischeri isolated from the tissues of squids and fishes and applied comparative genomics approaches to look for patterns between symbiont lineages and host colonization behavior. In addition, we identified the only two genes that were exclusively present in all V. fischeri strains isolated from the light organs of sepiolid squid species. Mutational studies of these genes indicated that they both played a role in colonization of the squid light organ, emphasizing the value of applying a comparative genomics approach in the study of symbioses., The luminous marine Gram-negative bacterium Vibrio (Aliivibrio) fischeri is the natural light organ symbiont of several squid species, including the Hawaiian bobtail squid, Euprymna scolopes, and the Japanese bobtail squid, Euprymna morsei. Work with E. scolopes has shown how the bacteria establish their niche in the light organ of the newly hatched host. Two types of V. fischeri strains have been distinguished based upon their behavior in cocolonization competition assays in juvenile E. scolopes, i.e., (i) niche-sharing or (ii) niche-dominant behavior. This study aimed to determine whether these behaviors are observed with other V. fischeri strains or whether they are specific to those isolated from E. scolopes light organs. Cocolonization competition assays between V. fischeri strains isolated from the congeneric squid E. morsei or from other marine animals revealed the same sharing or dominant behaviors. In addition, whole-genome sequencing of these strains showed that the dominant behavior is polyphyletic and not associated with the presence or absence of a single gene or genes. Comparative genomics of 44 squid light organ isolates from around the globe led to the identification of symbiosis-specific candidates in the genomes of these strains. Colonization assays using genetic derivatives with deletions of these candidates established the importance of two such genes in colonization. This study has allowed us to expand the concept of distinct colonization behaviors to strains isolated from a number of squid and fish hosts.

Phytoplankton blooms taking place during the warm season drive high productivity in Antarctic coastal seawaters. Important temporal and spatial variations exist in productivity patterns, indicating local constraints influencing the phototrophic community. Surface water in Chile Bay (Greenwich Island, South Shetlands) is influenced by freshwater from the melting of sea ice and surrounding glaciers; however, it is not a widely studied system. The phyto- and bacterioplankton communities in Chile Bay were studied over two consecutive summers; during a low productivity period (chlorophyll a

Copepods harbor diverse bacterial communities, which collectively carry out key biogeochemical transformations in the ocean. However, bulk copepod sampling averages over the variability in their associated bacterial communities, thereby limiting our understanding of the nature and specificity of copepod–bacteria associations. Here, we characterize the bacterial communities associated with nearly 200 individual Calanus finmarchicus copepods transitioning from active growth to diapause. We find that all individual copepods sampled share a small set of “core” operational taxonomic units (OTUs), a subset of which have also been found associated with other marine copepod species in different geographic locations. However, most OTUs are patchily distributed across individual copepods, thereby driving community differences across individuals. Among patchily distributed OTUs, we identified groups of OTUs correlated with common ecological drivers. For instance, a group of OTUs positively correlated with recent copepod feeding served to differentiate largely active growing copepods from those entering diapause. Together, our results underscore the power of individual-level sampling for understanding host–microbiome relationships.

Microbial communities often exhibit incredible taxonomic diversity, raising questions regarding the mechanisms enabling species coexistence and the role of this diversity in community functioning. On the one hand, many coexisting but taxonomically distinct microorganisms can encode the same energy-yielding metabolic functions, and this functional redundancy contrasts with the expectation that species should occupy distinct metabolic niches. On the other hand, the identity of taxa encoding each function can vary substantially across space or time with little effect on the function, and this taxonomic variability is frequently thought to result from ecological drift between equivalent organisms. Here, we synthesize the powerful paradigm emerging from these two patterns, connecting the roles of function, functional redundancy and taxonomy in microbial systems. We conclude that both patterns are unlikely to be the result of ecological drift, but are inevitable emergent properties of open microbial systems resulting mainly from biotic interactions and environmental and spatial processes.

Summary Viruses are traditionally thought to be under selective pressure to maintain compact genomes and thus depend on host cell translational machinery for reproduction. However, some viruses encode abundant tRNA and other translation-related genes, potentially optimizing for codon usage differences between phage and host. Here, we systematically interrogate selective advantages that carrying 18 tRNAs may convey to a T4-like Vibriophage. Host DNA and RNA degrade upon infection, including host tRNAs, which are replaced by those of the phage. These tRNAs are expressed at levels slightly better adapted to phage codon usage, especially that of late genes. The phage is unlikely to randomly acquire as diverse an array of tRNAs as observed (p = 0.0017). Together, our results support that the main driver behind phage tRNA acquisition is pressure to sustain translation as host machinery degrades, a process resulting in a dynamically adapted codon usage strategy during the course of infection.

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.

Heterotrophic bacteria exploit diverse microhabitats in the ocean, from particles to transient gradients. Yet the degree to which genes and pathways can contribute to an organism's fitness on such complex and variable natural resource landscapes remains poorly understood. Here, we determine the gene-by-gene fitness of a generalist saprophytic marine bacterium (Vibrio sp. F13 9CS106) on complex resources derived from its natural habitats - copepods (Apocyclops royi) and brown algae (Fucus vesiculosus) - and as reference substrates, glucose and the polysaccharide alginate, derived from brown algal cell walls. We find that resource complexity strongly buffers fitness costs of mutations, and that anabolic rather than catabolic pathways are more stringently required, likely due to functional redundancy in the latter. Moreover, while carbohydrate-rich algae requires several synthesis pathways, protein-rich Apocyclops does not, suggesting this ancestral habitat for Vibrios is a replete medium with metabolically redundant substrates. We also identify a candidate fitness trade-off for algal colonization: deletion of mshA increases mutant fitness. Our results demonstrate that gene fitness depends on habitat composition, and suggest that this generalist uses distinct resources in different natural habitats. The results further indicate that substrate replete conditions may lead to relatively relaxed selection on catabolic genes.

Cyanobacteria are widely distributed primary producers with significant implications for the global biogeochemical cycles of carbon and nitrogen. Diazotrophic cyanobacteria of subsection V (Order Stigonematales) are particularly ubiquitous in photoautotrophic microbial mats of hot springs. The Stigonematal cyanobacterium strain CHP1 isolated from the Porcelana hot spring (Chile) was one of the major contributors of the new nitrogen through nitrogen fixation. Further morphological and genetic characterization verified that the strain CHP1 belongs to Stigonematales, and it formed a separate clade together with other thermophiles of the genera Fischerella and Mastigocladus. Strain CHP1 fixed maximum N2 in the light, independent of the temperature range. At 50°C nifH gene transcripts showed high expression during the light period, whereas the nifH gene expression at 45°C was arrhythmic. The strain displayed a high affinity for nitrate and a low tolerance for high ammonium concentrations, whereas the narB and glnA genes showed higher expression in light and at the beginning of the dark phase. It is proposed that Mastigocladus sp. strain CHP1 would represent a good model for the study of subsection V thermophilic cyanobacteria, and for understanding the adaptations of these photoautotrophic organisms inhabiting microbial mats in hot springs globally.

The genetic code evolved to reduce the impact of nutrient limitations

Due to the promiscuous exchange of genetic material and asexual reproduction, delineating microbial species (and, by extension, populations) remains challenging. Because of this, the vast majority of microbial studies assessing population structure often compare divergent strains from disparate environments under varied selective pressures. Here, we investigated the population structure within a single bacterial ecotype, a unit equivalent to a eukaryotic species, defined as highly clustered genotypic and phenotypic strains with the same ecological niche. Using a combination of genomic and computational analyses, we assessed the phylogenetic structure, extent of recombination, and flexible gene content of this genomic diversity to infer patterns of gene flow. To our knowledge, this study is the first to do so for a dominant soil bacterium. Our results indicate that bacterial soil populations, similarly to those in other environments, are structured by gene flow discontinuities and exhibit distributional patterns consistent with both isolation by distance and isolation by environment. Thus, both dispersal limitation and local environments contribute to the divergence among closely related soil bacteria as observed in macroorganisms., For free-living bacteria and archaea, the equivalent of the biological species concept does not exist, creating several obstacles to the study of the processes contributing to microbial diversification. These obstacles are particularly high in soil, where high bacterial diversity inhibits the study of closely related genotypes and therefore the factors structuring microbial populations. Here, we isolated strains within a single Curtobacterium ecotype from surface soil (leaf litter) across a regional climate gradient and investigated the phylogenetic structure, recombination, and flexible gene content of this genomic diversity to infer patterns of gene flow. Our results indicate that microbial populations are delineated by gene flow discontinuities, with distinct populations cooccurring at multiple sites. Bacterial population structure was further delineated by genomic features allowing for the identification of candidate genes possibly contributing to local adaptation. These results suggest that the genetic structure within this bacterium is maintained both by ecological specialization in localized microenvironments (isolation by environment) and by dispersal limitation between geographic locations (isolation by distance).

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.

Although Clostridium difficile is widely considered an antibiotic- and hospital-associated pathogen, recent evidence indicates that this is an insufficient depiction of the risks and reservoirs. A common thread that links all major risk factors of infection is their association with gastrointestinal disturbances, but this relationship to C. difficile colonization has never been tested directly. Here, we show that disturbances caused by diarrhoeal events trigger susceptibility to C. difficile colonization. Using survey data of the human gut microbiome, we detected C. difficile colonization and blooms in people recovering from food poisoning and Vibrio cholerae infections. Carriers remained colonized for year-long time scales and experienced highly variable patterns of C. difficile abundance, where increased shedding over short periods of 1-2 d interrupted week-long periods in which C. difficile was undetectable. Given that short shedding events were often linked to gastrointestinal disturbances, our results help explain why C. difficile is frequently detected as a co-infecting pathogen in patients with diarrhoea. To directly test the impact of diarrhoea on susceptibility to colonization, we developed a mouse model of variable disturbance intensity, which allowed us to monitor colonization in the absence of disease. As mice exposed to avirulent C. difficile spores ingested increasing quantities of laxatives, more individuals experienced C. difficile blooms. Our results indicate that the likelihood of colonization is highest in the days immediately following acute disturbances, suggesting that this could be an important window during which transmission could be interrupted and the incidence of infection lowered.

Populations of the bacterium Vibrio cholerae consist of dozens of distinct lineages, with primarily (but not exclusively) members of the pandemic generating lineage capable of causing the diarrhoeal disease cholera. Assessing the composition and temporal dynamics of such populations requires extensive isolation efforts and thus only rarely covers large geographic areas or timeframes exhaustively. We developed a culture-independent amplicon sequencing strategy based on the protein-coding gene viuB (vibriobactin utilization) to study the structure of a V. cholerae population over the course of a summer. We show that the 26 co-occurring V. cholerae lineages continuously compete for limited space on nutrient-rich particles where only a few of them can grow to large numbers. Differential abundance of lineages between locations and size-fractions associated with a particle-attached or free-swimming lifestyle could reflect adaptation to various environmental niches. In particular, a major V. cholerae lineage occasionally grows to large numbers on particles but remain undetectable using isolation-based methods, indicating selective culturability for some members of the species. We thus demonstrate that isolation-based studies may not accurately reflect the structure and complex dynamics of V. cholerae populations and provide a scalable high-throughput method for both epidemiological and ecological approaches to studying this species.

In free-living bacteria and archaea, the equivalent of the biological species concept does not exist, creating several barriers to the study of the processes contributing to microbial diversification. As such, microorganisms are often operationally defined using conserved marker genes (i.e., 16S rRNA gene) or whole-genome measurements (i.e., ANI) to interpret intra-specific processes. However, as in eukaryotes, investigations into microbial populations must consider the potential for interacting genotypes among individuals that are subjected to similar environmental selective pressures. Therefore, we isolated 26 strains within a single bacterial ecotype (equivalent to a eukaryotic species definition) from a common habitat (leaf litter) across a regional climate gradient and asked whether the genetic diversity in a free-living soil bacterium (Curtobacterium) was consistent with patterns of allopatric or sympatric differentiation. By examining patterns of gene flow, our results indicate that microbial populations are delineated by gene flow discontinuities and exhibit evidence for population-specific adaptation. We conclude that the genetic structure within this bacterium is due to both adaptation within localized microenvironments (isolation-by-environment) as well as dispersal limitation between geographic locations (isolation-by-distance).

For tolerating extreme desiccation, cyanobacteria are known to produce both compatible solutes at intracellular level and a copious amount of exopolysaccharides as a protective coat. However, these molecules make cyanobacterial cells refractory to a broad spectrum of cell disruption methods, hindering genome sequencing, and molecular studies. In fact, few genomes are already available from cyanobacteria from extremely desiccated environments such as deserts. In this work, we report the 5.4 Mbp draft genome (with 100% of completeness in 105 contigs) of Gloeocapsopsis sp. UTEX B3054 (subsection I; Order Chroococcales), a cultivable sugar-rich and hardly breakable hypolithic cyanobacterium from the Atacama Desert. Our in silico analyses focused on genomic features related to sugar-biosynthesis and adaptation to dryness. Among other findings, screening of Gloeocapsopsis genome revealed a unique genetic potential related to the biosynthesis and regulation of compatible solutes and polysaccharides. For instance, our findings showed for the first time a novel genomic arrangement exclusive of Chroococcaceae cyanobacteria associated with the recycling of trehalose, a compatible solute involved in desiccation tolerance. Additionally, we performed a comparative genome survey and analyses to entirely predict the highly diverse pool of glycosyltransferases enzymes, key players in polysaccharide biosynthesis and the formation of a protective coat to dryness. We expect that this work will set the fundamental genomic framework for further research on microbial tolerance to desiccation and to a wide range of other extreme environmental conditions. The study of microorganisms like Gloeocapsopsis sp. UTEX B3054 will contribute to expand our limited understanding regarding water optimization and molecular mechanisms allowing extremophiles to thrive in xeric environments such as the Atacama Desert.

Adaptive radiations are important drivers of niche filling, since they rapidly adapt a single clade of organisms to ecological opportunities. Although thought to be common for animals and plants, adaptive radiations have remained difficult to document for microbes in the wild. Here we describe a recent adaptive radiation leading to fine-scale ecophysiological differentiation in the degradation of an algal glycan in a clade of closely related marine bacteria. Horizontal gene transfer is the primary driver in the diversification of the pathway leading to several ecophysiologically differentiated Vibrionaceae populations adapted to different physical forms of alginate. Pathway architecture is predictive of function and ecology, underscoring that horizontal gene transfer without extensive regulatory changes can rapidly assemble fully functional pathways in microbes., United States. Department of Energy (DE-SC0008743), National Defense Science and Engineering Graduate (NDSEG) Fellowship

Cultivated common beans are the primary protein source for millions of people around the world who subsist on low-input agriculture, enabled by the symbiotic N2 -fixation these legumes perform in association with rhizobia. Within a single agricultural plot, multiple Rhizobium species can nodulate bean roots, but it is unclear how genetically isolated these species remain in sympatry. To better understand this issue, we sequenced and compared the genomes of 33 strains isolated from the rhizosphere and root nodules of a particular bean variety grown in the same agricultural plot. We found that the Rhizobium species we observed coexist with low genetic recombination across their core genomes. Accessory plasmids thought to be necessary for the saprophytic lifestyle in soil show similar levels of genetic isolation, but with higher rates of recombination than the chromosomes. However, the symbiotic plasmids are extremely similar, with high rates of recombination and do not appear to have co-evolved with the chromosome or accessory plasmids. Therefore, while Rhizobium species are genetically isolated units within the microbial community, a common symbiotic plasmid allows all Rhizobium species to engage in symbiosis with the same host in a single agricultural plot.

Newly hatched Euprymna scolopes squid obtain their specific light-organ symbionts from an array of Vibrio (Allivibrio) fischeri strains present in their environment. Two genetically distinct populations of this squid species have been identified, one in Kaneohe Bay (KB), and another in Maunaloa Bay (MB), Oahu. We asked whether symbionts isolated from squid in each of these populations outcompete isolates from the other population in mixed-infection experiments. No relationship was found between a strain's host source (KB or MB) and its ability to competitively colonize KB or MB juveniles in a mixed inoculum. Instead, two colonization behaviors were identified among the 11 KB and MB strains tested: a 'dominant' outcome, in which one strain outcompetes the other for colonization, and a 'sharing' outcome, in which two strains co-colonize the squid. A genome-level comparison of these and other V. fischeri strains suggested that the core genomic structure of this species is both syntenous and highly conserved over time and geographical distance. We also identified ~250 Kb of sequence, encoding 194 dispersed orfs, that was specific to those strains that expressed the dominant colonization behavior. Taken together, the results indicate a link between the genome content of V. fischeri strains and their colonization behavior when initiating a light-organ symbiosis.

Populations are fundamental units of ecology and evolution, but can we define them for bacteria and archaea in a biologically meaningful way? Here, we review why population structure is difficult to recognize in microbes and how recent advances in measuring contemporary gene flow allow us to identify clearly delineated populations among collections of closely related genomes. Such structure can arise from preferential gene flow caused by coexistence and genetic similarity, defining populations based on biological mechanisms. We show that such gene flow units are sufficiently genetically isolated for specific adaptations to spread, making them also ecological units that are differentially adapted compared to their closest relatives. We discuss the implications of these observations for measuring bacterial and archaeal diversity in the environment. We show that operational taxonomic units defined by 16S rRNA gene sequencing have woefully poor resolution for ecologically defined populations and propose monophyletic clusters of nearly identical ribosomal protein genes as an alternative measure for population mapping in community ecological studies employing metagenomics. These population-based approaches have the potential to provide much-needed clarity in interpreting the vast microbial diversity in human and environmental microbiomes. This article is part of the theme issue ‘Conceptual challenges in microbial community ecology’.

Larger volumes of sea ice have been thawing in the Central Arctic Ocean (CAO) during the last decades than during the past 800,000 years. Brackish brine (fed by meltwater inside the ice) is an expanding sympagic habitat in summer all over the CAO. We report for the first time the structure of bacterial communities in this brine. They are composed of psychrophilic extremophiles, many of them related to phylotypes known from Arctic and Antarctic regions. Community structure displayed strong habitat segregation between brackish ice brine (IB; salinity 2.4-9.6) and immediate sub-ice seawater (SW; salinity 33.3-34.9), expressed at all taxonomic levels (class to genus), by dominant phylotypes as well as by the rare biosphere, and with specialists dominating IB and generalists SW. The dominant phylotypes in IB were related to Candidatus Aquiluna and Flavobacterium, those in SW to Balneatrix and ZD0405, and those shared between the habitats to Halomonas, Polaribacter and Shewanella. A meta-analysis for the oligotrophic CAO showed a pattern with Flavobacteriia dominating in melt ponds, Flavobacteriia and Gammaproteobacteria in solid ice cores, Flavobacteriia, Gamma- and Betaproteobacteria, and Actinobacteria in brine, and Alphaproteobacteria in SW. Based on our results, we expect that the roles of Actinobacteria and Betaproteobacteria in the CAO will increase with global warming owing to the increased production of meltwater in summer. IB contained three times more phylotypes than SW and may act as an insurance reservoir for bacterial diversity that can act as a recruitment base when environmental conditions change.

While most Vibrionaceae are considered generalists that thrive on diverse substrates, including animal-derived material, we show that Vibrio breoganii has specialized for the consumption of marine macroalga-derived substrates. Genomic and physiological comparisons of V. breoganii with other Vibrionaceae isolates revealed the ability to degrade alginate, laminarin, and additional glycans present in algal cell walls. Moreover, the widely conserved ability to hydrolyze animal-derived polymers, including chitin and glycogen, was lost, along with the ability to efficiently grow on a variety of amino acids. Ecological data showing associations with particulate algal material but not zooplankton further support this shift in niche preference, and the loss of motility appears to reflect a sessile macroalga-associated lifestyle. Together, these findings indicate that algal polysaccharides have become a major source of carbon and energy in V. breoganii, and these ecophysiological adaptations may facilitate transient commensal associations with marine invertebrates that feed on algae.IMPORTANCE Vibrios are often considered animal specialists or generalists. Here, we show that Vibrio breoganii has undergone massive genomic changes to become specialized on algal carbohydrates. Accompanying genomic changes include massive gene import and loss. These vibrios may help us better understand how algal biomass is degraded in the environment and may serve as a blueprint on how to optimize the conversion of algae to biofuels.

Viruses are highly discriminating in their interactions with host cells and are thought to play a major role in maintaining diversity of environmental microbes. However, large-scale ecological and genomic studies of co-occurring virus-host pairs, required to characterize the mechanistic and genomic foundations of virus-host interactions, are lacking. Here, we present the largest dataset of cultivated and sequenced co-occurring virus-host pairs that captures ecologically representative fine-scale diversity. Using the ubiquitous and ecologically diverse marine Vibrionaceae as a host platform, we isolate and sequence 251 dsDNA viruses and their hosts from three time points within a 93-day time-series study. The virus collection includes representatives of the three Caudovirales tailed virus morphotypes, a novel family of nontailed viruses, and the smallest (10,046 bp) and largest (348,911 bp) Vibrio virus genomes described. We provide general characterization and annotation of the viruses and describe read-mapping protocols to standardize genome presentation. The rich ecological and genomic contextualization of hosts and viruses make the Nahant Collection a unique platform for high-resolution studies of environmental virus-host infection networks., National Science Foundation (U.S.) (OCE 1435993)

How the diversity of organisms competing for or sharing resources influences community production is an important question in ecology but has rarely been explored in natural microbial communities. These generally contain large numbers of species making it difficult to disentangle how the effects of different interactions scale with diversity. Here, we show that changing diversity affects measures of community function in relatively simple communities but that increasing richness beyond a threshold has little detectable effect. We generated self-assembled communities with a wide range of diversity by growth of cells from serially diluted seawater on brown algal leachate. We subsequently isolated the most abundant taxa from these communities via dilution-to-extinction in order to compare productivity functions of the entire community to those of individual taxa. To parse the effect of different types of organismal interactions, we developed relative total function (RTF) as an index for positive or negative effects of diversity on community function. Our analysis identified three overall regimes with increasing diversity. At low richness (

Prophages are known to encode important virulence factors in the human pathogen Vibrio cholerae. However, little is known about the occurrence and composition of prophage-encoded traits in environmental vibrios. A database of 5,674 prophage-like elements constructed from 1,874 Vibrio genome sequences, covering sixty-four species, revealed that prophage-like elements encoding possible properties such as virulence and antibiotic resistance are widely distributed among environmental vibrios, including strains classified as non-pathogenic. Moreover, we found that 45% of Vibrio species harbored a complete prophage-like element belonging to the Inoviridae family, which encode the zonula occludens toxin (Zot) previously described in the V. cholerae. Interestingly, these zot-encoding prophages were found in a variety of Vibrio strains covering both clinical and marine isolates, including strains from deep sea hydrothermal vents and deep subseafloor sediments. In addition, the observation that a spacer from the CRISPR locus in the marine fish pathogen V. anguillarum strain PF7 had 95% sequence identity with a zot gene from the Inoviridae prophage found in V. anguillarum strain PF4, suggests acquired resistance to inoviruses in this species. Altogether, our results contribute to the understanding of the role of prophages as drivers of evolution and virulence in the marine Vibrio bacteria., Natioanal Science Foundation (U.S.) (Grant OCE-1435993)

Advances in sequencing technologies have led to a rapid increase in available bacterial and archaeal genome information, but for much of this diversity, little ecological information is available. Reverse ecology provides a potential path forward by using genomic information to gain insight into the ecological associations and niche spaces of organisms. A crucial first step is to predict population structure, which provides the basis for analyzing genomes for evidence of ecological differentiation. Although delineation of bacterial and archaeal populations remains difficult, we outline how gene flow information can be used to identify populations as genetic units, which also are ecological units because adaptations can spread through them in a specific manner. This approach is particularly powerful when closely related populations are analyzed for signatures of differential selection that indicate recent ecological differentiation. Genome-wide association studies can also help identify mutations and genes underlying ecologically relevant traits. Albeit still in their infancy, reverse ecology approaches have the potential to order microbial diversity into genetically and ecologically cohesive units and hence provide the opportunity to test hypotheses about the evolutionary mechanisms creating and maintaining diversity within and between populations.

A universal tool in the culture-based study of bacterial viruses (bacteriophages, or phages) is the agar overlay, which is used in the isolation of new viruses, and in their quantification and purification. Here, simple optimizations that increase efficiency and throughput in agar overlay based isolation and cultivation of virus-host systems are presented. The agar overlay is streamlined to minimize steps and materials. Serial purification of viruses from viral colonies (plaques) is optimized to eliminate steps by combining purification by serial re-streaking with the optimized agar overlay approach. Finally, recommendations are made for efficient archival and storage of virus plaques. In sum, this work presents: • Tube-free Agar Overlays: rapid plaque assays with fewer steps and materials• Molten Streaking for Singles: rapid tube-free serial purification of viruses• Archiving Plaques: saving virus purification for later Keywords: Agar overlay; Agar layer; Top agar; Bottom agar; Plaque; Virus; Phage; Isolation; Cultivation; Purification, National Science Foundation (U.S.) (Grant OCE-1435993)

Because microbial plankton in the ocean comprise diverse bacteria, algae, and protists that are subject to environmental forcing on multiple spatial and temporal scales, a fundamental open question is to what extent these organisms form ecologically cohesive communities. Here we show that although all taxa undergo large, near daily fluctuations in abundance, microbial plankton are organized into clearly defined communities whose turnover is rapid and sharp. We analyze a time series of 93 consecutive days of coastal plankton using a technique that allows inference of communities as modular units of interacting taxa by determining positive and negative correlations at different temporal frequencies. This approach shows both coordinated population expansions that demarcate community boundaries and high frequency of positive and negative associations among populations within communities. Our analysis thus highlights that the environmental variability of the coastal ocean is mirrored in sharp transitions of defined but ephemeral communities of organisms., Whether marine microbes form strongly differentiated communities over time remains unknown. Here, Martin-Platero and colleagues develop a time series analysis to characterize marine bacteria and Eukarya communities at a fine temporal grain, revealing cohesive but rapidly changing communities.

International audience; Vibrios are frequently associated with oyster mortality; however whether they are the primary causative agent or secondary opportunistic colonizers is not well understood. Here we combine analysis of natural infection dynamics, population genomics and molecular genetics to ask (i) to what extent oysters are passively colonized by Vibrio population present in the surrounding water, (ii) how populations turn over during pathogenicity events and (iii) what genetic factors are responsible for pathogenicity. We identified several populations of Vibrio preferentially associated with oyster tissues. Among these, Vibrio crassostreae is particularly abundant in diseased animals while nearly absent in the surrounding water, and its pathogenicity is correlated with the presence of a large mobilizable plasmid. We further demonstrate that the plasmid is essential for killing but not necessary for survival in tissues of oysters. Our results suggest that V. crassostreae first differentiated into a benign oyster colonizer that was secondarily turned into a pathogen by introgression of a virulence plasmid into the population, possibly facilitated by elevated host density in farming areas.

Rich animal and microbial communities have been found at deep-sea hydrothermal vents. Although the biogeography of vent macrofauna is well understood, the corresponding knowledge about vent microbial biogeography is lacking. Here, we apply the multilocus sequence analysis (MLSA) to assess the genetic variation of 109 Sulfurimonas strains with greater than or equal to98% 16S rRNA gene sequence similarity, which were isolated from four different geographical regions (Okinawa Trough (OT), Mariana Volcanic Arc and Trough (MVAT), Central Indian Ridge (CIR) and Mid-Atlantic Ridge (MAR)). Sequence typing based on 11 protein-coding genes revealed high genetic variation, including some allele types that are widespread within regions, resulting in 102 nucleotide sequence types (STs). This genetic variation was predominantly due to mutation rather than recombination. Phylogenetic analysis of the 11 concatenated genes showed a clear geographical isolation corresponding to the hydrothermal regions they originated from, suggesting limited dispersal. Genetic differentiation among Sulfurimonas populations was primarily influenced by geographical distance rather than gas composition of vent fluid or habitat, although in situ environmental conditions of each microhabitat could not be examined. Nevertheless, Sulfurimonas may possess a higher dispersal capability compared with deep-sea hydrothermal vent thermophiles. This is the first report on MLSA of deep-sea hydrothermal vent Epsilonproteobacteria, which is indicative of allopatric speciation.

Bacteria face a trade-off between genetic fidelity, which reduces deleterious mistakes in the genome, and genetic innovation, which allows organisms to adapt. Evidence suggests that many bacteria balance this trade-off by modulating their mutation rates, but few mechanisms have been described for such modulation. Following experimental evolution and whole-genome resequencing of the marine bacterium Vibrio splendidus 12B01, we discovered one such mechanism, which allows this bacterium to switch to an elevated mutation rate. This switch is driven by the excision of a mobile element residing in mutS, which encodes a DNA mismatch repair protein. When integrated within the bacterial genome, the mobile element provides independent promoter and translation start sequences for mutS—different from the bacterium’s original mutS promoter region—which allow the bacterium to make a functional mutS gene product. Excision of this mobile element rejoins the mutS gene with host promoter and translation start sequences but leaves a 2-bp deletion in the mutS sequence, resulting in a frameshift and a hypermutator phenotype. We further identified hundreds of clinical and environmental bacteria across Betaproteobacteria and Gammaproteobacteria that possess putative mobile elements within the same amino acid motif in mutS. In a subset of these bacteria, we detected excision of the element but not a frameshift mutation; the mobile elements leave an intact mutS coding sequence after excision. Our findings reveal a novel mechanism by which one bacterium alters its mutation rate and hint at a possible evolutionary role for mobile elements within mutS in other bacteria., IMPORTANCE DNA mutations are a double-edged sword. Most mutations are harmful; they can scramble precise genetic sequences honed over thousands of generations. But in rare cases, mutations also produce beneficial new traits that allow populations to adapt to changing environments. Recent evidence suggests that some bacteria balance this trade-off by altering their mutation rates to suit their environment. To date, however, we know of few mechanisms that allow bacteria to change their mutation rates. We describe one such mechanism, driven by the action of a mobile element, in the marine bacterium Vibrio splendidus 12B01. We also found similar mobile genetic sequences in the mutS genes of many different bacteria, including clinical and agricultural pathogens. These mobile elements might play an as yet unknown role in the evolution of these important bacteria.

Bacterial and archaeal communities inhabiting the subsurface seabed live under strong energy limitation and have growth rates that are orders of magnitude slower than laboratory-grown cultures. It is not understood how subsurface microbial communities are assembled and whether populations undergo adaptive evolution or accumulate mutations as a result of impaired DNA repair under such energy-limited conditions. Here we use amplicon sequencing to explore changes of microbial communities during burial and isolation from the surface to the >5,000-y-old subsurface of marine sediment and identify a small core set of mostly uncultured bacteria and archaea that is present throughout the sediment column. These persisting populations constitute a small fraction of the entire community at the surface but become predominant in the subsurface. We followed patterns of genome diversity with depth in four dominant lineages of the persisting populations by mapping metagenomic sequence reads onto single-cell genomes. Nucleotide sequence diversity was uniformly low and did not change with age and depth of the sediment. Likewise, therewas no detectable change inmutation rates and efficacy of selection. Our results indicate that subsurface microbial communities predominantly assemble by selective survival of taxa able to persist under extreme energy limitation.

Marine microbes use alginate lyases to degrade and catabolize alginate, a major cell wall matrix polysaccharide of brown seaweeds. Microbes frequently contain multiple, apparently redundant alginate lyases, raising the question of whether these enzymes have complementary functions. We report here on the molecular cloning and functional characterization of three exo-type oligoalginate lyases (OalA, OalB, and OalC) from Vibrio splendidus 12B01 (12B01), a marine bacterioplankton species. OalA was most active at 16°C, had a pH optimum of 6.5, and displayed activities toward poly-β- d -mannuronate [poly(M)] and poly-α- l -guluronate [poly(G)], indicating that it is a bifunctional enzyme. OalB and OalC were most active at 30 and 35°C, had pH optima of 7.0 and 7.5, and degraded poly(M·G) and poly(M), respectively. Detailed kinetic analyses of oligoalginate lyases with poly(G), poly(M), and poly(M·G) and sodium alginate as substrates demonstrated that OalA and OalC preferred poly(M), whereas OalB preferred poly(M·G). The catalytic efficiency ( k cat / K m ) of OalA against poly(M) increased with decreasing size of the substrate. OalA showed k cat / K m from 2,130 mg −1 ml s −1 for the trisaccharide to 224 mg −1 ml s −1 for larger oligomers of ∼50 residues, and 50.5 mg −1 ml s −1 for high-molecular-weight alginate. Although OalA was most active on the trisaccharide, OalB and OalC preferred dimers. Taken together, our results indicate that these three Oals have complementary substrate scopes and temperature and pH adaptations.