The ISME Journal

How a single gram of soil harbors billions of microorganisms, each with distinct genomic variants that collectively maintain coherent ecological function(s), is one of microbiology's grand unsolved problems. A key obstacle is determining which variants contribute to individual- and community-level fitness, in which contexts, and how co-occurring ecotypes interact to divide niche space. Here, using nitrate (NO3-)-contaminated subsurface sediment as inoculum, we have performed high throughput enrichments in laboratory media of defined carbon source compositions across ecologically relevant gradients of pH and NO3-. Long-read metagenomics and link-community decomposition of co-occurrence networks of taxa across these enrichments has revealed context-specific functional interactions among dominant generalist and lower-abundance specialist denitrifier ecotypes that comprise 53 distinct enriched communities (EnComs) across 288 enrichments derived from a single sediment sample. We identified a single enzymatic difference of alternative NO3- reductases (NapAB vs. NarGHI) with differing substrate affinities that provided a mechanistic explanation for competitive niche partitioning between the two dominant taxa, Neorhizobium spp. and Allorhizobium spp., along the NO3- gradient. Genome-wide polymorphism ratios (pN/pS) revealed that selective pressures vary systematically with carbon source availability and gradients of pH and NO3-, which helps explain the natural biodiversity and functional interactions of ecotypes within denitrifying communities in the subsurface sediment. Our findings show that controlled enrichments along ecological gradients can thus uncover eco-evolutionary forces of selection, drift, and diversification that sculpt the biodiversity of microbial populations in the natural environment.

Dissimilatory nitrate reduction to ammonium (DNRA) is a key biological nitrogen retention pathway, yet the evolutionary mechanisms remain poorly understood. Here, we combined phylogenomic analysis, codon usage bias assessment, horizontal gene transfer (HGT) detection, and gene tree--species tree reconciliation to investigate the evolutionary history of nrfA. Analysis of 103 DNRA-capable taxa and 45 curated nrfA sequences revealed that DNRA capability evolved polyphyletically. Reconciliation analysis identified six discrete HGT events, establishing {delta}-Proteobacteria as the primary donor reservoir. Extreme GC3 differentiation between {delta}- and {varepsilon}-Proteobacteria and uniformly negative {Delta}ENC values indicated strong lineage-specific translational selection. We identified inter-phylum HGT to Planctomycetes and Bacteroidetes, one inter-domain transfer (Archaea [->] {delta}-Proteobacteria), and provided direct molecular evidence for IS-element-mediated transfer. To validate the generalizability, we expanded the analysis to 180 nrfA sequences, identifying 112 cross-phylum phylogenetic clusters that further support HGT as a widespread dissemination. These findings established a predictive framework linking molecular evolutionary signatures to DNRA capability, with implications for understanding nitrogen cycling.

Microbes in the animal gut compete to colonize spatially restricted host niches. However, whether competition for space drives inter-individual variation among hosts, and which factors determine the outcome of such competition, remain poorly understood. Here, we investigated competitive interactions of the western honeybee gut symbiont Frischella perrara with other members of the bee gut microbiota. Shotgun metagenomics analysis of individual bees revealed that F. perrara is negatively correlated with a specific species of the genus Gilliamella. Co-colonization of microbiota-depleted bees with these two bacteria resulted in their competitive exclusion. The outcome of this competition depended on the relative number of bacteria each bee received and benefited Gilliamella when one of the two type VI secretion systems of F. perrara was mutated. Using fluorescently tagged strains, high-resolution microscopy, and gut region-specific quantification, we show that both bacteria localize to the same host niche in the ileum of mono-colonized bees, indicating competition in a spatially restricted host niche. Moreover, both microbes protected against infection, promoting bee health. This competition provides an explanatory mechanism underlying variation in the occurrence of F. perrara across honeybee colonies and highlights the importance of gut spatial structure and microbial competition in shaping microbiome composition and inter-individual variability.

Microbial communities spontaneously colonize pristine environments, yet how species growth kinetics and individual cell variation shape community assembly remains poorly understood. Here, we use time-lapse microscopy imaging to track the division of individual founder cells in communities composed of up to seven soil bacterial isolates grown on nutrient surfaces. With cell lineage tracking, we quantify species-specific absolute biomass formation and growth kinetics from early growth through stationary phase. The reproductive success of individual founder cells depended on the timing of their first cell division, which determined their access to the primary substrate and their maximum growth rates. In mixed-species communities, founder cell success also depended on species-specific, substrate-dependent growth rates and yields. In addition, spatial factors such as cell positioning, distances to non-kin neighbours, and identities of co-occurring species, further influenced outcomes. In spatially structured communities, interspecific interactions were globally governed by competition for primary substrates. We also observed cross-feeding of leaked metabolites, reflected in fluctuating paired interaction strengths and interaction signs. Species-pair interactions differed locally, with cells within distances of less than 15 mum exhibiting opposite interaction behaviours. Global pairwise interactions predicted from monoculture growth kinetics were observed in approximately half of the measured pairs, whereas measured paired interactions generally weakened in combinations of three or more species. Using a spatially explicit agent-based Monod growth model that includes interspecific interactions, we accurately predicted the compositions of seven-member communities. Overall, our results indicate that emergent, spatially mediated interspecific interactions between cells of different bacterial species primarily drive local and temporal changes in individual cell growth rates, which in turn determine final biomass formation. Because most natural microbial habitats are spatially structured, stochastic founder-cell positioning and fitness differences are key determinants of locally formed interaction patterns and species coexistence.

Bacterial chitin degradation contributes to global carbon cycling, particularly in marine environments where it is a highly abundant polysaccharide. Despite the taxonomic diversity of co-occurring chitin-degrading bacteria, the influence of individual traits on interactions between them remains poorly understood. Here, we measured key physiological traits of seven chitin degraders and investigated how these traits shape interaction outcomes and chitin degradation in pairwise cocultures. We found mainly negative and neutral interactions among degraders, contrasting with the synergistic dynamics observed with other complex polysaccharides. However, chitin degradation was not consistently diminished. These interaction types could be attributed to the limited partitioning of degradation products, alongside variations in enzyme repertoires and attachment behaviours that help some degraders to prevail over others. Further, we showed that one degrader can strongly inhibit the growth of others, even those possessing favourable physiological traits, likely due to the secretion of inhibitory compounds. These findings extend our understanding of the breadth of interactions among primary polysaccharide degraders and their implications for the degradation process. One-sentence SummaryThe physiological traits of bacteria that degrade chitin, a highly abundant biopolymer in marine environments, promote a range of neutral and negative interactions among them.

The roles of reactive oxygen species (ROS) as signaling molecules and inhibitors of phytoplankton growth are well documented. While phytoplankton physiological mechanisms for ROS detoxification are well characterized, the role of heterotrophic bacterial partners in ROS alleviation and outcomes for these bacteria remain poorly understood. Here, we examined how extracellular hydrogen peroxide (H2O2) shapes nutrient exchange between the diatom Phaeodactylum tricornutum and two phycosphere bacteria. From an initial screen of 20 bacteria, we identified a "helper" (Muricauda sp.) that enabled P. tricornutum to survive acute H2O2 stress and a "non-helper" (Algoriphagus sp.) that did not. Using nanoscale secondary ion mass spectrometry (nanoSIMS), we tracked diatom-derived carbon and nitrogen (13C and 15N) transfer to each bacterial partner under ROS stress. Oxidative stress disrupted diatom metabolism and altered nutrient transfer: diatom-derived carbon and nitrogen incorporation was significantly reduced in the helper but increased in the non-helper under H2O2 stress. Growth assays revealed that the helper preferentially utilized exudates from healthy, intact hosts, whereas the non-helper did not grow on exudates but thrived on lysates from damaged or lysed cells. Together, these findings indicate the helper was better adapted to accessing resources from living hosts, while the non-helper relied on nutrients released through ROS-induced host damage. Our results highlight oxidative stress as a key driver of algal-bacterial interactions and suggest that bacterial resource-acquisition strategy underlies host protection: bacteria utilizing healthy-host exudates are more likely to protect hosts from oxidative stress, while those benefiting from host damage are not, despite retaining ROS detoxification capacity.

Bacillus velezensis is a widely used plant growth-promoting rhizobacterium whose effectiveness under natural conditions is strongly influenced by interactions with surrounding microorganisms. While bacterial secondary metabolites are known to shape these interactions, little is known about their long-term evolutionary consequences. Here, we show that repeated exposure of B. velezensis GA1 to secondary metabolites produced by the competing rhizobacterium Pseudomonas sessilinigenes CMR12a drives the emergence of an adapted subpopulation with enhanced ecological fitness. Multi-omics analyses revealed extensive metabolomic and transcriptional changes associated with altered growth dynamics, sporulation, motility, and biofilm formation. Importantly, the evolved variant exhibited improved tomato root colonization and reduced the abundance of the competing Pseudomonas strain in planta. Together, our results demonstrate that prolonged exposure to diffusible bacterial metabolites can drive rapid adaptive diversification in rhizosphere-associated bacteria and highlight the importance of long-term interbacterial interactions in shaping the outcome of plant microbiome assembly and biocontrol performance.

Microbial communities are structured by environmental gradients and metabolic interactions, yet the genomic characteristics and metabolic functions of co-occurring populations remain underexplored. Here, we investigated co-occurring microbial cohorts across the Baltic Sea, a system characterized by strong salinity, temperature, and oxygen gradients. For this, we used a genomic catalog consisting of 701 species-representative genomes to recruit reads from 112 metagenomes and infer cohort structure, environmental distributions, and metabolic potential. We identified nine microbial cohorts that showed strong associations with environmental gradients, indicating deterministic assembly. Cohorts differed markedly in genomic traits, with the most abundant and prevalent taxa associated with smaller, streamlined genomes, while a low-oxygen cohort with larger genomes contributed disproportionately to nitrogen and sulfur transformations. Across cohorts, biosynthetic potential was unevenly distributed. Amino acid biosynthesis pathways were frequently complete, whereas B-vitamin pathways were typically incomplete and rarely encoded in full by individual genomes. Metabolites with low pathway completeness showed consistent taxonomic partitioning, with biosynthetic capabilities distributed across taxa rather than collectively encoded within cohorts. Together, these results show that Baltic Sea microbial cohorts are ecologically structured assemblages whose genomic repertoires reflect catabolic specialization and anabolic interdependencies. Our findings highlight microbial cohorts as a useful framework for linking environmental gradients, genome traits, and the organization of metabolic functions in natural microbial communities.

Most microbes that live in or on hosts are not obligate symbionts. Instead, they often cycle between host-associated and free-living phases and experience selection in both environments. The benefits that microbes confer to hosts are often assumed to be a product of host-microbe (co)evolution, but microbial benefits to hosts may also evolve independent of the host-microbe interaction, while the microbe is free-living. We investigated this hypothesis by experimentally evolving a beneficial Allorhizobium bacterium we had previously isolated from duckweed (Lemna japonica). We evolved this Allorhizobium strain in the absence of any host at high and low salinity and at high and low nitrogen, and then tested how the evolved strains performed as symbionts when host-associated. Microbial salinity adaptation drove the emergence of novel benefits to host plants in high-salinity environments. However, these locally adaptive benefits emerged only when microbes evolved under low-nitrogen conditions; microbial adaptation to high nitrogen reduced plant growth. Bacterial phenotyping indicated that the same microbial traits that underlie salinity adaptation mediate host benefits. Whole-genome sequencing of the evolved strains revealed significant genomic shifts between selective treatments, including plasmid variation and point mutations associated with osmotic regulation. The emergence of the microbial benefits to hosts, as a byproduct of microbial adaptation, highlights that these benefits do not require targeted host-microbe co-evolution. Rather, predicting the evolutionary trajectory of these symbioses may require understanding both the abiotic and biotic selective agents acting on key microbial traits mediating the host-microbe interaction.

Ammonia oxidation is a rate-limiting step in the nitrogen cycle, yet viral contributions to this process remain largely unresolved. Here, we identify three genomically distinct groups of amoC-encoding phages (155-338 kilobases in length; termed as amoC-phages) from multiple freshwater lakes in Europe and North America, including the Laurentian Great Lakes. These phages are highly divergent in phylogeny, genome architecture, and gene content, and are predicted to infect two distinct Nitrosomonadaceae ammonia-oxidizing bacterial lineages. The placement of phage-encoded amoC genes across these divergent viral clades indicates independent acquisition of amoC. Time-series and depth-resolved metagenomes and metatranscriptomes reveal persistent and depth-structured distributions of amoC-phages and their predicted hosts, with seasonal mixing periodically reshaping their co-occurrence patterns. Furthermore, virome data from Lake Mendota show that some of the amoC-phages occur as free viral particles, supporting active viral lysis and particle redistribution along the water column. Metatranscriptomes of the Laurentian Great Lakes reveal coordinated expression of phage structural genes (e.g., major capsid protein) together with phage-encoded amoC, indicating active infection in situ. Together, these results support a framework in which amoC-phage infection is depth-structured, seasonally dynamic, and coupled to ammonia-oxidizing bacterial host activity, highlighting viruses as previously overlooked components of freshwater nitrogen cycling.

Marine microbes metabolize a wide range of carbon and nitrogen sources, shaping global biogeochemical cycles. Despite being crucial at the global scale, the coupling between carbon and nitrogen remains poorly understood for individual metabolites and bacteria. By phenotyping a library of marine heterotrophic bacteria across increasingly complex carbon and nitrogen sources, we generated a snapshot of this coupling. Growth phenotypes were weakly explained by phylogeny, but could be organized around substrate properties, including C:N stoichiometry and degree of reduction, reflecting the interplay between carbon, nitrogen and energy constraints. Beyond these patterns, we found strain-specific characteristics of ecological relevance, including differences in nitrogen use efficiency, and instances where yields on individual substrates are surprisingly not predictive of yields on more complex nutrient combinations. Overall, our results point to a few organizing principles for the role of carbon-nitrogen substrates in marine heterotrophic bacteria, with implications for understanding microbial interactions and modeling global biogeochemical cycles.

Halogenated one-carbon (C1) compounds, such as dichloromethane (DCM), drive critical fluxes in global carbon and halogen cycles. While the genus Dehalobacter is canonically defined by obligate organohalide respiration, its physiological and ecological roles in anaerobic C1 metabolism have remained fundamentally ambiguous. Here, we document a paradigm-shifting metabolic capacity within a sediment-derived microbial consortium: the autonomous fermentation of DCM by a novel population, Candidatus Dehalobacter formatiformans strain J1. Over successive transfers, strain J1 outcompeted co-existing Dehalobacterium formicoaceticum to become the overwhelmingly dominant population (>80% relative abundance), converting DCM stoichiometrically to acetate and formate (4:1) without auxiliary substrates. Genome-resolved metagenomics revealed that strain J1 couples a distinct mec gene cassette--mediating methyl-transfer reactions during DCM activation--to a complete Wood-Ljungdahl pathway for efficient C1 assimilation. Crucially, strain J1 lacks the complete genetic repertoire for de novo cobamide biosynthesis. Physiological validation confirmed that this fermentative pathway is strictly dependent on exogenous cobamides, exposing a profound reliance on community cross-feeding. These findings reveal an unexpected acetogenic lifestyle within Dehalobacter, a lineage historically viewed as comprising obligate organohalide-respiring bacteria. More broadly, this work identifies cobamide-dependent methyl-transfer metabolism as an ecological control on anaerobic DCM fermentation and expands the known roles of Dehalobacter in carbon-halogen cycling in anoxic environments.

1)Siderophores are classically viewed as shared iron-scavenging public goods, yet their ecological roles in multispecies communities remain poorly defined. Here, we establish a synthetic microbial community to dissect how different siderophores, their uptake compatibility and spatial structure shape iron competition. Using Corynebacterium glutamicum as a model, we show that this siderophore non-producer accesses diverse xenosiderophores, including enterobactin secreted by Escherichia coli. However, exploitation was constrained and co-cultures converged to stable compositions. Dose-response experiments combined with mathematical modelling indicated that the producer retains more effective access to enterobactin than the exploiter. Presence of Pseudomonas putida altered this interaction, as it exploited enterobactin while producing pyoverdine, a siderophore inaccessible to the other community members that restricted their iron access. Across different cultivation scales, community dynamics was strongly influenced by spatial organization and initial composition. These findings identify siderophores as context-dependent iron-allocation agents that can promote microbial coexistence or exclusion.

Host-associated microbiomes are typically maintained in stable configurations that support host fitness, yet the mechanisms by which metabolic perturbations destabilize these communities remain poorly understood. Using the freshwater cnidarian Hydra vulgaris AEP, we systematically assessed microbiome responses to 326 single-metabolite perturbations. Only 17 metabolites, mostly amino acid-related compounds, induced significant compositional shifts in the microbial community. Most shifts are accompanied by transitions from Curvibacter- to Pseudomonas-dominated or Legionella-dominated states, indicating the existence of three alternative community states which can be induced by metabolic triggers. Integrating 16S sequences with functional genomic information, we found that {beta}-diversity strongly predicted functional shifts, whereas reduced -diversity was associated with loss of metabolic functions. The metabolite perturbations also altered host-microbe interactions, affecting pathogenicity-, glycocalyx-, and nitrogen-related functions. In particular, nitrogen metabolism shifted from ammonia oxidation in Curvibacter-dominated communities to ammonia reduction in Pseudomonas-dominated states. Experimental validation confirmed that Pseudomonas metabolizes L-arginine and drives environmental ammonia accumulation to levels that could impair Hydras fitness and induce disease phenotypes. Conversely, Limnobacter was found to scavenge the environmental ammonia, potentially mitigating the adverse effects. These results demonstrate that metabolite-driven niche reconfiguration can destabilize host-associated microbiomes by coupling compositional shifts to functional change and host pathology, identifying metabolite-driven niche restructuring as a mechanism linking microbial community instability to host dysfunction.

Cross-feeding interactions, in which a producer species release by-products that serve as resources for a consumer species, play an important role in shaping microbial community diversity. Producers create opportunities for consumers by supplying high-energy resources that are often scarce in the environment. However, they also exert strong top-down effects by releasing metabolites in pulses and generating spatial gradients of resource availability. How these spatiotemporal constraints shape consumer evolution remains poorly understood. To address this question, we used a two-species cross-feeding system in which Acinetobacter johnsonii excretes benzoate (a by-product of benzyl alcohol oxidation) into the external environment where it is consumed by Pseudomonas putida. To assess how the origin of benzoate (externally supplied or produced by cross-feeding) shapes consumer evolution, we evolved P. putida for 200 generations in monoculture or in co-culture with A. johnsonii. Populations evolved in monoculture exhibited improved growth relative to the ancestor, whereas populations evolved under cross-feeding showed little to no growth improvement. Whole-genome sequencing revealed pervasive loss-of-function mutations in flagellar genes among populations evolved in monoculture, but not under cross-feeding conditions. High-throughput imaging assays showed that populations evolved under cross-feeding not only maintained but also enhanced functional motility. Competition experiments with single mutants revealed context-dependent fitness effects: loss-of-function mutations were highly beneficial when benzoate was externally supplied but deleterious when benzoate was supplied by A. johnsonii, highlighting the importance of motility in cross-feeding interactions. Together, our results show that resource origin fundamentally reshapes selective pressures and alters evolutionary outcomes in microbial communities. Significance StatementCross-feeding interactions are pervasive in microbial communities, often arising from metabolite leakage into the environment. Our study focused on understanding how cross-feeding interactions shape the evolution of populations that cross-feed on these resources. Using a highly trackable cross-feeding system and linking genotypic to phenotypic changes, we showed that cross-feeding interactions reshape the evolutionary trajectories of consumer species. Notably, selection acts on flagellar genes, but their effects on function depend on the origin of resources. When resources are externally supplied, selection favors loss of flagellar motility, whereas when resources are generated through cross-feeding, selection not only maintains functional motility but also enhance it. These findings highlight that navigating the environment is essential for exploiting high-energy resources generated through cross-feeding.

As key drivers of nitrification, ammonia-oxidizing archaea (AOA) play a central role in the global nitrogen cycle and contribute significantly to the emissions of the potent greenhouse gas nitrous oxide (N2O). However, the ecological implications of AOA growth as biofilms, remain poorly understood. Since nitrite production can be used to follow cellular activities directly we were able to compare biofilms with planktonic cells of the terrestrial model AOA Nitrososphaera viennensis at ecologically and agriculturally relevant conditions. Biofilms were more resistant across nearly all tested conditions and remained active at lower temperatures, acidic pH, and high ammonium concentrations. Collectively, activities in biofilm help reconcile discrepancies between earlier laboratory and environmental observations of soil AOA. Additionally, biofilms showed a high general resilience and lowered sensitivities to nitrification inhibitors. Although in situ biofilms grown in microrespiratory chambers exhibited activity and ammonia affinity similar to planktonic cells, biofilm cultures produced only half as much N2O. The enhanced fitness of biofilms across all tested conditions vastly expands the potential ecophysiological niche of AOA and supports the hypothesis that biofilm growth represents the in situ phenotype of AOA in soil environments.

Fungal hyphae form spatially confined interfaces in soil that mediate close associations with bacteria, collectively referred to as the hyphosphere. Despite its recognized ecological importance, experimental access to hyphosphere-associated microbial communities under realistic soil and plant-associated conditions has remained limited. Here, we present a soil-mimetic microcosm that enables controlled reconstruction and recovery of hyphosphere bacterial communities embedded within plant-associated soil. The system integrates field-derived soil, a native soil microbial inoculum, living cotton seedlings, and a spatially constrained fungal inoculum housed within sterile cell-strainer assemblies, permitting hyphal extension into soil while preserving a recoverable fungal-soil boundary. Using the soil-borne plant pathogen Fusarium oxysporum f. sp. vasinfectum as a model filamentous fungus, we show that the microcosm enables reproducible recovery of hypha-associated soil microaggregates containing physically attached bacterial cells. Full-length 16S rRNA profiling revealed pronounced reductions in bacterial richness and evenness in hyphosphere samples relative to bulk and rhizosphere soils, consistent with recruitment of a restricted subset of the surrounding microbiota. Ordination analyses demonstrated clear compositional separation between soil and hyphosphere compartments, with convergence of hypha-associated communities across bulk and rhizosphere contexts. Phylogenetic turnover analyses indicated phylogenetic structuring, whereas taxonomic analyses identified a conserved set of bacterial genera consistently associated with hyphae alongside compartment-specific taxa influenced by soil and plant context. Together, these findings establish the novel hyphal release-and-capture microcosm as a reproducible, ecologically grounded platform for studying hyphosphere-associated bacterial communities in plant-associated soils.

Protistan predators are key regulators of microbial food webs, yet most are considered to occupy relatively narrow trophic niches. Here, we demonstrate that Mayorella spp. (Amoebozoa), isolated from marine and freshwater environments, exhibits exceptional trophic breadth spanning multiple trophic levels. Live-cell imaging revealed predation on bacteria, algae, dinoflagellates, diatoms, flagellates, ciliates, and multicellular prey including rotifers. Large or filamentous prey were engulfed whole or mechanically fragmented during ingestion. Notably, Mayorella consumed both trophozoites and cysts of free-living amoebae (Naegleria and Acanthamoeba), with clear digestion of cyst contents. Dense cultures showed aggregation around large prey and facultative cannibalism. Ingestion of microplastic-like particles occurred without evidence of digestion. Predator cell size and population density increased markedly when feeding on protist or mixed prey relative to bacterial diets, indicating pronounced trophic plasticity. These findings establish Mayorella as a broad-spectrum, cross-trophic predator with the capacity to exert top-down effects across microbial food webs and suggest a previously underappreciated role in the suppression of pathogenic free-living amoebae.

1Consortia of archaea and partner bacteria couple the anaerobic oxidation of alkanes to sulfate reduction. While catabolic pathways in anaerobic alkane-oxidizing archaea (ANKA) are increasingly understood, their anabolic capacities remain poorly characterized. Here, we examined nine enrichment cultures dominated by ANKA and their partner bacteria for small-molecular compounds using solvent extraction and gas chromatographic analysis of derivatized extracts. All hydrocarbon-degrading cultures contained substantial amounts of disaccharides in their metabolite pools. Cold-adapted methane-oxidizing cultures dominated by ANME-2c and Seep-SRB2 contained up to 1.5 mg of trehalose per mg soluble protein. Trehalose was also abundant in ethane-oxidizing cultures of Candidatus Ethanoperedens and its distinct partner SRBs, accounting for up to 75 % of the extracted metabolites. In contrast, thermophilic ANKA cultures dominated by ANME-1 or Ca. Syntropharchaeum and Ca. Desulfofervidus contained an abundant as-yet-unidentified glucose-containing disaccharide. Metagenomic analysis revealed widespread trehalose metabolism genes among partner Desulfobacterota and in ANME-2c and Ca. Ethanoperedens, but a lower potential in ANME-1 and Syntropharchaeum, consistent with metabolite profiles. If exogenous trehalose was added to the Ethane50 culture, we observed rapid metabolization by heterotrophic microorganisms, but poor assimilation by the Ca. Ethanoperedens/ Ca. Desulfofervidus core community, indicating that ANKA/SRB consortia do not consume externally supplied trehalose. Instead, Ca. Ethanoperedens/ Ca. Desulfofervidus, as well as other ANKA/SRB consortia, may use the disaccharides as energy-storage molecules, osmolytes, or components of the extracellular matrix. Notably, the disaccharides produced by the consortia also sustain ancillary heterotrophs, thereby linking alkane oxidation to broader sedimentary carbon cycling.

Understanding the kinetic processes that govern bacterial population dynamics within hosts is critical in developing effective strategies to control microbiota. However, inferring population dynamics is challenging due to large host-to-host bacterial population variability stemming from stochastic colonization events, as well as the inability to continuously monitor the bacterial population without disturbing the host. Using C. elegans fed E. coli under different diets, we show that early colonization acts as a stochastic bottleneck that drives substantial divergence in host-level bacterial loads, and that the spreading of bacteria from colonized worms to sterile ones regulates this variability by altering effective colonization pressure. These conclusions are drawn using a simulation-based inference framework that quantifies stochastic within-host population dynamics from discrete snapshot data, enabling inference of effective colonization and growth rates across heterogeneous hosts with variable carrying capacities. Applying this framework, we further demonstrate that the bacterial predator B. bacteriovorus reduces average gut bacterial loads by two orders of magnitude, primarily by suppressing environmental recolonization and subsequent host-to-host transmission rather than eliminating established intra-host populations. Together, these results reveal that host-associated microbial population dynamics are strongly impacted by environmental colonization processes that modulate stochastic entry events.