Competition and niche separation of pelagic bacteria in freshwater habitats

Characterization of bacterial community dynamics dominated by salinity in lakes of the Inner Mongolian Plateau, China

Microorganisms in lakes are sensitive to salinity fluctuations. Despite extensive prior research on bacterial communities, our understanding of their characteristics and assembly mechanisms in lakes, especially in desert lakes with different salinities. To address this issue, we collected three samples from freshwater lakes, six from brackish lakes, and five from salt lakes in the Badanjilin Desert. The 16S rRNA gene sequencing was applied to investigate the bacterial interactions with rising salinity, community coexistence patterns, and assembly mechanisms. Our findings suggested that the increased lake salinity significantly reduces the bacterial community diversity and enhanced the community differentiation. Significant variations were observed in the contribution of biomarkers from Cyanobacteria, Chloroflexi, and Halobacterota to the composition of the lake bacterial communities. The bacterial communities in the salt lakes exhibited a higher susceptibility to salinity limitations than those in the freshwater and brackish lakes. In addition, the null modeling analyses confirmed the quantitative biases in the stochastic assembly processes of bacterial communities across freshwater, brackish, and saline lakes. With the increasing lake salinity, the significance of undominated and diffusion limitation decreased slightly, and the influence of homogenizing dispersal on community assembly increased. However, the stochasticity remained the dominant process across all lakes in the Badanjilin Desert. The analysis of co-occurring networks revealed that the rising salinity reduced the complexity of bacterial network structures and altered the interspecific interactions, resulting in the increased interspecies collaboration with increasing salinity levels. Under the influence of salinity stress, the key taxon Cyanobacteria in freshwater lakes (Schizothrix_LEGE_07164) was replaced by Proteobacteria (Thalassobaculum and Polycyclovorans) in brackish lakes, and Thermotogota (SC103) in salt lakes. The results indicated the symbiotic patterns of bacterial communities across varying salinity gradients in lakes and offer insights into potential mechanisms of community aggregation, thereby enhancing our understanding of bacterial distribution in response to salinity changes.

Biogeography and dynamics of prokaryotic and microeukaryotic community assembly across 2600 km in the coastal and shelf ecosystems of the China Seas

Marine prokaryotes and microeukaryotes are essential components of microbial food webs, and drive the biogeochemical cycling. However, the underlying ecological mechanisms driving prokaryotic and microeukaryotic community assembly in large-scale coastal ecosystems remain unclear. In this study, we studied biogeographic patterns of prokaryotic and microeukaryotic communities in the coastal and shelf ecosystem of the China Seas. Results showed that prokaryotic richness was the highest in the Yangtze River Plume, whereas microeukaryotic richness decreased from south to north. Prokaryotic-microeukaryotic co-occurrence networks display greater complexity in the Yangtze River Plume compared to other regions, potentially indicating higher environmental heterogeneity. Furthermore, the cross-domain networks revealed that prokaryotes were more interconnected with each other than with microeukaryotes or between microeukaryotes, and all hub nodes were bacterial taxa, suggesting that prokaryotes may be more important for sustaining the stability and multifunctionality of coastal ecosystem than microeukaryotes. Variation Partitioning Analysis revealed that approximately equal proportions of environmental, biotic and spatial factors contribute to variations in microbial community composition. Temperature was the primary environmental driver of both prokaryotic and microeukaryotic communities across the China Seas. Additionally, stochastic processes (dispersal limitation) and deterministic processes (homogeneous selection) were two major ecological factors in shaping microeukaryotic and prokaryotic assemblages, respectively, suggesting their different environmental plasticity and evolutionary mechanisms. Overall, these results demonstrate both prokaryotic and microeukaryotic communities displayed a latitude-driven distribution pattern and different assembly mechanisms, improving our understanding of microbial biogeography patterns under global change and anthropogenic activity driven habitat diversification in the coastal and shelf ecosystem.

Metagenomic insight into taxonomic composition, environmental filtering and functional redundancy for shaping worldwide modern non-lithifying microbial mats

Modern microbial mats are relictual communities mostly found in extreme environments worldwide. Despite their significance as representatives of the ancestral Earth and their important roles in biogeochemical cycling, research on microbial mats has largely been localized, focusing on site-specific descriptions and environmental change experiments. Here, we present a global comparative analysis of non-lithifying microbial mats, integrating environmental measurements with metagenomic data from 62 samples across eight sites, including two new samples from the recently discovered Archaean Domes from Cuatro Ciénegas, Mexico. Our results revealed a notable influence of environmental filtering on both taxonomic and functional compositions of microbial mats. Functional redundancy appears to confer resilience to mats, with essential metabolic pathways conserved across diverse and highly contrasting habitats. We identified six highly correlated clusters of taxa performing similar ecological functions, suggesting niche partitioning and functional specialization as key mechanisms shaping community structure. Our findings provide insights into the ecological principles governing microbial mats, and lay the foundation for future research elucidating the intricate interplay between environmental factors and microbial community dynamics.

Evolutionary consequences of microbiomes for hosts: impacts on host fitness, traits, and heritability

An organism's phenotypes and fitness often depend on the interactive effects of its genome (Gh⁢o⁢s⁢t), microbiome (Gm⁢i⁢c⁢r⁢o⁢b⁢e), and environment (E). These G × G, G × E, and G × G × E effects fundamentally shape host-microbiome (co)evolution and may be widespread, but are rarely compared within a single experiment. We collected and cultured L⁢e⁢m⁢n⁢am⁢i⁢n⁢o⁢r (duckweed) and its associated microbiome from 10 sites across an urban-to-rural ecotone. We factorially manipulated host genotype and microbiome in two environments (low and high zinc, an urban aquatic stressor) in an experiment with 200 treatments: 10 host genotypes × 10 microbiomes × 2 environments. Host genotype explained the most variation in L.m⁢i⁢n⁢o⁢r fitness and traits, while microbiome effects often depended on host genotype (G × G). Microbiome composition predicted G × G effects: when compared in more similar microbiomes, duckweed genotypes had more similar effects on traits. Further, host fitness increased and microbes grew faster when applied microbiomes more closely matched the host's field microbiome, suggesting some local adaptation between hosts and microbiota. Finally, selection on and heritability of host traits shifted across microbiomes and zinc exposure. Thus, we found that microbiomes impact host fitness, trait expression, and heritability, with implications for host-microbiome evolution and microbiome breeding.

Dynamics of carbon substrate competition among heterotrophic microorganisms

Growing evidence suggests that interactions among heterotrophic microorganisms influence the efficiency and rate of organic matter turnover. These interactions are dynamic and shaped by the composition and availability of resources in their surrounding environment. Heterotrophic microorganisms inhabiting marine environments often encounter fluctuations in the quality and quantity of carbon inputs, ranging from simple sugars to large, complex compounds. Here, we experimentally tested how the chemical complexity of carbon substrates affects competition and growth dynamics between two heterotrophic marine isolates. We tracked cell density using species-specific polymerase chain reaction (PCR) assays and measured rates of microbial CO2 production along with associated isotopic signatures (13C and 14C) to quantify the impact of these interactions on organic matter remineralization. The observed cell densities revealed substrate-driven interactions: one species exhibited a competitive advantage and quickly outgrew the other when incubated with a labile compound whereas both species seemed to coexist harmoniously in the presence of more complex organic matter. Rates of CO2 respiration revealed that coincubation of these isolates enhanced organic matter turnover, sometimes by nearly 2-fold, compared to their incubation as mono-cultures. Isotopic signatures of respired CO2 indicated that coincubation resulted in a greater remineralization of macromolecular organic matter. These results demonstrate that simple substrates promote competition whereas high substrate complexity reduces competitiveness and promotes the partitioning of degradative activities into distinct niches, facilitating coordinated utilization of the carbon pool. Taken together, this study yields new insight into how the quality of organic matter plays a pivotal role in determining microbial interactions within marine environments.

A genome catalogue of lake bacterial diversity and its drivers at continental scale

Lakes are heterogeneous ecosystems inhabited by a rich microbiome whose genomic diversity is poorly defined. We present a continental-scale study of metagenomes representing 6.5 million km2 of the most lake-rich landscape on Earth. Analysis of 308 Canadian lakes resulted in a metagenome-assembled genome (MAG) catalogue of 1,008 mostly novel bacterial genomospecies. Lake trophic state was a leading driver of taxonomic and functional diversity among MAG assemblages, reflecting the responses of communities profiled by 16S rRNA amplicons and gene-centric metagenomics. Coupling the MAG catalogue with watershed geomatics revealed terrestrial influences of soils and land use on assemblages. Agriculture and human population density were drivers of turnover, indicating detectable anthropogenic imprints on lake bacteria at the continental scale. The sensitivity of bacterial assemblages to human impact reinforces lakes as sentinels of environmental change. Overall, the LakePulse MAG catalogue greatly expands the freshwater genomic landscape, advancing an integrative view of diversity across Earth's microbiomes.

May microbial ecological baseline exist in continental groundwater?

BACKGROUND

Microbes constitute almost the entire biological community in subsurface groundwater and play an important role in ecological evolution and global biogeochemical cycles. Ecological baseline as a fundamental reference with less human interference has been investigated in surface ecosystems such as soils, rivers, and ocean, but the existence of groundwater microbial ecological baseline (GMEB) is still an open question so far.

RESULTS

Based on high-throughput sequencing information derived from national monitoring of 733 newly constructed wells, we find that bacterial communities in pristine groundwater exhibit a significant lateral diversity gradient and gradually approach the topsoil microbial latitudinal diversity gradient with decreasing burial depth of phreatic water. Among 74 phyla dominated by Proteobacteria in groundwater, Patescibacteria act as keystone taxa that harmonize microbes in shallower aquifers and accelerate decline in bacterial diversity with increasing well-depth. Decreasing habitat niche breadth with increasing well-depth suggests a general change in the relationship among key microbes from closer cooperation in shallow to stronger competition in deep groundwater. Unlike surface-water microbes, microbial communities in pristine groundwater are predominantly shaped by deterministic processes, potentially associated with nutrient sequestration under dark and anoxic environments in aquifers.

CONCLUSIONS

By unveiling the biogeographic patterns and mechanisms controlling the community assembly of microbes in pristine groundwater throughout China, we firstly confirm the existence of GMEB in shallower aquifers and propose Groundwater Microbial Community Index (GMCI) to evaluate anthropogenic impact, which highlights the importance of GMEB in groundwater water security and health diagnosis. Video Abstract.

Presence of an ultra-small microbiome in fermented cabbages

Background

Ultramicrobacteria (UMB), also known as ultra-small bacteria, are tiny bacteria with a size less than 0.1 µm³. They have a high surface-to-volume ratio and are found in various ecosystems, including the human body. UMB can be classified into two types: one formed through cell contraction and the other that maintains a small size. The ultra-small microbiome (USM), which may contain UMB, includes all bacteria less than 0.2 µm in size and is difficult to detect with current methods. However, it poses a potential threat to food hygiene, as it can pass through sterilization filters and exist in a viable but non-culturable (VBNC) state. The data on the USM of foods is limited. Some bacteria, including pathogenic species, are capable of forming UMB under harsh conditions, making it difficult to detect them through conventional culture techniques.

Methods

The study described above focused on exploring the diversity of USM in fermented cabbage samples from three different countries (South Korea, China, and Germany). The samples of fermented cabbage (kimchi, suancai, and sauerkraut) were purchased and stored in chilled conditions at approximately 4 °C until filtration. The filtration process involved two steps of tangential flow filtration (TFF) using TFF cartridges with different pore sizes (0.2 µm and 100 kDa) to separate normal size bacteria (NM) and USM. The USM and NM isolated via TFF were stored in a refrigerator at 4 °C until DNA extraction. The extracted DNA was then amplified using PCR and the full-length 16S rRNA gene was sequenced using single-molecule-real-time (SMRT) sequencing. The transmission electron microscope (TEM) was used to confirm the presence of microorganisms in the USM of fermented cabbage samples.

Results

To the best of our knowledge, this is the first study to identify the differences between USM and NM in fermented cabbages. Although the size of the USM (average 2,171,621 bp) was smaller than that of the NM (average 15,727,282 bp), diversity in USM (average H' = 1.32) was not lower than that in NM (average H' = 1.22). In addition, some members in USM probably underwent cell shrinkage due to unfavorable environments, while others maintained their size. Major pathogens were not detected in the USM in fermented cabbages. Nevertheless, several potentially suspicious strains (genera Cellulomonas and Ralstonia) were detected. Our method can be used to screen food materials for the presence of USM undetectable via conventional methods. USM and NM were efficiently separated using tangential flow filtration and analyzed via single-molecule real-time sequencing. The USM of fermented vegetables exhibited differences in size, diversity, and composition compared with the conventional microbiome. This study could provide new insights into the ultra-small ecosystem in fermented foods, including fermented cabbages.

Impact of nutrients and trace elements on freshwater microbial communities in Croatia: identifying bacterial bioindicator taxa

Since aquatic microbial communities promptly respond to environmental changes, it is now evident that they can complement traditional taxa such as fish, macroinvertebrates and algae as bioindicators of water quality. The aim of this study was to correlate the physico-chemical parameters of water with the microbial community structure and the occurrence of putative bioindicator taxa. Thirty-five water samples were collected throughout Croatia and their physico-chemical parameters, including the concentration of trace elements using the high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS), and the composition of the microbial communities by high-throughput sequencing of the 16S rRNA marker gene, were analysed in parallel. Partial least squares regression (PLS-R) modelling revealed that a number of microbial taxa were positively correlated with some of the water parameters. For example, some taxa from the phylum Proteobacteria were positively correlated with the ion content of the water (e.g. Erythrobacter, Rhodobacteraceae, Alteromonadaceae), while some Firmicutes taxa, such as the well-known faecal indicators Enterococcus and Clostridium, were correlated with nutrient content (ammonium and total phosphorus). Among the trace elements, uranium was positively correlated with a highest number of microbial taxa. The results obtained will aid in development of protocols for eDNA-based biological assessment of water quality.

From the Sunlit to the Aphotic Zone: Assembly Mechanisms and Co-Occurrence Patterns of Protistan-Bacterial Microbiotas in the Western Pacific Ocean

We know little about the assembly processes and association patterns of microbial communities below the photic zone. In marine pelagic systems, there are insufficient observational data regarding why and how the microbial assemblies and associations vary from photic to aphotic zones. In this study, we investigated size-fractionated oceanic microbiotas, specifically free-living (FL; 0.22 to 3 μm) and particle-associated (PA; >3 μm) bacteria and protists (0.22 to 200 μm) collected from the surface to 2,000 m in the western Pacific Ocean, to see how assembly mechanisms and association patterns changed from photic to aphotic zones. Taxonomic analysis revealed a distinct community composition between photic and aphotic zones that was largely driven by biotic associations rather than abiotic factors. Aphotic community co-occurrence was less widespread and robust than its photic counterparts, and biotic associations were crucial in microbial co-occurrence, having a higher influence on photic than aphotic co-occurrences. The decrease in biotic associations and the increase in dispersal limitation from the photic to the aphotic zone affect the deterministic-stochastic balance, leading to a more stochastic-process-driven community assembly for all three microbial groups in the aphotic zone. Our findings significantly contribute to our understanding of how and why microbial assembly and co-occurrence vary from photic to aphotic zones, offering insight into the dynamics of the protistan-bacterial microbiota in the western Pacific's photic and aphotic zones. IMPORTANCE We know little about the assembly processes and association patterns of microbial communities below the photic zone in marine pelagic systems. We discovered that community assembly processes differed between photic and aphotic zones, with all three microbial groups studied (protists and FL and PA bacteria) being more influenced by stochastic processes than in the photic zone. The decrease in organismic associations and the increase in dispersal limitation from the photic to the aphotic zone both have an impact on the deterministic-stochastic balance, resulting in a more stochastic process-driven community assembly for all three microbial groups in the aphotic zone. Our findings significantly contribute to the understanding of how and why microbial assembly and co-occurrence change between photic and aphotic zones, offering insight into the dynamics of the protist-bacteria microbiota in the western Pacific oceans.

Bacterioplankton seasonality in deep high-mountain lakes

Due to global warming, shorter ice cover duration might drastically affect the ecology of lakes currently undergoing seasonal surface freezing. High-mountain lakes show snow-rich ice covers that determine contrasting conditions between ice-off and ice-on periods. We characterized the bacterioplankton seasonality in a deep high-mountain lake ice-covered for half a year. The lake shows a rich core bacterioplankton community consisting of three components: (i) an assemblage stable throughout the year, dominated by Actinobacteria, resistant to all environmental conditions; (ii) an ice-on-resilient assemblage dominating during the ice-covered period, which is more diverse than the other components and includes a high abundance of Verrucomicrobia; the deep hypolimnion constitutes a refuge for many of the typical under-ice taxa, many of which recover quickly during autumn mixing; and (iii) an ice-off-resilient assemblage, which members peak in summer in epilimnetic waters when the rest decline, characterized by a dominance of Flavobacterium, and Limnohabitans. The rich core community and low random elements compared to other relatively small cold lakes can be attributed to its simple hydrological network in a poorly-vegetated catchment, the long water-residence time (ca. 4 years), and the long ice-cover duration; features common to many headwater deep high-mountain lakes.

Seasonal bacterial niche structures and chemolithoautotrophic ecotypes in a North Atlantic fjord

Quantifying the temporal change of bacterial communities is essential to understanding how both natural and anthropogenic pressures impact the functions of coastal marine ecosystems. Here we use weekly microbial DNA sampling across four years to show that bacterial phyla have distinct seasonal niches, with a richness peak in winter (i.e., an inverse relationship with daylength). Our results suggest that seasonal fluctuations, rather than the kinetic energy or resource hypotheses, dominated the pattern of bacterial diversity. These findings supplement those from global analyses which lack temporal replication and present few data from winter months in polar and temperate regions. Centered log-ratio transformed data provided new insights into the seasonal niche partitioning of conditionally rare phyla, such as Modulibacteria, Verrucomicrobiota, Synergistota, Deinococcota, and Fermentibacterota. These patterns could not be identified using the standard practice of ASV generation followed by rarefaction. Our study provides evidence that five globally relevant ecotypes of chemolithoautotrophic bacteria from the SUP05 lineage comprise a significant functional group with varying seasonal dominance patterns in the Bedford Basin.

Genome Analysis of Enterobacter asburiae and Lelliottia spp. Proliferating in Oligotrophic Drinking Water Reservoirs and Lakes

Surface waters are one of the main sources for drinking water production, and thus microbial contamination should be as minimal as possible. However, high concentrations of coliform bacteria were detected in reservoirs and lakes used for drinking water production during summer months due to autochthonous proliferation processes. Here, we present the genomic analyses of 17 strains of Enterobacter asburiae and Lelliottia spp. proliferating in reservoirs and lakes with special focus on the hygienic relevance, antibiotic resistance, and adaptations to the oligotrophic environments. The genomes contain neither genes for the type III secretion system nor cytotoxins or hemolysins, which are considered typical virulence factors. Examination of antibiotic resistance genes revealed mainly efflux pumps and β-lactamase class C (ampC) genes. Phenotypically, single isolates of Enterobacter asburiae showed resistance to fosfomycin and ceftazidime. The genome analyses further suggest adaptations to oligotrophic and changing environmental conditions in reservoirs and lakes, e.g., genes to cope with low nitrate and phosphate levels and the ability to utilize substances released by algae, like amino acids, chitin, alginate, rhamnose, and fucose. This leads to the hypothesis that the proliferation of the coliform bacteria could occur at the end of summer due to algae die-off. IMPORTANCE Certain strains of coliform bacteria have been shown to proliferate in the oligotrophic water of drinking water reservoirs and lakes, reaching values above 10⁴ per 100 mL. Such high concentrations challenge drinking water treatment, and occasionally the respective coliform bacteria have been detected in the treated drinking water. Thus, the question of their hygienic relevance is of high importance for water suppliers and authorities. Our genomic analyses suggest that the strains are not hygienically relevant, as typical virulence factors are absent and antibiotic resistance genes in the genomes most likely are of natural origin. Furthermore, their presence in the water is not related to fecal contamination. The proliferation in reservoirs and lakes during stable summer stratification is an autochthonic process of certain E. asburiae and Lelliottia strains that are well adapted to the surrounding oligotrophic environment.

Water masses and their associated temperature and cross-domain biotic factors co-shape upwelling microbial communities

Disentangling the drivers and mechanisms that shape microbial communities in a river-influenced coastal upwelling system requires considering a hydrologic dimension that can drive both deterministic and stochastic community assembly by generating hydrological heterogeneity and dispersal events. Additionally, ubiquitous and complex microbial interactions can play a significant role in community structuring. However, how the hydrology, biotic, and abiotic factors collectively shape microbial distribution in the hydrologically complicated river plume-upwelling coupling system remains unknown. Through underway sampling and daily observations, we employed 16S and 18S ribosomal RNA sequencing to disentangle drivers and mechanisms shaping the protist-bacteria microbiota in a river-influenced coastal upwelling system. Our findings indicate that the composition of microbial communities was water mass specific. Collectively, water mass, local water chemistry (mostly temperature) and biotic interaction (mostly cross-domain biotic interaction) shaped the protistan-bacterial communities. In comparison to protists, bacteria were more influenced by abiotic factors such as temperature than by cross-domain biotic factors, implying a stronger coupling of geochemical cycles. Both deterministic and stochastic processes had an effect on the distribution of microbial communities, but deterministic processes were more important for bacteria and were especially pronounced for upwelling communities. The co-occurrence network revealed strong associations between the protistan assemblages Orchrophyta and Ciliophora and the bacterial assemblages Alphaproteobacteria, Deltaproteobacteria, and Bacteroidetes, which may reflect predation and mutualism interactions. Overall, this study emphasizes the importance of taking water masses, temperature and domains of life into account when seeking to understand the drivers and assemblies of protist-bacteria microbiome dynamics in coastal upwelling systems, which is especially true given the complex and dynamic nature of the coastal ecosystem.

Effects of bioturbation by tubificid worms on biogeochemical processes, bacterial community structure and diversity in heterotrophic wetland sediments

Bioturbation activity of tubificid worms has been recognized as a key process influencing organic matter processing and nutrient cycling in benthic aquatic ecosystems. This activity is expected to modify benthic microbial communities by affecting the physical and chemical environment in sediments. Nevertheless, quantifications of bacterial community changes associated with bioturbation in freshwater ecosystems are still lacking. The present study aimed at evaluating the impact of tubificid worms on bacterial community structure using NGS approach (16S metabarcoding) and long (6 months) laboratory experiments on four heterotrophic wetland sediments. Worm bioturbation activity significantly stimulated biogeochemical processes at the water-sediment interface but only had a marginally significant effect on bacterial community structures. Yet, bacterial diversity was consistently reduced in presence of worms. Such decrease could be associated with the stimulation of organic matter mineralization by worms, leading to a reduction of the diversity of trophic niches available for bacterial species. The slight changes in bacterial community structures induced by bioturbation did not appear to control biogeochemical processes. Thus, the stimulation of biogeochemical processes by worm bioturbation was more associated with a stimulation of the initial bacterial community than with a drastic change in bacterial communities induced by worms.

Spatial and Temporal Distribution of Bacterioplankton Molecular Ecological Networks in the Yuan River under Different Human Activity Intensity

Bacterioplankton communities play a crucial role in freshwater ecosystem functioning, but it is unknown how co-occurrence networks within these communities respond to human activity disturbances. This represents an important knowledge gap because changes in microbial networks could have implications for their functionality and vulnerability to future disturbances. Here, we compare the spatiotemporal and biogeographical patterns of bacterioplankton molecular ecological networks using high-throughput sequencing of Illumina HiSeq and multivariate statistical analyses from a subtropical river during wet and dry seasons. Results demonstrated that the lower reaches (high human activity intensity) network had less of an average degree (10.568/18.363), especially during the dry season, when compared with the upper reaches (low human activity intensity) network (10.685/37.552) during the wet and dry seasons, respectively. The latter formed more complexity networks with more modularity (0.622/0.556) than the lower reaches (high human activity intensity) network (0.505/0.41) during the wet and dry seasons, respectively. Bacterioplankton molecular ecological network under high human activity intensity became significantly less robust, which is mainly caused by altering of the environmental conditions and keystone species. Human activity altered the composition of modules but preserved their ecological roles in the network and environmental factors (dissolved organic carbon, temperature, arsenic, oxidation-reduction potential and Chao1 index) were the best parameters for explaining the variations in bacterioplankton molecular ecological network structure and modules. Proteobacteria, Actinobacteria and Bacteroidetes were the keystone phylum in shaping the structure and niche differentiations in the network. In addition, the lower reaches (high human activity intensity) reduce the bacterioplankton diversity and ecological niche differentiation, which deterministic processes become more important with increased farmland and constructed land area (especially farmland) with only 35% and 40% of the community variation explained by the neutral community model during the wet season and dry season, respectively. Keystone species in high human activity intensity stress habitats yield intense functional potentials and Bacterioplankton communities harbor keystone taxa in different human activity intensity stress habitats, which may exert their influence on microbiome network composition regardless of abundance. Therefore, human activity plays a crucial role in shaping the structure and function of bacterioplankton molecular ecological networks in subtropical rivers and understanding the mechanisms of this process can provide important information about human-water interaction processes, sustainable uses of freshwater as well as watershed management and conservation.

Effect of Water Chemistry, Land Use Patterns, and Geographic Distances on the Spatial Distribution of Bacterioplankton Communities in an Anthropogenically Disturbed Riverine Ecosystem

The spatial distribution of bacterioplankton communities in rivers is driven by multiple environmental factors, including local and regional factors. Local environmental condition is associated with effect of river water chemistry (through species sorting); ecological process in region is associated with effects of land use and geography. Here, we investigated variation in bacterioplankton communities (free-living, between 0.22 and 5 μm) in an anthropogenically disturbed river using high-throughput DNA sequencing of community 16S rRNA genes in order to investigate the importance of water chemistry, land use patterns, and geographic distance. Among environmental factors, sulfate (SO₄ ^2-), manganese (Mn), and iron (Fe) concentrations were the water chemistry parameters that best explained bacterioplankton community variation. In addition, forest and freshwater areas were the land use patterns that best explained bacterioplankton community variation. Furthermore, cumulative dendritic distance was the geographic distance parameter that best explained bacterial community variation. Variation partitioning analysis revealed that water chemistry, land use patterns, and geographic distances strongly shaped bacterioplankton communities. In particular, the direct influence of land use was prominent, which alone contributed to the highest proportion of variation (26.2% in wet season communities and 36.5% in dry season communities). These results suggest that the mechanisms of species sorting and mass effects together control bacterioplankton communities, although mass effects exhibited higher contributions to community variation than species sorting. Given the importance of allochthonous bacteria input from various land use activities (i.e., mass effects), these results provide new insights into the environmental factors and determinant mechanisms that shape riverine ecosystem communities.

High biodiversity in a benzene-degrading nitrate-reducing culture is sustained by a few primary consumers

A key question in microbial ecology is what the driving forces behind the persistence of large biodiversity in natural environments are. We studied a microbial community with more than 100 different types of species which evolved in a 15-years old bioreactor with benzene as the main carbon and energy source and nitrate as the electron acceptor. Using genome-centric metagenomics plus metatranscriptomics, we demonstrate that most of the community members likely feed on metabolic left-overs or on necromass while only a few of them, from families Rhodocyclaceae and Peptococcaceae, are candidates to degrade benzene. We verify with an additional succession experiment using metabolomics and metabarcoding that these few community members are the actual drivers of benzene degradation. As such, we hypothesize that high species richness is maintained and the complexity of a natural community is stabilized in a controlled environment by the interdependencies between the few benzene degraders and the rest of the community members, ultimately resulting in a food web with different trophic levels.

Expanding ecological assessment by integrating microorganisms into routine freshwater biomonitoring

Bioindication has become an indispensable part of water quality monitoring in most countries of the world, with the presence and abundance of bioindicator taxa, mostly multicellular eukaryotes, used for biotic indices. In contrast, microbes (bacteria, archaea and protists) are seldom used as bioindicators in routine assessments, although they have been recognized for their importance in environmental processes. Recently, the use of molecular methods has revealed unexpected diversity within known functional groups and novel metabolic pathways that are particularly important in energy and nutrient cycling. In various habitats, microbial communities respond to eutrophication, metals, and natural or anthropogenic organic pollutants through changes in diversity and function. In this review, we evaluated the common trends in these changes, documenting that they have value as bioindicators and can be used not only for monitoring but also for improving our understanding of the major processes in lotic and lentic environments. Current knowledge provides a solid foundation for exploiting microbial taxa, community structures and diversity, as well as functional genes, in novel monitoring programs. These microbial community measures can also be combined into biotic indices, improving the resolution of individual bioindicators. Here, we assess particular molecular approaches complemented by advanced bioinformatic analysis, as these are the most promising with respect to detailed bioindication value. We conclude that microbial community dynamics are a missing link important for our understanding of rapid changes in the structure and function of aquatic ecosystems, and should be addressed in the future environmental monitoring of freshwater ecosystems.

From pine to pasture: land use history has long-term impacts on soil bacterial community composition and functional potential

Bacterial communities are crucial to soil ecosystems and are known to be sensitive to environmental changes. However, our understanding of how present-day soil bacterial communities remain impacted by historic land uses is limited; implications for their functional potential are especially understudied. Through 16S rRNA gene amplicon and shotgun metagenomic sequencing, we characterized the structure and functional potential of soil bacterial communities after land use conversion. Sites converted from pine plantations to dairy pasture were sampled five- and eight-years post conversion. The bacterial community composition and functional potential at these sites were compared to long-term dairy pastures and pine forest reference sites. Bacterial community composition and functional potential at the converted sites differed significantly from those at reference sites (P = 0.001). On average, they were more similar to those in the long-term dairy sites and showed gradual convergence (P = 0.001). Differences in composition and functional potential were most strongly related to nutrients such as nitrogen, Olsen P and the carbon to nitrogen ratio. Genes related to the cycling of nitrogen, especially denitrification, were underrepresented in converted sites compared to long-term pasture soils. Together, our study highlights the long-lasting impacts land use conversion can have on microbial communities, and the implications for future soil health and functioning.

Sediment Metagenomes as Time Capsules of Lake Microbiomes

The reconstruction of ecological time series from lake sediment archives can retrace the environmental impact of human activities. Molecular genetic approaches in paleolimnology have provided unprecedented access to DNA time series, which record evidence of the microbial ecologies that underlaid historical lake ecosystems. Such studies often rely on single-gene surveys, and consequently, the full diversity of preserved microorganisms remains unexplored. In this study, we probed the diversity archived in contemporary and preindustrial sediments by comparative shotgun metagenomic analysis of surface water and sediment samples from three eastern Canadian lakes. In a strategy that was aimed at disentangling historical DNA from the indigenous sediment background, microbial preservation signals were captured by mapping sequence similarities between sediment metagenome reads and reference surface water metagenome assemblies. We detected preserved Cyanobacteria, diverse bacterioplankton, microeukaryotes, and viruses in sediment metagenomes. Among the preserved microorganisms were important groups never before reported in paleolimnological reconstructions, including bacteriophages (Caudovirales) and ubiquitous freshwater Betaproteobacteria (Polynucleobacter and Limnohabitans). In contrast, ultramicroscopic Actinobacteria ("Candidatus Nanopelagicales") and Alphaproteobacteria (Pelagibacterales) were apparently not well preserved in sediment metagenomes even though they were numerically dominant in surface water metagenomes. Overall, our study explored a novel application of whole-metagenome shotgun sequencing for discovering the DNA remains of a broad diversity of microorganisms preserved in lake sediments. The recovery of diverse microbial time series supports the taxonomic expansion of microbiome reconstructions and the development of novel microbial paleoindicators.IMPORTANCE Lakes are critical freshwater resources under mounting pressure from climate change and other anthropogenic stressors. The reconstruction of ecological time series from sediment archives with paleolimnological techniques has been shown to be an effective means of understanding how humans are modifying lake ecosystems over extended timescales. In this study, we combined shotgun DNA sequencing with a novel comparative analysis of surface water and sediment metagenomes to expose the diversity of microorganisms preserved in lake sediments. The detection of DNA from a broad diversity of preserved microbes serves to more fully reconstruct historical microbiomes and describe preimpact lake conditions.

New insights into the pelagic microorganisms involved in the methane cycle in the meromictic Lake Pavin through metagenomics

Advances in metagenomics have given rise to the possibility of obtaining genome sequences from uncultured microorganisms, even for those poorly represented in the microbial community, thereby providing an important means to study their ecology and evolution. In this study, metagenomic sequencing was carried out at four sampling depths having different oxygen concentrations or environmental conditions in the water column of Lake Pavin. By analyzing the sequenced reads and matching the contigs to the proxy genomes of the closest cultivated relatives, we evaluated the metabolic potential of the dominant planktonic species involved in the methane cycle. We demonstrated that methane-producing communities were dominated by the genus Methanoregula while methane-consuming communities were dominated by the genus Methylobacter, thus confirming prior observations. Our work allowed the reconstruction of a draft of their core metabolic pathways. Hydrogenotrophs, the genes required for acetate activation in the methanogen genome, were also detected. Regarding methanotrophy, Methylobacter was present in the same areas as the non-methanotrophic, methylotrophic Methylotenera, which could suggest a relationship between these two groups. Furthermore, the presence of a large gene inventory for nitrogen metabolism (nitrate transport, denitrification, nitrite assimilation and nitrogen fixation, for instance) was detected in the Methylobacter genome.

Temperature and Nutrient Levels Correspond with Lineage-Specific Microdiversification in the Ubiquitous and Abundant Freshwater Genus Limnohabitans

Most freshwater bacterial communities are characterized by a few dominant taxa that are often ubiquitous across freshwater biomes worldwide. Our understanding of the genomic diversity within these taxonomic groups is limited to a subset of taxa. Here, we investigated the genomic diversity that enables Limnohabitans, a freshwater genus key in funneling carbon from primary producers to higher trophic levels, to achieve abundance and ubiquity. We reconstructed eight putative Limnohabitans metagenome-assembled genomes (MAGs) from stations located along broad environmental gradients existing in Lake Michigan, part of Earth's largest surface freshwater system. De novo strain inference analysis resolved a total of 23 strains from these MAGs, which strongly partitioned into two habitat-specific clusters with cooccurring strains from different lineages. The largest number of strains belonged to the abundant LimB lineage, for which robust in situ strain delineation had not previously been achieved. Our data show that temperature and nutrient levels may be important environmental parameters associated with microdiversification within the Limnohabitans genus. In addition, strains predominant in low- and high-phosphorus conditions had larger genomic divergence than strains abundant under different temperatures. Comparative genomics and gene expression analysis yielded evidence for the ability of LimB populations to exhibit cellular motility and chemotaxis, a phenotype not yet associated with available Limnohabitans isolates. Our findings broaden historical marker gene-based surveys of Limnohabitans microdiversification and provide in situ evidence of genome diversity and its functional implications across freshwater gradients.IMPORTANCE Limnohabitans is an important bacterial taxonomic group for cycling carbon in freshwater ecosystems worldwide. Here, we examined the genomic diversity of different Limnohabitans lineages. We focused on the LimB lineage of this genus, which is globally distributed and often abundant, and its abundance has shown to be largely invariant to environmental change. Our data show that the LimB lineage is actually comprised of multiple cooccurring populations for which the composition and genomic characteristics are associated with variations in temperature and nutrient levels. The gene expression profiles of this lineage suggest the importance of chemotaxis and motility, traits that had not yet been associated with the Limnohabitans genus, in adapting to environmental conditions.

Factors shaping community patterns of protists and bacteria on a European scale

Factors shaping community patterns of microorganisms are controversially discussed. Physical and chemical factors certainly limit the survival of individual taxa and maintenance of diversity. In recent years, a contribution of geographic distance and dispersal barriers to distribution patterns of protists and bacteria has been demonstrated. Organismic interactions such as competition, predation and mutualism further modify community structure and maintenance of distinct taxa. Here, we address the relative importance of these different factors in shaping protists and bacterial communities on a European scale using high-throughput sequencing data obtained from lentic freshwater ecosystems. We show that community patterns of protists are similar to those of bacteria. Our results indicate that cross-domain organismic factors are important variables with a higher influence on protists as compared with bacteria. Abiotic physical and chemical factors also contributed significantly to community patterns. The contribution of these latter factors was higher for bacteria, which may reflect a stronger biogeochemical coupling. The contribution of geographical distance was similar for both microbial groups.

Microdiversity ensures the maintenance of functional microbial communities under changing environmental conditions

Microdiversity can lead to different ecotypes within the same species. These are assumed to provide stability in time and space to those species. However, the role of microdiversity in the stability of whole microbial communities remains underexplored. Understanding the drivers of microbial community stability is necessary to predict community response to future disturbances. Here, we analyzed 16S rRNA gene amplicons from eight different temperate bog lakes at the 97% OTU and amplicon sequence variant (ASV) levels and found ecotypes within the same OTU with different distribution patterns in space and time. We observed that these ecotypes are adapted to different values of environmental factors such as water temperature and oxygen concentration. Our results showed that the existence of several ASVs within a OTU favored its persistence across changing environmental conditions. We propose that microdiversity aids the stability of microbial communities in the face of fluctuations in environmental factors.

Bacterial community composition shaped by water chemistry and geographic distance in an anthropogenically disturbed river

'Core bacterial communities', bacterial species that are found consistently throughout a river continuum, have previously been identified. However, variations in core and non-core bacterial community structure, as well as the relationships between these communities and water chemistry or geographic distance have not been well studied. Here, we sampled in the entire course of the Le'an River, China, and explored the bacterial community composition at each site using Illumina high-throughput sequencing. The proportion of sequence reads assigned to the core community was ~95% in the upper and middle reaches, gradually decreasing below 90% in the lower reaches. Both the Chao1 richness index and the Shannon diversity index of the bacterial communities were significantly higher in the wet season than in the dry season, and both indices increased slightly from upstream to downstream. The variation in the non-core community was more aggregated from upstream to downstream in the wet season than in the dry season, while the aggregation of the core community was similar between the dry season and the wet season. The proportion of typical freshwater bacterial was significantly higher in the core community than in the non-core community. NO₃^--N was the subset of water chemistry parameters that best explained bacterial community dissimilarities, while 'river length' was the subset of geographic distance parameters that best explained bacterial community dissimilarities. Water chemistry parameters explained more of the variations in the bacterial communities than did geographic distance, especially in the dry season. However, the correlation between water chemistry and bacteria was primarily due to collective allochthonous input (mass effects), not because of any nutritious or toxic effects on bacterial growth competition (species sorting). The greater influence of the mass effects, as compared to species sorting, on bacterial community structure was due to the allochthonous input of bacteria from anthropogenic sources.

Nano-Sized and Filterable Bacteria and Archaea: Biodiversity and Function

Nano-sized and filterable microorganisms are thought to represent the smallest living organisms on earth and are characterized by their small size (50-400 nm) and their ability to physically pass through <0.45 μm pore size filters. They appear to be ubiquitous in the biosphere and are present at high abundance across a diverse range of habitats including oceans, rivers, soils, and subterranean bedrock. Small-sized organisms are detected by culture-independent and culture-dependent approaches, with most remaining uncultured and uncharacterized at both metabolic and taxonomic levels. Consequently, their significance in ecological roles remain largely unknown. Successful isolation, however, has been achieved for some species (e.g., Nanoarchaeum equitans and "Candidatus Pelagibacter ubique"). In many instances, small-sized organisms exhibit a significant genome reduction and loss of essential metabolic pathways required for a free-living lifestyle, making their survival reliant on other microbial community members. In these cases, the nano-sized prokaryotes can only be co-cultured with their 'hosts.' This paper analyses the recent data on small-sized microorganisms in the context of their taxonomic diversity and potential functions in the environment.

Environmental determinants of pyoverdine production, exploitation and competition in natural Pseudomonas communities

Many bacteria rely on the secretion of siderophores to scavenge iron from the environment. Laboratory studies revealed that abiotic and biotic factors together determine how much siderophores bacteria make, and whether siderophores can be exploited by non-producing cheaters or be deployed by producers to inhibit competitors. Here, we explore whether these insights apply to natural communities, by comparing the production of the siderophore pyoverdine among 930 Pseudomonas strains from 48 soil and pond communities. We found that pH, iron content, carbon concentration and community diversity determine pyoverdine production levels, and the extent to which strains are either stimulated or inhibited by heterologous (non-self) pyoverdines. While pyoverdine non-producers occurred in both habitats, their prevalence was higher in soils. Environmental and genetic analyses suggest that non-producers can evolve as cheaters, exploiting heterologous pyoverdine, but also due to pyoverdine disuse in environments with increased iron availability. Overall, we found that environmental factors explained between-strain variation in pyoverdine production much better in soils than in ponds, presumably because high strain mixing in ponds impedes local adaption. Our study sheds light on the complexity of natural bacterial communities, and provides first insights into the multivariate nature of siderophore-based iron acquisition and competition among environmental pseudomonads.

Synchrony of Eukaryotic and Prokaryotic Planktonic Communities in Three Seasonally Sampled Austrian Lakes

Freshwater systems are characterized by an enormous diversity of eukaryotic protists and prokaryotic taxa. The community structures in different lakes are thereby influenced by factors such as habitat size, lake chemistry, biotic interactions, and seasonality. In our study, we used high throughput 454 sequencing to study the diversity and temporal changes of prokaryotic and eukaryotic planktonic communities in three Austrian lakes during the ice-free season. In the following year, one lake was sampled again with a reduced set of sampling dates to observe reoccurring patterns. Cluster analyses (based on SSU V9 (eukaryotic) and V4 (prokaryotic) OTU composition) grouped samples according to their origin followed by separation into seasonal clusters, indicating that each lake has a unique signature based on OTU composition. These results suggest a strong habitat-specificity of microbial communities and in particular of community patterns at the OTU level. A comparison of the prokaryotic and eukaryotic datasets via co-inertia analysis (CIA) showed a consistent clustering of prokaryotic and eukaryotic samples, probably reacting to the same environmental forces (e.g., pH, conductivity). In addition, the shifts in eukaryotic and bacterioplanktonic communities generally occurred at the same time and on the same scale. Regression analyses revealed a linear relationship between an increase in Bray-Curtis dissimilarities and elapsed time. Our study shows a pronounced coupling between bacteria and eukaryotes in seasonal samplings of the three analyzed lakes. However, our temporal resolution (biweekly sampling) and data on abiotic factors were insufficient to determine if this was caused by direct biotic interactions or by reacting to the same seasonally changing environmental forces.