Taxonomic and environmental distribution of bacterial amino acid auxotrophies

Many microorganisms are auxotrophic-unable to synthesize the compounds they require for growth. With this work, we quantify the prevalence of amino acid auxotrophies across a broad diversity of bacteria and habitats. We predicted the amino acid biosynthetic capabilities of 26,277 unique bacterial genomes spanning 12 phyla using a metabolic pathway model validated with empirical data. Amino acid auxotrophy is widespread across bacterial phyla, but we conservatively estimate that the majority of taxa (78.4%) are able to synthesize all amino acids. Our estimates indicate that amino acid auxotrophies are more prevalent among obligate intracellular parasites and in free-living taxa with genomic attributes characteristic of 'streamlined' life history strategies. We predicted the amino acid biosynthetic capabilities of bacterial communities found in 12 unique habitats to investigate environmental associations with auxotrophy, using data compiled from 3813 samples spanning major aquatic, terrestrial, and engineered environments. Auxotrophic taxa were more abundant in host-associated environments (including the human oral cavity and gut) and in fermented food products, with auxotrophic taxa being relatively rare in soil and aquatic systems. Overall, this work contributes to a more complete understanding of amino acid auxotrophy across the bacterial tree of life and the ecological contexts in which auxotrophy can be a successful strategy.

Colonization-persistence trade-offs in natural bacterial communities

Fitness equalizing mechanisms, such as trade-offs, are recognized as one of the main factors promoting species coexistence in community ecology. However, they have rarely been explored in microbial communities. Although microbial communities are highly diverse, the coexistence of their multiple taxa is largely attributed to niche differences and high dispersal rates, following the principle 'everything is everywhere, but the environment selects'. We use a dynamical stochastic model based on the theory of island biogeography to study highly diverse bacterial communities over time across three different systems (soils, alpine lakes and shallow saline lakes). Assuming fitness equalization mechanisms, here we newly analytically derive colonization-persistence trade-offs, and report a signal of such trade-offs in natural bacterial communities. Moreover, we show that different subsets of species in the community drive this trade-off. Rare taxa, which are occasional and more likely to follow independent colonization/extinction dynamics, drive this trade-off in the aquatic communities, while the core sub-community did it in the soils. We conclude that equalizing mechanisms may be more important than previously recognized in bacterial communities. Our work also emphasizes the fundamental value of dynamical models for understanding temporal patterns and processes in highly diverse communities.

Exploring the potential links between gut microbiota composition and natural populations management in wild boar (Sus scrofa)

We surveyed wild boar (Sus scrofa) populations using 16S rRNA gene analysis of the gut microbiota in fresh faeces taken from 88 animals hunted in 16 hunting estates. The wild boar is a very convenient model system to explore how environmental factors including game management, food availability, disease prevalence, and behaviour may affect different biological components of wild individuals with potential implications in management and conservation. We tested the hypotheses that diet (according to stable carbon isotopes analyses), gender (i.e., animal behaviour studying males and females), and both health (analyses of serum samples to detect exposure to several diseases) and form statutes (i.e., thoracic circumference in adults) are reflected in changes in the intestinal microbiota. We focused on a gut functional biomarker index combining Oscillospiraceae and Ruminococcaceae vs. Enterobacteriaceae. We found that gender and the estate (population) were explanatory variables (c.a. 28% of the variance), albeit a high degree of overlapping among individuals was observed. The individuals with higher abundance of Enterobacteriaceae showed a gut microbiota with low diversity, mostly in males. Significant statistical differences for thoracic circumference were not found between males and females. Interestingly, the thoracic circumference was significantly and inversely related to the relative abundance of Enterobacteriaceae in males. Overall, we found that diet, gender, and form status were major factors that could be related to the composition and diversity of the gut microbiota. A high variability was observed in the biomarker index for populations with natural diet (rich in C3 plants). Although, we noticed a marginally significant negative trend between the index (higher abundance of Enterobacteriaceae) and the continuous feeding of C4 plants (i.e., supplementary maize) in the diet of males. This result suggests that continuous artificial feeding in hunting estates could be one of the factors negatively influencing the gut microbiota and the form status of wild boars that deserves further investigations.

Understanding atmospheric intercontinental dispersal of harmful microorganisms

The atmosphere is a major route for microbial intercontinental dispersal, including harmful microorganisms, antibiotic resistance genes, and allergens, with strong implications in ecosystem functioning and global health. Long-distance dispersal is facilitated by air movement at higher altitudes in the free troposphere and is affected by anthropogenic forcing, climate change, and by the general atmospheric circulation, mainly in the intertropical convergence zone. The survival of microorganisms during atmospheric transport and their remote invasive potential are fundamental questions, but data are scarce. Extreme atmospheric conditions represent a challenge to survival that requires specific adaptive strategies, and recovery of air samples from the high altitudes relevant to study harmful microorganisms can be challenging. In this paper, we highlight the scope of the problem, identify challenges and knowledge gaps, and offer a roadmap for improved understanding of intercontinental microbial dispersal and their outcomes. Greater understanding of long-distance dispersal requires research focus on local factors that affect emissions, coupled with conditions influencing transport and survival at high altitudes, and eventual deposition at sink locations.

Understanding stochastic and deterministic assembly processes in microbial communities along temporal, spatial and environmental scales

Identifying the main drivers of community assembly remains an open fundamental question in ecology. Dispersal processes introduce randomness in community composition while selection for particular environments creates predictable assemblages. However, the interaction between selection and dispersal processes is still poorly understood. Here, we address this question in bacterial and microeukaryotic communities inhabiting a highly dynamic system of ephemeral (hyper)saline lakes. We show that the combination of beta-diversity decomposition methods and a temporal approach based on colonization and extinction dynamics yields new insights into the relative effect of selection and dispersal along environmental gradients. Selective pressure and dispersal-related processes simultaneously shape each local community with variable strength and effect. The dominance of selection vs. dispersal shifted from stochastic to deterministic assembly as salinity increased along the gradient. This transition also had an impact on the temporal dynamics of the lakes as community turnover decreased at high salinities because both colonization and extinction rates slowed down. Only microeukaryotic richness decreased along the gradient due to lower effective colonization at higher salinities, suggesting that the net effect of selection and dispersal is determined by both environmental conditions and the idiosyncrasy of the different microbial ecologies. Our results emphasize the use of temporal approaches in combination with standard statistical methods for a better understanding of the dynamic processes underlying community assembly.

Microbial metabolic routes in metagenome assembled genomes are mirrored by the mass balance of polycyclic aromatic hydrocarbons in a high altitude lake

Semivolatile organic pollutants have potential for long range atmospheric transport and can thus reach pristine remote lakes by atmospheric deposition. Polycyclic aromatic hydrocarbons (PAHs) are among the most abundant and toxic semivolatile pollutants affecting lakes, however, the main factors controlling their fate are still poorly known. Here we show two contrasting lines of evidence for the importance of microbial degradation on the environmental fate of PAHs in a high altitude deep lake. The first evidence is given by an assessment of the metagenomes from surface and deep waters from Lake Redon (Pyrenees Mountains), which shows the occurrence of the initial ring hydroxylating dioxygenases as well as other PAH degrading genes from the complete metabolic route of PAH degradation. The second line of evidence is by the application of an environmental fate model for PAHs to Lake Redon under two contrasting scenarios considering the inclusion or not of degradation. When degradation is included in the model, PAH concentrations in the sediment are predicted within a factor of two of those measured in Lake Redon. Finally, the extent of the degradation sink is quantified and compared to other cycling PAH fluxes in the lake.

Absence of stress‐promoted facilitation coupled with a competition decrease in the microbiome of ephemeral saline lakes

Salinity fluctuations constitute a well-known high stress factor strongly shaping global biological distributions and abundances. However, there is a knowledge gap regarding how increasing saline stress affects microbial biological interactions. We applied the combination of a probabilistic method for estimating significant co-occurrences/exclusions and a conceptual framework for filtering out associations potentially linked to environmental and/or spatial factors, in a series of connected ephemeral (hyper) saline lakes. We carried out a network analysis over the full aquatic microbiome-bacteria, eukarya, and archaea-under severe salinity fluctuations. Most of the observed co-occurrences/exclusions were potentially explained by environmental niche and/or dispersal limitation. Co-occurrences assigned to potential biological interactions remained stable, suggesting that the salt gradient was not promoting interspecific facilitation processes. Conversely, co-exclusions assigned to potential biological interactions decreased along the gradient both in number and network complexity, pointing to a decrease of interspecies competition as salinity increased. Overall, higher saline stress reduced microbial co-exclusions while co-occurrences remained stable suggesting decreasing competition coupled with lack of stress-gradient promoted facilitation in the microbiome of ephemeral saline lakes.

Unifying the known and unknown microbial coding sequence space

Genes of unknown function are among the biggest challenges in molecular biology, especially in microbial systems, where 40–60% of the predicted genes are unknown. Despite previous attempts, systematic approaches to include the unknown fraction into analytical workflows are still lacking. Here, we present a conceptual framework, its translation into the computational workflow AGNOSTOS and a demonstration on how we can bridge the known-unknown gap in genomes and metagenomes. By analyzing 415,971,742 genes predicted from 1749 metagenomes and 28,941 bacterial and archaeal genomes, we quantify the extent of the unknown fraction, its diversity, and its relevance across multiple organisms and environments. The unknown sequence space is exceptionally diverse, phylogenetically more conserved than the known fraction and predominantly taxonomically restricted at the species level. From the 71 M genes identified to be of unknown function, we compiled a collection of 283,874 lineage-specific genes of unknown function for Cand. Patescibacteria (also known as Candidate Phyla Radiation, CPR), which provides a significant resource to expand our understanding of their unusual biology. Finally, by identifying a target gene of unknown function for antibiotic resistance, we demonstrate how we can enable the generation of hypotheses that can be used to augment experimental data.

It is estimated that scientists do not know what half of microbial genes actually do. When these genes are discovered in microorganisms grown in the lab or found in environmental samples, it is not possible to identify what their roles are. Many of these genes are excluded from further analyses for these reasons, meaning that the study of microbial genes tends to be limited to genes that have already been described.

These limitations hinder research into microbiology, because information from newly discovered genes cannot be integrated to better understand how these organisms work. Experiments to understand what role these genes have in the microorganisms are labor-intensive, so new analytical strategies are needed.

To do this, Vanni et al. developed a new framework to categorize genes with unknown roles, and a computational workflow to integrate them into traditional analyses. When this approach was applied to over 400 million microbial genes (both with known and unknown roles), it showed that the share of genes with unknown functions is only about 30 per cent, smaller than previously thought. The analysis also showed that these genes are very diverse, revealing a huge space for future research and potential applications. Combining their approach with experimental data, Vanni et al. were able to identify a gene with a previously unknown purpose that could be involved in antibiotic resistance.

This system could be useful for other scientists studying microorganisms to get a more complete view of microbial systems. In future, it may also be used to analyze the genetics of other organisms, such as plants and animals.

Global dispersal and potential sources of antibiotic resistance genes in atmospheric remote depositions

Antibiotic resistance has become a major Global Health concern and a better understanding on the global spread mechanisms of antibiotic resistant bacteria (ARB) and intercontinental ARB exchange is needed. We measured atmospheric depositions of antibiotic resistance genes (ARGs) by quantitative (q)PCR in rain/snow collected fortnightly along 4 y. at a remote high mountain LTER (Long-Term Ecological Research) site located above the atmospheric boundary layer (free troposphere). Bacterial composition was characterized by 16S rRNA gene sequencing, and air mass provenances were determined by modelled back trajectories and rain/snow chemical composition. We hypothesize that the free troposphere may act as permanent reservoir and vector for ARB and ARGs global dispersal. We aimed to i) determine whether ARGs are long-range intercontinental and persistently dispersed through aerosols, ii) assess ARGs long-term atmospheric deposition dynamics in a remote high mountain area, and iii) unveil potential diffuse ARGs pollution sources. We showed that the ARGs sul1 (resistance to sulfonamides), tetO (resistance to tetracyclines), and intI1 (a proxy for horizontal gene transfer and anthropogenic pollution) were long-range and persistently dispersed in free troposphere aerosols. Major depositions of tetracyclines resistance matched with intensification of African dust outbreaks. Potential ARB mostly traced their origin back into agricultural soils. Our study unveils that air masses pathways are shaping ARGs intercontinental dispersal and global spread of antibiotic resistances, with potential predictability for interannual variability and remote deposition rates. Because climate regulates aerosolization and long-range air masses movement patterns, we call for a more careful evaluation of the connections between land use, climate change and ARB long-range intercontinental dispersal.

OUP accepted manuscript

The atmosphere connects habitats across multiple spatial scales via airborne dispersal of microbial cells, propagules and biomolecules. Atmospheric microorganisms have been implicated in a variety of biochemical and biophysical transformations. Here, we review ecological aspects of airborne microorganisms with respect to their dispersal, activity and contribution to climatic processes. Latest studies utilizing metagenomic approaches demonstrate that airborne microbial communities exhibit pronounced biogeography, driven by a combination of biotic and abiotic factors. We quantify distributions and fluxes of microbial cells between surface habitats and the atmosphere and place special emphasis on long-range pathogen dispersal. Recent advances have established that these processes may be relevant for macroecological outcomes in terrestrial and marine habitats. We evaluate the potential biological transformation of atmospheric volatile organic compounds and other substrates by airborne microorganisms and discuss clouds as hotspots of microbial metabolic activity in the atmosphere. Furthermore, we emphasize the role of microorganisms as ice nucleating particles and their relevance for the water cycle via formation of clouds and precipitation. Finally, potential impacts of anthropogenic forcing on the natural atmospheric microbiota via emission of particulate matter, greenhouse gases and microorganisms are discussed.

A long-term atmospheric baseline for intercontinental exchange of airborne pathogens

The atmosphere is a potential pathway for global-scale and long-range dispersal of viable microorganisms, promoting biological interconnections among the total environment. We aimed to provide relevant baseline information for long-range long-term intercontinental exchange of potentially infectious airborne microorganisms of major interest in environmental and health-related disciplines. We used an interannual survey (7-y) with wet depositions fortnightly collected above the boundary layer (free troposphere) at a remote high-elevation LTER (Long-Term-Ecological-Research) site, analyzed by 16S and 18S rRNA genes, and compared to a database of 475 well-known pathogens. We applied a conservative approach on close relatives of pathogenic species (>98% identity) standing their theoretical upper limit for atmospheric baseline relative abundances. We identified c. 2-3% of the total airborne microbiota as potential pathogens. Their most frequent environmental origins were soil, aquatic, and anthropogenic sources. Phytopathogens (mostly fungi) were the potential infectious agents most widely present. We uncovered consistent interannual dynamics with taxa foreseeable over time (i.e., predictable seasonal behavior) and under recurrent environmental scenarios (e.g., Saharan dust intrusions), respectively, being highly valuable microbial forensic environmental indicators. Up to 8 bacterial and 21 fungal genera consistently showed temporal abundances and recurrences unevenly distributed. Incidence of allergenic fungi was lower in summer, and significantly higher in spring. Close relatives to Coccidioides posadasii consistently showed higher signals (i.e., high specificity and high fidelity) in winter, whereas Cryptococcus neoformans had a significant signal in spring. Along Saharan dust intrusions, the bacterial phytopathogens Acidovorax avenae and Agrobacterium tumefaciens and the fungal phytopathogens Pseudozyma hubeiensis and Peniophora sp. consistently showed higher signals. Potential human pathogens showed low proportion, being mostly fungal allergens. Microorganisms related to obligated human, amphibian and fish pathogens were commonly found in winter. More studies in remote field sites above the boundary layer will unveil whether or not a similar trend is found globally.

Ecological and Metabolic Thresholds in the Bacterial, Protist, and Fungal Microbiome of Ephemeral Saline Lakes (Monegros Desert, Spain)

We studied the 16S and 18S rRNA genes of the bacterial, protist, and fungal microbiomes of 131 samples collected in 14 ephemeral small inland lakes located in the endorheic area of the Monegros Desert (NE Spain). The sampling covered different temporal flooding/desiccation cycles that created natural salinity gradients between 0.1% (w/v) and salt saturation. We aimed to test the hypothesis of a lack of competitive advantage for microorganisms using the "salt-in" strategy in highly fluctuating hypersaline environments where temperature and salinity transitions widely vary within short time periods, as in ephemeral inland lakes. Overall, 5653 bacterial zOTUs and 2658 eukaryal zOTUs were detected heterogeneously distributed with significant variations on taxonomy and general energy-yielding metabolisms and trophic strategies along the gradient. We observed a more diverse bacterial assembly than initially expected at extreme salinities and a lack of dominance of a few "salt-in" organisms. Microbial thresholds were unveiled for these highly fluctuating hypersaline environments with high selective pressures. We conclude that the extremely high dynamism observed in the ephemeral lakes of Monegros may have given a competitive advantage for more versatile ("salt-out") organisms compared to those better adapted to stable high salinities usually more common in solar salterns. Ephemeral inland saline lakes offered a well-suited natural framework for highly detailed evolutionary and ecological studies.

The characteristic time of ecological communities

A simple description of temporal dynamics of ecological communities may help us understand how community assembly proceeds, predict ecological responses to environmental disturbances, and improve the performance of biological conservation actions. Although community changes take place at multiple temporal scales, the variation of species composition and richness over time across communities and habitats shows general patterns that may potentially reveal the main drivers of community dynamics. We used the simplest stochastic model of island biogeography to propose two quantities to characterize community dynamics: the community characteristic time, as a measure of the typical time scale of species‐richness change, and the characteristic Jaccard index, as a measure of temporal β diversity, that is, the variation of community composition over time. In addition, the community characteristic time, which sets the temporal scale at which null, noninteracting species assemblages operate, allowed us to define a relative sampling frequency (to the characteristic time). Here we estimate these quantities across microbial and macroscopic species assemblages to highlight two related results. First, we illustrated both characteristic time and Jaccard index and their relation with classic time‐series in ecology, and found that the most thoroughly sampled communities, relative to their characteristic time, presented the largest similarity between consecutive samples. Second, our analysis across a variety of habitats and taxa show that communities span a large range of species turnover, from potentially very fast (short characteristic times) to rather slow (long characteristic times) communities. This was in agreement with previous knowledge, but indicated that some habitats may have been sampled less frequently than required. Our work provides new perspectives to explore the temporal component in ecological studies and highlights the usefulness of simple approximations to the complex dynamics of ecological communities.

Individual fate and gut microbiome composition in the European wild rabbit (Oryctolagus cuniculus)

Studies connecting microbiome composition and functional performance in wildlife have received little attention and understanding their connections with wildlife physical condition are sorely needed. We studied the variation in gut microbiota (hard fecal pellets) between allopatric subspecies of the European wild rabbit in wild populations and in captured individuals studied under captivity. We evaluated the influence of environmental and host-specific factors. The microbiome of wild rabbit populations reduced its heterogeneity under controlled conditions. None of the host-specific factors tested correlated with the microbiota composition. We only observed significant intra-group dispersion for the age factor. The most diverse microbiomes were rich in Ruminococcaceae potentially holding an enriched functional profile with dominance of cellulases and xylanases, and suggesting higher efficiency in the digestion of fiber-rich food. Conversely, low diversity gut microbiomes showed dominance of Enterobacteriaceae potentially rich in amylases. We preliminary noticed geographical variations in field populations with higher dominance of Ruminococcaceae in south-western than in north-eastern Spain. Spatial differences appeared not to be subspecies driven, since they were lost in captivity, but environmentally driven, although differences in social structure and behavior may also play a role that deserve further investigations. A marginally significant relationship between the Ruminococcaceae/Enterobacteriaceae ratio and potential life expectancy was observed in captive rabbits. We hypothesize that the gut microbiome may determine the efficiency of feeding resource exploitation, and can also be a potential proxy for life expectancy, with potential applications for the management of declining wild herbivorous populations. Such hypotheses remain to be explored in the future.

Dynamics and ecological distributions of the Archaea microbiome from inland saline lakes (Monegros Desert, Spain)

We characterized the rich Archaea microbiome of shallow inland lakes (Monegros Desert, NE Spain) by 16S rRNA gene tag sequencing covering a wide salinity range (0.1%-40% w/v) along 3 years. Up to 990 operational taxonomic units (OTUs; >97% identity) were detected allocated in 14 major archaeal phyla and heterogeneously distributed along the salt gradient. Dynamics and idiosyncratic ecological distributions were uncovered for the different phyla. A high genetic richness was observed for Woesearchaeota and Pacearchaeota (>370 OTUs each), followed by Halobacteria (105), Nanohaloarchaeota (62) and Thermoplasmata (19). Overall, the distribution of genetic richness was strongly correlated with environmental niche amplitude, but not with occurrence. We unveiled high occurrence for a very rich Woesearchaeota assemblage, and an unexpected positive correlation of Pacearchaeota abundance with salinity at >15% dissolved salt content. The estimated dynamic behaviour (temporal 'turnover' rates of presence/absence data) unveiled Thaumarchaeota and Halobacteria as the most dynamic groups, and Aenigmarchaeota and Thermoplasmata as the most stable. The DPANN Pacearchaeota, Woesearchaeota, and Nanohaloarchaeota showed intermediate rates, suggesting higher resilience to environmental perturbations. A rich and dynamic Archaea microbiome was unveiled, including unseen ecological traits for relevant members of the still largely unknown DPANN group, supporting a strong ecological differentiation between Pacearchaeota and Woesearchaeota.

Climate mediates continental scale patterns of stream microbial functional diversity

BACKGROUND

Understanding the large-scale patterns of microbial functional diversity is essential for anticipating climate change impacts on ecosystems worldwide. However, studies of functional biogeography remain scarce for microorganisms, especially in freshwater ecosystems. Here we study 15,289 functional genes of stream biofilm microbes along three elevational gradients in Norway, Spain and China.

RESULTS

We find that alpha diversity declines towards high elevations and assemblage composition shows increasing turnover with greater elevational distances. These elevational patterns are highly consistent across mountains, kingdoms and functional categories and exhibit the strongest trends in China due to its largest environmental gradients. Across mountains, functional gene assemblages differ in alpha diversity and composition between the mountains in Europe and Asia. Climate, such as mean temperature of the warmest quarter or mean precipitation of the coldest quarter, is the best predictor of alpha diversity and assemblage composition at both mountain and continental scales, with local non-climatic predictors gaining more importance at mountain scale. Under future climate, we project substantial variations in alpha diversity and assemblage composition across the Eurasian river network, primarily occurring in northern and central regions, respectively.

CONCLUSIONS

We conclude that climate controls microbial functional gene diversity in streams at large spatial scales; therefore, the underlying ecosystem processes are highly sensitive to climate variations, especially at high latitudes. This biogeographical framework for microbial functional diversity serves as a baseline to anticipate ecosystem responses and biogeochemical feedback to ongoing climate change. Video Abstract.

Taxonomy and functional interactions in upper and bottom waters of an oligotrophic high-mountain deep lake (Redon, Pyrenees) unveiled by microbial metagenomics

High mountain lakes are, in general, highly sensitive systems to external forcing and good sentinels of global environmental changes. For a better understanding of internal lake processes, we examined microbial biodiversity and potential biogeochemical interactions in the oligotrophic deep high-mountain Lake Redon (Pyrenees, 2240 m altitude) using shotgun metagenomics. We analyzed the two ends of the range of environmental conditions found in Lake Redon, at 2 and 60 m depths. Bacteria were the most abundant component of the metagenomic reads (>90%) and the diversity indices of both taxonomic (16S and 18S rRNA) and functional (carbon-, nitrogen-, sulfur-, and phosphorous-cycling) related genes were higher in the bottom dark layer than in the upper compartment. A marked segregation was observed both in biodiversity and in the dominant energy and biomass generating pathways between the extremes. The aerobic respiration was mainly dominated by heterotrophic Burkholderiales at the top and Actinobacteria and Burkholderiales at the lake bottom. The potential for an active nitrogen cycle (nitrogen fixation, nitrification, nitrite oxidation, and nitrate reduction) was mainly found at 60 m, and potential for methanogenesis, anaerobic ammonia oxidation and dissimilatory sulfur pathways were only observed there. Some unexpected and mostly unseen energy and biomass pathways were found relevant for the biogeochemical cycling in lake Redon, i.e., those related to carbon monoxide oxidation and phosphonates processing. We provide a general scheme of the main biogeochemical processes that may operate in the sentinel deep Lake Redon. This framework may help for a better understanding of the whole lake metabolism.

A Randomized Trait Community Clustering approach to unveil consistent environmental thresholds in community assembly

Similarities and differences of phenotypes within local co-occurring species hold the key to inferring the contribution of stochastic or deterministic processes in community assembly. Developing both phylogenetic-based and trait-based quantitative methods to unravel these processes is a major aim in community ecology. We developed a trait-based approach that: (i) assesses if a community trait clustering pattern is related to increasing environmental constraints along a gradient; and (ii) determines quantitative thresholds for an environmental variable along a gradient to interpret changes in prevailing community assembly drivers. We used a regional set of natural shallow saline ponds covering a wide salinity gradient (0.1-40% w/v). We identify a consistent discrete salinity threshold (ca. 5%) for microbial community assembly drivers. Above 5% salinity a strong environmental filtering prevailed as an assembly force, whereas a combination of biotic and abiotic factors dominated at lower salinities. This method provides a conceptual approach to identify consistent environmental thresholds in community assembly and enables quantitative predictions for the ecological impact of environmental changes.

Sponges and Their Microbiomes Show Similar Community Metrics Across Impacted and Well-Preserved Reefs

Sponge diversity has been reported to decrease from well-preserved to polluted environments, but whether diversity and intra-species variation of their associated microbiomes also change as function of environmental quality remains unknown. Our study aimed to assess whether microbiome composition and structure are related to the proliferation of some sponges and not others under degraded conditions. We characterized the most frequent sponges and their associated bacteria in two close areas (impacted and well-preserved) of Nha Trang Bay (Indo-Pacific). Sponge assemblages were richer and more diverse in the well-preserved reefs, but more abundant (individuals/m. transect) in the impacted environments, where two species (Clathria reinwardti and Amphimedon paraviridis) dominated. Sponge microbiomes from the polluted zones had, in general, lower bacterial diversity and core size and consequently, higher intra-species dispersion than microbiomes of sponges from the well-preserved environments. Microbial communities reflect the reduction of diversity and richness shown by their host sponges. In this sense, sponges with less complex and more variable microbiomes proliferate under degraded environmental conditions, following the ecological paradigm that negatively correlates community diversity and environmental degradation. Thereby, the diversity and structure of sponge microbiomes might indirectly determine the presence and proliferation of sponge species in certain habitats.

The DNRA-Denitrification Dichotomy Differentiates Nitrogen Transformation Pathways in Mountain Lake Benthic Habitats

Effects of nitrogen (N) deposition on microbially-driven processes in oligotrophic freshwater ecosystems are poorly understood. We quantified guilds in the main N-transformation pathways in benthic habitats of 11 mountain lakes along a dissolved inorganic nitrogen gradient. The genes involved in denitrification (nirS, nirK, nosZ), nitrification (archaeal and bacterial amoA), dissimilatory nitrate reduction to ammonium (DNRA, nrfA) and anaerobic ammonium oxidation (anammox, hdh) were quantified, and the bacterial 16S rRNA gene was sequenced. The dominant pathways and associated bacterial communities defined four main N-transforming clusters that differed across habitat types. DNRA dominated in the sediments, except in the upper layers of more productive lakes where nirS denitrifiers prevailed with potential N₂O release. Loss as N₂ was more likely in lithic biofilms, as indicated by the higher hdh and nosZ abundances. Archaeal ammonia oxidisers predominated in the isoetid rhizosphere and rocky littoral sediments, suggesting nitrifying hotspots. Overall, we observed a change in potential for reactive N recycling via DNRA to N losses via denitrification as lake productivity increases in oligotrophic mountain lakes. Thus, N deposition results in a shift in genetic potential from an internal N accumulation to an atmospheric release in the respective lake systems, with increased risk for N₂O emissions from productive lakes.

Microbial composition, potential functional roles and genetic novelty in gypsum-rich and hypersaline soils of Monegros and Gallocanta (Spain)

Soil microbial communities (both Bacteria and Archaea) were studied after 16S rRNA genes massive sequencing in two hypersaline and gypsum-rich contrasted sites located in NE Spain. Soil microbial communities were also locally analysed according to environmental variables, including geological, physico-chemical, biogeochemically, and climatic data. Typical soil characteristics, climate data, and plant composition clearly split the two sites and major differences among the microbial communities for the areas were initially expected. Overall, high values of microbial species richness (up to 2300 taxa) and ecological diversity was detected in both sites. High genetic novelty levels were found mostly to environmental sequences, highlighting the high potential for microbiological studies. In contrast to the initial expectations, a substantial overlapping between Monegros and Gallocanta microbes was observed, indicating a high similarity despite of the geographical, botanical and environmental distances between sites, in agreement with both high dispersal and local selection inherent to the microbial world. The potential biogeochemical cycling showed small differences between sites, with presence of photosynthetic green and purple sulfur bacteria, cyanobacteria and aerobic and anaerobic chemolitotrophs. Potential for aerobic methane oxidation and anaerobic methanogenesis was observed in both sites, with predominance of potential nitrification mostly by ammonia-oxidizing archaea, nitrite oxidation and denitrification, and minor contribution for nitrate reduction and nitrate ammonification. The predicted functions based on the taxonomic composition showed high overlapping between the two studied regions, despite their difference in gypsum richness.

Showcasing the role of seawater in bacteria recruitment and microbiome stability in sponges

We studied the core bacterial communities of 19 sponge species from Nha Trang Bay (Central Vietnam), with particular emphasis on the contribution of planktonic seawater bacteria to the sponge core microbiomes. To ensure consistent sponge-microbe associations and accurate identification of planktonic bacteria transmitted from seawater, we were very restrictive with the definition of the sponge core microbiomes (present in all the replicates), and with the identification of valid biological 16S rRNA gene sequences (100% sequence identity) that belonged to potentially different bacterial taxa. We found a high overlap (>50% relative abundance) between the sponge species core microbiome and the seawater bacterial core in ca. a half of the studied species, including representatives of both, HMA and LMA sponges. From our restrictive analysis, we point to horizontal transmission as a relevant way of symbiont acquisition in sponges. Some species-specific recognition mechanisms may act in sponges to enrich specific seawater bacteria in their tissues. These mechanisms would allow the maintenance of bacterial communities in a species across geographical ranges. Moreover, besides contrasting preferences in bacteria selection from seawater, divergent physiological traits may also account for the different microbiomes in species of HMA and LMA sponges.

Stream drying drives microbial ammonia oxidation and first-flush nitrate export

Stream microbial communities and associated processes are influenced by environmental fluctuations that may ultimately dictate nutrient export. Discharge fluctuations caused by intermittent stream flow are increasing worldwide in response to global change. We examined the impact of flow cessation and drying on in-stream nitrogen cycling. We determined archaeal (AOA) and bacterial ammonia oxidizer (AOB) abundance and ammonia oxidation activity in surface and deep sediments from different sites along the Fuirosos stream (Spain) subjected to contrasting hydrological conditions (i.e., running water, isolated pools, and dry streambeds). AOA were more abundant than AOB, with no major changes across hydrological conditions or sediment layers. However, ammonia oxidation activity and sediment nitrate content increased with the degree of stream drying, especially in surface sediments. Upscaling of our results shows that ammonia oxidation in dry streambeds can contribute considerably (~50%) to the high nitrate export typically observed in intermittent streams during first-flush events following flow reconnection. Our study illustrates how the dry channels of intermittent streams can be potential hotspots of ammonia oxidation. Consequently, shifts in the duration, spatial extent and severity of intermittent flow can play a decisive role in shaping nitrogen cycling and export along fluvial networks in response to global change.

Consistent changes in the taxonomic structure and functional attributes of bacterial communities during primary succession

Ecologists have long studied primary succession, the changes that occur in biological communities after initial colonization of an environment. Most of this work has focused on succession in plant communities, laying the conceptual foundation for much of what we currently know about community assembly patterns over time. Because of their prevalence and importance in ecosystems, an increasing number of studies have focused on microbial community dynamics during succession. Here, we conducted a meta-analysis of bacterial primary succession patterns across a range of distinct habitats, including the infant gut, plant surfaces, soil chronosequences, and aquatic environments, to determine whether consistent changes in bacterial diversity, community composition, and functional traits are evident over the course of succession. Although these distinct habitats harbor unique bacterial communities, we were able to identify patterns in community assembly that were shared across habitat types. We found an increase in taxonomic and functional diversity with time while the taxonomic composition and functional profiles of communities became less variable (lower beta diversity) in late successional stages. In addition, we found consistent decreases in the rRNA operon copy number and in the high-efficient phosphate assimilation process (Pst system) suggesting that reductions in resource availability during succession select for taxa adapted to low-resource conditions. Together, these results highlight that, like many plant communities, microbial communities also exhibit predictable patterns during primary succession.

Development of a 16S rRNA-targeted fluorescence in situ hybridization probe for quantification of the ammonia-oxidizer Nitrosotalea devanaterra and its relatives

The Thaumarchaeota SAGMCG-1 group and, in particular, members of the genus Nitrosotalea have high occurrence in acidic soils, the rhizosphere, groundwater and oligotrophic lakes, and play a potential role in nitrogen cycling. In this study, the specific oligonucleotide fluorescence in situ hybridization probe SAG357 was designed for this Thaumarchaeota group based on the available 16S rRNA gene sequences in databases, and included the ammonia-oxidizing species Nitrosotalea devanaterra. Cell permeabilization for catalyzed reporter deposition fluorescence in situ detection and the hybridization conditions were optimized on enrichment cultures of the target species N. devanaterra, as well as the non-target ammonia-oxidizing archaeon Nitrosopumilus maritimus. Probe specificity was improved with a competitor oligonucleotide, and fluorescence intensity and cell visualization were enhanced by the design and application of two adjacent helpers. Probe performance was tested in soil samples along a pH gradient, and counting results matched the expected in situ distributions. Probe SAG357 and the CARD-FISH protocol developed in the present study will help to improve the current understanding of the ecology and physiology of N. devanaterra and its relatives in natural environments.

High planktonic diversity in mountain lakes contains similar contributions of autotrophic, heterotrophic and parasitic eukaryotic life forms

A rich eukaryotic planktonic community exists in high-mountain lakes despite the diluted, oligotrophic and cold, harsh prevailing conditions. Attempts of an overarching appraisal have been traditionally hampered by observational limitations of small, colorless, and soft eukaryotes. We aimed to uncover the regional eukaryotic biodiversity of a mountain lakes district to obtain general conclusions on diversity patterns, dominance, geographic diversification, and food-web players common to oligotrophic worldwide distributed freshwater systems. An unprecedented survey of 227 high-altitude lakes comprising large environmental gradients was carried out using Illumina massive tag sequencing of the 18S rRNA gene. We observed a large Chrysophyceae dominance in richness, abundance and novelty, and unveiled an unexpected richness in heterotrophic phagotrophs and parasites. In particular, Cercozoa and Chytridiomycota showed diversity features similar to the dominant autotrophic groups. The prominent beta-dispersion shown by parasites suggests highly specific interactions and a relevant role in food webs. Interestingly, the freshwater Pyrenean metacommunity contained more diverse specific populations than its closest marine oligotrophic equivalent, with consistently higher beta-diversity. The relevance of unseen groups opens new perspectives for the better understanding of planktonic food webs. Mountain lakes, with remarkable environmental idiosyncrasies, may be suitable environments for the genetic diversification of microscopic eukaryotic life forms.

Degradation of sulfonamides as a microbial resistance mechanism

Two of the main mechanisms of bacterial resistance to sulfonamides in aquatic systems, spread of antibiotic resistance genes (ARG) among the microbial community and in-situ bacterial sulfonamide degradation, were studied in mesocosms experiments using water and cobble biofilms from upstream (pristine waters) and downstream (polluted waters) from the Llobregat river, NE Iberian Peninsula. Mesocosms were prepared at two different concentrations (5000 ng/L and 1000 ng/L) of sulfonamides antibiotics (sulfamethazine and sulfamethoxazole). Concentrations of ARG, nutrients, sulfonamides and their degradation products were measured during the time course of the experiments. Sulfonamides were efficiently degraded by the biofilms during the first four weeks of the experiment. The abundance of ARG in biofilms sharply decreased after addition of high concentrations of sulfonamides, but this was not observed in the mesocosms treated with low concentrations of sulfonamides. Sulfonamide degradation was faster in polluted waters and at high concentrations of sulfonamide (and lower ARG abundances), suggesting that both degradation and ARG are two complementary resistance strategies employed by the microbial community. This study shows that microbial degradation of antibiotics is an efficient resistance mechanism coupled with the presence of ARG, and suggests that in situ degradation prevails at high concentrations of antibiotics whereas physiological adaptation by ARG spread would be more important under relatively lower concentrations of antibiotics.

Bioaerosols in the Barcelona subway system

Subway systems worldwide transport more than 100 million people daily; therefore, air quality on station platforms and inside trains is an important urban air pollution issue. We examined the microbiological composition and abundance in space and time of bioaerosols collected in the Barcelona subway system during a cold period. Quantitative PCR was used to quantify total bacteria, Aspergillus fumigatus, influenza A and B, and rhinoviruses. Multitag 454 pyrosequencing of the 16S rRNA gene was used to assess bacterial community composition and biodiversity. The results showed low bioaerosol concentrations regarding the targeted microorganisms, although the bacterial bioburden was rather high (10⁴ bacteria/m³ ). Airborne bacterial communities presented a high degree of overlap among the different subway environments sampled (inside trains, platforms, and lobbies) and were dominated by a few widespread taxa, with Methylobacterium being the most abundant genus. Human-related microbiota in sequence dataset and ascribed to potentially pathogenic bacteria were found in low proportion (maximum values below 2% of sequence readings) and evenly detected. Hence, no important biological exposure marker was detected in any of the sampled environments. Overall, we found that commuters are not the main source of bioaerosols in the Barcelona subway system.

The dominant detritus-feeding invertebrate in Arctic peat soils derives its essential amino acids from gut symbionts

Supplementation of nutrients by symbionts enables consumers to thrive on resources that might otherwise be insufficient to meet nutritional demands. Such nutritional subsidies by intracellular symbionts have been well studied; however, supplementation of de novo synthesized nutrients to hosts by extracellular gut symbionts is poorly documented, especially for generalists with relatively undifferentiated intestinal tracts. Although gut symbionts facilitate degradation of resources that would otherwise remain inaccessible to the host, such digestive actions alone cannot make up for dietary insufficiencies of macronutrients such as essential amino acids (EAA). Documenting whether gut symbionts also function as partners for symbiotic EAA supplementation is important because the question of how some detritivores are able to subsist on nutritionally insufficient diets has remained unresolved. To answer this poorly understood nutritional aspect of symbiont-host interactions, we studied the enchytraeid worm, a bulk soil feeder that thrives in Arctic peatlands. In a combined field and laboratory study, we employed stable isotope fingerprinting of amino acids to identify the biosynthetic origins of amino acids to bacteria, fungi and plants in enchytraeids. Enchytraeids collected from Arctic peatlands derived more than 80% of their EAA from bacteria. In a controlled feeding study with the enchytraeid Enchytraeus crypticus, EAA derived almost exclusively from gut bacteria when the worms fed on higher fibre diets, whereas most of the enchytraeids' EAA derived from dietary sources when fed on lower fibre diets. Our gene sequencing results of gut microbiota showed that the worms harbour several taxa in their gut lumen absent from their diets and substrates. Almost all gut taxa are candidates for EAA supplementation because almost all belong to clades capable of biosynthesizing EAA. Our study provides the first evidence of extensive symbiotic supplementation of EAA by microbial gut symbionts and demonstrates that symbiotic bacteria in the gut lumen appear to function as partners both for symbiotic EAA supplementation and for digestion of insoluble plant fibres.

High occurrence of Pacearchaeota and Woesearchaeota (Archaea superphylum DPANN) in the surface waters of oligotrophic high-altitude lakes

We carried out a regional survey on the archaea composition from surface waters of > 300 high-altitude Pyrenean lakes (average altitude 2300 m, pH range 4.4-10.1) by 16S rRNA gene tag sequencing. Relative Archaea abundances ranged between 0% and 6.3% of total prokaryotes amplicons in the polymerase chain reaction (PCR) mixture, and we detected 769 operational taxonomic units (OTUs; grouped at 97% identity) that split into 13 different lineages, with altitude and pH having a significant effect on the community composition. Woesearchaeota and Pacearchaeota (formerly Euryarchaeota DHVEG-6 cluster) dominated the data set (83% of total OTUS), showed a high occurrence (presence in c. 75% of the lakes) and had relative abundances significantly and positively correlated with the phylogenetic diversity of bacterial communities. Micrarchaeota-Diapherotrites (formerly Euryarchaeota MEG cluster), Methanomicrobia, Thermoplasmata and ammonia-oxidizing thaumarchaeota (AOA) showed relative abundances between 1% and 3% and occurrences between 14% and 26%. Minor lineages were SM1K20, Aenigmarchaeota (formerly Euryarchaeota DSEG cluster), Methanobacteria, Bathyarchaeota and SCG. Environmental preferences substantially differed among lineages, with Aenigmarchaeota and Methanomicrobia having the largest habitat breadth, and Thermoplasmata, AOA and Micrarchaeota having the smallest. Pacearchaeota and Woesearchaeota had been mostly reported from saline habitats and sediments, but surface waters of oligotrophic alpine lakes are suitable environments for such ecologically spread and genetically diverse archaeal lineages.

High Bacterial Diversity and Phylogenetic Novelty in Dark Euxinic Freshwaters Analyzed by 16S Tag Community Profiling

Microbial communities growing under extreme low redox conditions are present in anoxic and sulfide-rich (euxinic) environments such as karstic lakes and experience limitation of electron acceptors. The fine natural chemical gradients and the large diversity of organic and inorganic compounds accumulated in bottom waters are impossible to mimic under laboratory conditions, and only a few groups have been cultured. We investigated the bacterial composition in the oxic-anoxic interface and in the deep waters of three sulfurous lakes from the Lake Banyoles karstic area (NE Spain) through 16S rRNA gene tag sequencing and identified the closest GenBank counterpart. High diversity indices were found in most of the samples with >15 phyla/classes and >45 bacterial orders. A higher proportion of operational taxonomic units (OTUs) of the "highest novelty" was found in the hypolimnia (38 % of total sequences) than in the metalimnia (17 %), whereas the percentage of OTUs closer to cultured counterparts (i.e., 97 % identity in the 16S rRNA gene) was 6 to 21 %, respectively. Elusimicrobia, Chloroflexi, Fibrobacteres, and Spirochaetes were the taxa with the highest proportion of novel sequences. Interestingly, tag sequencing results comparison with metagenomics data available from the same dataset, showed a systematic underestimation of sulfur-oxidizing Epsilonproteobacteria with the currently available 907R "universal" primer. Overall, despite the limitation of electron acceptors, a highly diverse and novel assemblage was present in dark and euxinic hypolimnetic freshwaters, unveiling a hotspot of microbial diversity with a remarkable gap with cultured counterparts.

Influence of edaphic, climatic, and agronomic factors on the composition and abundance of nitrifying microorganisms in the rhizosphere of commercial olive crops

The microbial ecology of the nitrogen cycle in agricultural soils is an issue of major interest. We hypothesized a major effect by farm management systems (mineral versus organic fertilizers) and a minor influence of soil texture and plant variety on the composition and abundance of microbial nitrifiers. We explored changes in composition (16S rRNA gene) of ammonia-oxidizing archaea (AOA), bacteria (AOB), and nitrite-oxidizing bacteria (NOB), and in abundance of AOA and AOB (qPCR of amoA genes) in the rhizosphere of 96 olive orchards differing in climatic conditions, agricultural practices, soil properties, and olive variety. Majority of archaea were 1.1b thaumarchaeota (soil crenarchaeotic group, SCG) closely related to the AOA genus Nitrososphaera. Most AOB (97%) were identical to Nitrosospira tenuis and most NOB (76%) were closely related to Nitrospira sp. Common factors shaping nitrifiers assemblage composition were pH, soil texture, and olive variety. AOB abundance was positively correlated with altitude, pH, and clay content, whereas AOA abundances showed significant relationships with organic nitrogen content and exchangeable K. The abundances of AOA differed significantly among soil textures and olive varieties, and those of AOB among soil management systems and olive varieties. Overall, we observed minor effects by orchard management system, soil cover crop practices, plantation age, or soil organic matter content, and major influence of soil texture, pH, and olive tree variety.

Microbial food web components, bulk metabolism, and single-cell physiology of piconeuston in surface microlayers of high-altitude lakes

Sharp boundaries in the physical environment are usually associated with abrupt shifts in organism abundance, activity, and diversity. Aquatic surface microlayers (SML) form a steep gradient between two contrasted environments, the atmosphere and surface waters, where they regulate the gas exchange between both environments. They usually harbor an abundant and active microbial life: the neuston. Few ecosystems are subjected to such a high UVR regime as high altitude lakes during summer. Here, we measured bulk estimates of heterotrophic activity, community structure and single-cell physiological properties by flow cytometry in 19 high-altitude remote Pyrenean lakes and compared the biological processes in the SML with those in the underlying surface waters. Phototrophic picoplankton (PPP) populations, were generally present in high abundances and in those lakes containing PPP populations with phycoerythrin (PE), total PPP abundance was higher at the SML. Heterotrophic nanoflagellates (HNF) were also more abundant in the SML. Bacteria in the SML had lower leucine incorporation rates, lower percentages of “live” cells, and higher numbers of highly-respiring cells, likely resulting in a lower growth efficiency. No simple and direct linear relationships could be found between microbial abundances or activities and environmental variables, but factor analysis revealed that, despite their physical proximity, microbial life in SML and underlying waters was governed by different and independent processes. Overall, we demonstrate that piconeuston in high altitude lakes has specific features different from those of the picoplankton, and that they are highly affected by potential stressful environmental factors, such as high UVR radiation.

Wastewater treatment plant effluents change abundance and composition of ammonia-oxidizing microorganisms in mediterranean urban stream biofilms

Streams affected by wastewater treatment plant (WWTP) effluents are hotspots of nitrification. We analyzed the influence of WWTP inputs on the abundance, distribution, and composition of epilithic ammonia-oxidizing (AO) assemblages in five Mediterranean urban streams by qPCR and amoA gene cloning and sequencing of both archaea (AOA) and bacteria (AOB). The effluents significantly modified stream chemical parameters, and changes in longitudinal profiles of both NH(4)(+) and NO(3)(-) indicated stimulated nitrification activity. WWTP effluents were an allocthonous source of both AOA, essentially from the Nitrosotalea cluster, and mostly of AOB, mainly Nitrosomonas oligotropha, Nitrosomonas communis, and Nitrosospira spp. changing the relative abundance and the natural composition of AO assemblages. Under natural conditions, Nitrososphaera and Nitrosopumilus AOA dominated AO assemblages, and AOB were barely detected. After the WWTP perturbation, epilithic AOB increased by orders of magnitude whereas AOA did not show quantitative changes but a shift in population composition to dominance of Nitrosotalea spp. The foraneous AOB successfully settled in downstream biofilms and probably carried out most of the nitrification activity. Nitrosotalea were only observed downstream and only in biofilms exposed to either darkness or low irradiance. In addition to other potential environmental limitations for AOA distribution, this result suggests in situ photosensitivity as previously reported for Nitrosotalea under laboratory conditions.

Environmental heterogeneity and microbial inheritance influence sponge-associated bacterial composition of Spongia lamella

Sponges are important components of marine benthic communities. High microbial abundance sponges host a large diversity of associated microbial assemblages. However, the dynamics of such assemblages are still poorly known. In this study, we investigated whether bacterial assemblages present in Spongia lamella remained constant or changed as a function of the environment and life cycle. Sponges were collected in multiple locations and at different times of the year in the western Mediterranean Sea and in nearby Atlantic Ocean to cover heterogeneous environmental variability. Co-occurring adult sponges and offsprings were compared at two of the sites. To explore the composition and abundance of the main bacteria present in the sponge mesohyl, embryos, and larvae, we applied both 16S rRNA gene-denaturing gradient gel electrophoresis (DGGE) and sequencing of excised DGGE bands and quantitative polymerase chain reactions (qPCR). On average, the overall core bacterial assemblage showed over 60 % similarity. The associated bacterial assemblage fingerprints varied both within and between sponge populations, and the abundance of specific bacterial taxa assessed by qPCR significantly differed among sponge populations and between adult sponge and offsprings (higher proportions of Actinobacteria in the latter). Sequences showed between 92 and 100 % identity to sequences previously reported in GenBank, and all were affiliated with uncultured invertebrate bacterial symbionts (mainly sponges). Sequences were mainly related to Chloroflexi and Acidobacteria and a few to Actinobacteria and Bacteroidetes. Additional populations may have been present under detection limits. Overall, these results support that both ecological and biological sponge features may shape the composition of endobiont bacterial communities in S. lamella.

Structure, inter-annual recurrence, and global-scale connectivity of airborne microbial communities

Dust coming from the large deserts on Earth, such as the Sahara, can travel long distances and be dispersed over thousands of square kilometers. Remote dust deposition rates are increasing as a consequence of global change and may represent a mechanism for intercontinental microbial dispersal. Remote oligotrophic alpine lakes are particularly sensitive to dust inputs and can serve as sentinels of airborne microbial transport and the ecological consequences of accelerated intercontinental microbial migration. In this study, we applied high-throughput sequencing techniques (16S rRNA amplicon pyrosequencing) to characterize the microbial communities of atmospheric deposition collected in the Central Pyrenees (NE Spain) along three years. Additionally, bacteria from soils in Mauritania and from the air-water interface of high altitude Pyrenean lakes were also examined. Communities in aerosol deposition varied in time with a strong seasonal component of interannual similarity. Communities from the same season tended to resemble more each other than those from different seasons. Samples from disparate dates, in turn, slightly tended to have more dissimilar microbial assemblages (i.e., temporal distance decay), overall suggesting that atmospheric deposition may influence sink habitats in a temporally predictable manner. The three habitats examined (soil, deposition, and air-water interface) harbored distinct microbial communities, although airborne samples collected in the Pyrenees during Saharan dust outbreaks were closer to Mauritian soil samples than those collected during no Saharan dust episodes. The three habitats shared c.a. 1.4% of the total number of microbial sequences in the dataset. Such successful immigrants were spread in different bacterial classes. Overall, this study suggests that local and regional features may generate global trends in the dynamics and distribution of airborne microbial assemblages, and that the diversity of viable cells in the high atmosphere is likely higher than previously expected.

Nitrogen-cycling genes in epilithic biofilms of oligotrophic high-altitude lakes (central Pyrenees, Spain)

Microbial biofilms in oligotrophic environments are the most reactive component of the ecosystem. In high-altitude lakes, exposed bedrock, boulders, gravel, and sand in contact with highly oxygenated water and where a very thin epilithic biofilm develops usually dominate the littoral zone. Traditionally, these surfaces have been considered unsuitable for denitrification, but recent investigations have shown higher biological diversity than expected, including diverse anaerobic microorganisms. In this study, we explored the presence of microbial N-cycling nirS and nirK (denitrification through the conversion of NO2(-) to NO), nifH (N2 fixation), anammox (anaerobic ammonium oxidation), and amoA (aerobic ammonia oxidation, both bacterial and archaeal) genes in epilithic biofilms of a set of high-altitude oligotrophic lakes in the Pyrenees. The concentrations of denitrifying genes determined by quantitative PCR were two orders of magnitude higher than those of ammonia-oxidizing genes. Both types of genes were significantly correlated, suggesting a potential tight coupling nitrification-denitrification in these biofilms that deserves further confirmation. The nifH gene was detected after nested PCR, and no signal was detected for the anammox-specific genes used. The taxonomic composition of denitrifying and nitrogen-fixing genes was further explored by cloning and sequencing. Interestingly, both microbial functional groups were richer and more genetically diverse than expected. The nirK gene, mostly related to Alphaproteobacteria (Bradyrhizobiaceae), dominated the denitrifying gene pool as expected for oxygen-exposed habitats, whereas Deltaproteobacteria (Geobacter like) and Cyanobacteria were the most abundant among nitrogen fixers. Overall, these results suggest an epilithic community more metabolically diverse than previously thought and with the potential to carry out an active role in the biogeochemical nitrogen cycling of high-altitude ecosystems. Measurements of activity rates should be however carried out to substantiate and further explore these findings.

The microbial contribution to macroecology

There has been a recent explosion of research within the field of microbial ecology that has been fueled, in part, by methodological improvements that make it feasible to characterize microbial communities to an extent that was inconceivable only a few years ago. Furthermore, there is increasing recognition within the field of ecology that microorganisms play a critical role in the health of organisms and ecosystems. Despite these developments, an important gap still persists between the theoretical framework of macroecology and microbial ecology. We highlight two idiosyncrasies of microorganisms that are fundamental to understanding macroecological patterns and their mechanistic drivers. First, high dispersal rates provide novel opportunities to test the relative importance of niche, stochastic, and historical processes in structuring biological communities. Second, high speciation rates potentially lead to the convergence of ecological and evolutionary time scales. After reviewing these unique aspects, we discuss strategies for improving the conceptual integration of microbes into macroecology. As examples, we discuss the use of phylogenetic ecology as an integrative approach to explore patterns across the tree of life. Then we demonstrate how two general theories of biodiversity (i.e., the recently developed theory of stochastic geometry and the neutral theory) can be adapted to microorganisms. We demonstrate how conceptual models that integrate evolutionary and ecological mechanisms can contribute to the unification of microbial ecology and macroecology.

Environmental controls and composition of anoxygenic photoheterotrophs in ultraoligotrophic high-altitude lakes (Central Pyrenees)

The phylogenetic composition of freshwater anoxygenic photoheterotrophs (APs) has been poorly investigated as compared with their marine counterparts. In this study, we explored a set of ultraoligotrophic cold high mountain lakes (Central Pyrenees, Spain) by both pufM gene denaturing gradient gel electrophoresis fingerprinting, and cloning and sequencing of selected lakes samples. Different ranges of limnological and physico-chemical values were explored as environmental drivers of APs richness and composition. We did not observe significant relationships between richness/diversity of pufM and any of the limnological characteristics measured or trophic status, but a negative correlation with ammonia concentration. Conductivity, pH and nitrate concentration were significantly related to changes in APs community composition, whereas lake area, altitude, temperature and trophic status did not. Most of the sequences (> 85%) had the pufM sequences of Limnohabitans (Betaproteobacteria) as the closest relative in databases, whereas less abundant clones were more closely related to Rhodobacter, Sulfitobacter and Brevundimonas (Alphaproteobacteria), in agreement with 16S rRNA gene sequences previously found in the area. Congregibacter-like Gammaproteobacteria were not detected. Comparison with available studies in inland waters showed taxonomic partitioning along salinity gradients, and Congregibacter-like sequences restricted to high saline conditions in continental water bodies.

The phylogenetic and ecological context of cultured and whole genome-sequenced planktonic bacteria from the coastal NW Mediterranean Sea

Microbial isolates are useful models for physiological and ecological studies and can also be used to reassemble genomes from metagenomic analyses. However, the phylogenetic diversity that can be found among cultured marine bacteria may vary significantly depending on the isolation. Therefore, this study describes a set of 136 bacterial isolates obtained by traditional isolation techniques from the Blanes Bay Microbial Observatory, of which seven strains have had the whole genome sequenced. The complete set was compared to a series of environmental sequences obtained by culture-independent techniques (60 DGGE sequences and 303 clone library sequences) previously obtained by molecular methods. In this way, each isolate was placed in both its "ecological" (time of year, nutrient limitation, chlorophyll and temperature values) context or setting, and its "phylogenetic" landscape (i.e. similar organisms that were found by culture-independent techniques, when they were relevant, and when they appeared). Nearly all isolates belonged to the Gammaproteobacteria, Alphaproteobacteria, or the Bacteroidetes (70, 40 and 20 isolates, respectively). Rarefaction analyses showed similar diversity patterns for sequences from isolates and molecular approaches, except for Alphaproteobacteria where cultivation retrieved a higher diversity per unit effort. Approximately 30% of the environmental clones and isolates formed microdiversity clusters constrained at 99% 16S rRNA gene sequence identity, but the pattern was different in Bacteroidetes (less microdiversity) than in the other main groups. Seventeen cases (12.5%) of nearly complete (98-100%) rRNA sequence identity between isolates and environmental sequences were found: nine in the Alphaproteobacteria, five in the Gammaproteobacteria, and three in the Bacteroidetes, indicating that cultivation could be used to obtain at least some organisms representative of the various taxa detected by molecular methods. Collectively, these results illustrated the largely unexplored potential of culturing on standard media for complementing the study of microbial diversity by culture-independent techniques and for obtaining phylogenetically distinct model organisms from natural seawater.

Environmental distribution of two widespread uncultured freshwater Euryarchaeota clades unveiled by specific primers and quantitative PCR

Quantitative environmental distribution of two widely distributed uncultured freshwater Euryarchaeota with unknown functional role was explored by newly designed quantitative PCR primers targeting the 16S rRNA gene of clades Miscellaneous Euryarchaeota Group (MEG, containing the groups pMC2A384 and VALII/Eury4) and Deep-Sea Euryarchaeotal Groups (DSEG, targeting the cluster named VALIII containing the DHVE-3/DSEG, BC07-2A-27/DSEG-3 and DSEG-2 groups), respectively. The summer surface plankton of 28 lakes was analysed, and one additional dimictic deep alpine lake, Lake Redon, was temporally and vertically surveyed covering seasonal limnological variability. A trophic range between 0.2 and 5.2 μg l(-1) Chl a, and pH span from 3.8 to 9.5 was explored at altitudes between 632 and 2590 m above sea level. The primers showed to be highly selective with c. 85% coverage and 100% specificity. Only pH significantly explained the changes observed in gene abundances and environment. In Lake Redon, DSEG bloomed in deep stratified waters both in summer and early spring, and MEG at intermediate depths during the ice-cover period. Overall, MEG and DSEG showed a differential ecological distribution although correlational analyses indicated lack of coupling of both Euryarchaeota with phytoplankton (chlorophyll a). However, an intriguing positive and significant relationship was found between DSEG and putative ammonia oxidizing thaumarchaeota.

Targeting spatiotemporal dynamics of planktonic SAGMGC-1 and segregation of ammonia-oxidizing thaumarchaeota ecotypes by newly designed primers and quantitative polymerase chain reaction

The annual dynamics of three different ammonia-oxidizing archaea (AOA) ecotypes (amoA gene) and of the SAGMGC-1 (Nitrosotalea-like aquatic Thaumarchaeota) group (16S rRNA gene) were studied by newly designed specific primers and quantitative polymerase chain reaction analysis in a deep oligotrophic high mountain lake (Lake Redon, Limnological Observatory of the Pyrenees, Spain). We observed segregated distributions of the main AOA populations, peaking separately in time and space, and under different ammonia concentrations and irradiance conditions. Strong positive correlation in gene abundances was found along the annual survey between 16S rRNA SAGMAGC-1 and one of the amoA ecotypes suggesting the potential for ammonia oxidation in the freshwater SAGMAGC-1 clade. We also observed dominance of Nitrosotalea-like ecotypes over Nitrosopumilus-like (Marine Group 1.1a) and not the same annual dynamics for the two thaumarchaeotal clades. The fine scale segregation in space and time of the different AOA ecotypes indicated the presence of phylogenetically close but ecologically segregated AOA species specifically adapted to specific environmental conditions. It remains to be elucidated what would be such environmental drivers.

Stream hydrological fragmentation drives bacterioplankton community composition

In Mediterranean intermittent streams, the hydrological fragmentation in summer and the successive water flow re-convergence in autumn allow exploring how local processes shape the microbial community within the same habitat. The objectives of this study were to determine how bacterial community composition responded to hydrological fragmentation in summer, and to evaluate whether the seasonal shifts in community composition predominate over the effects of episodic habitat fragmentation. The bacterial community was assessed along the intermittent stream Fuirosos (Spain), at different levels of phylogenetic resolution by in situ hybridization, fingerprinting, and 16S rRNA gene sequencing. The hydrological fragmentation of the stream network strongly altered the biogeochemical conditions with the depletion of oxidized solutes and caused changes in dissolved organic carbon characteristics. In the isolated ponds, beta-Proteobacteria and Actinobacteria increased their abundance with a gradual reduction of the alpha-diversity as pond isolation time increased. Moreover, fingerprinting analysis clearly showed a shift in community composition between summer and autumn. In the context of a seasonal shift, the temporary stream fragmentation simultaneously reduced the microbial dispersion and affected local environmental conditions (shift in redox regime and quality of the dissolved organic matter) tightly shaping the bacterioplankton community composition.

Contrasting activity patterns determined by BrdU incorporation in bacterial ribotypes from the Arctic Ocean in winter

The winter Arctic Ocean is one of the most unexplored marine environments from a microbiological perspective. Heterotrophic bacteria maintain their activity at a baseline level during the extremely low-energy conditions of the winter, but little is known about the specific phylotypes that have the potential to survive and grow in such harsh environment. In this study, we aimed at identifying actively growing ribotypes in winter Arctic Ocean seawater cultures by experimental incubations with the thymidine analog bromodeoxyuridine (BrdU), followed by immunocapturing, terminal restriction fragment length polymorphism fingerprinting, cloning, and sequencing the 16S rRNA gene. We incubated water collected at different months over the Arctic winter and showed that the actively growing bacterial fraction, taking up BrdU, represented only a subset of the total community. Among the BrdU-labeled bacterial taxa we identified the Flavobacteria Polaribacter, the Alphaproteobacteria SAR11, the Gammaproteobacteria Arctic 96B-16 cluster and, predominately, members of Colwellia spp. Interestingly, Colwellia sequences formed three clusters (93 and 97% pairwise 16S rRNA identity) that contributed in contrasting ways to the active communities in the incubations. Polaribacter, Arctic 96B-16 and one cluster of Colwellia were more abundant in the active community represented by the BrdU-labeled DNA. In contrast, SAR11 and two other Colwellia clusters were underrepresented in the BrdU-labeled community compared to total communities. Despite the limitation of the long incubations needed to label slow growing arctic communities, the BrdU approach revealed the potential for active growth in low-energy conditions in some relevant groups of polar bacteria, including Polaribacter and Arctic 96B-16. Moreover, under similar incubation conditions, the growth of different Colwellia ribotypes varied, suggesting that related clusters of Colwellia may have distinct metabolic features.