Biology subjects

Amann, R.

Publications and source records attributed to Amann, R..

18 recordsLinked to original sources

bioRxiv2026

Metagenomic insights into microbial mediation of Caulerpa prolifera invasion in a coastal lagoon

Opportunistic seaweeds of the genus Caulerpa have benefited from human-driven environmental change to expand within coastal lagoons, frequently displacing native seagrasses. In the Mar Menor lagoon (southeastern Spain), Caulerpa prolifera has progressively expanded since the 1970s, coinciding with the decline of the native seagrass Cymodocea nodosa. Sediments colonized by Ca. prolifera are characterized by elevated sulfide concentrations, which are toxic to seagrasses. Because sulfide is microbially produced, these sediment communities likely shape the competition between Ca. prolifera and Cy. nodosa. However, their metabolic potential and activity remain poorly understood. Here, we examined the taxonomic composition and functional capabilities of sediment microbial communities using 12 paired metagenomes and metatranscriptomes from areas dominated by either Ca. prolifera or Cy. nodosa. Caulerpa-associated sediments displayed a higher potential for organic matter degradation and sulfate reduction. Several metagenome-assembled genomes (MAGs) encoded enzymes putatively involved in the degradation of pectin-like polysaccharides characteristic of the Caulerpa cell wall. Comparison of metagenomic and metatranscriptomic data further revealed that the most abundant MAGs were not necessarily the most transcriptionally active. This finding challenges the common assumption that ecological importance is primarily determined by abundance and highlights the potential contribution of less abundant microorganisms to biogeochemical cycling. Further, 97 of the 100 most highly expressed genes lacked functional annotation. Recombinant production and functional characterization of the protein encoded by the most highly expressed gene in the analyzed sediments revealed that it is an enzyme able to breakdown endogenous bacterial peptidoglycan. Collectively, these results underscore the importance of integrating metagenomics and metatranscriptomics with experimental validation to achieve a more comprehensive understanding of microbial functional capabilities, an integration that is still uncommon in the microbial ecology field.

microbiology↗

bioRxiv2025

Metagenomics reveal allopatric speciation and higher connectivity among coastal vs. inland hypersaline lakes and solar salterns

Hypersaline environments, due to their discrete and geographically isolated nature, constitute ideal systems for studying evolutionary patterns and microbial diversification, and especially here when contrasting coastal with inland systems. Based on metagenomic comparisons of 25 hypersaline sites across 11 countries, we explored the influence of environmental factors, ionic composition, and geographic distance on their microbial community structures and taxa diversification. Our results revealed that microbial communities from coastal environments were taxonomically and functionally more similar to each other than to those from inland sites. A distance-decay relationship in the genetic relatedness, significantly more pronounced for the coastal sites, was observed among reconstructed metagenome-assembled genomes (MAGs), clearer at distances below 400 km, but still detectable across global scales up to 20,000 km. The 484 MAGs recovered, representing 284 distinct species, revealed a striking global ubiquity, with 62.5% of the species showed cosmopolitanism as were detected across multiple sites. The higher taxonomic and genetic similarity of coastal environments over the inland sites seems to reflect an environmental connection that may be related to the ocean current dynamics. Most cosmopolitan species showed clear allopatric differentiation, although few cases of a single globally dominant genomovar (average nucleotide identity, ANI > 99.8%) were also observed, especially for some Haloquadratum species. The findings suggest that coastal hypersaline systems are loosely constrained by geographic isolation, with clear signals of allopatric speciation at the mesoscale (tenths to hundreds of kilometers) that become blurrier at larger scales.

microbiology↗

bioRxiv2025

Identification of key bacteria for ecological dynamics in a coastal marine observatory

Bacteria are essential for ecosystem maintenance, drive biogeochemical cycles, and influence responses to climate change. Additionally, many are involved in blooms and toxin production, affecting environmental quality. Despite their crucial role, some of their characteristics are only now being understood with the development of molecular techniques independent of cultivation. Thus, the recent effort to unify classical and microbial ecology studies has led to a better understanding of their dynamics. Here, we investigate the temporal processes that influence bacterial abundance and persistence, as well as their role in promoting or inhibiting the presence of certain organisms in the core of a metacommunity. Using molecular approaches such as amplicon and metagenomic analyses, we retrieved information on key bacteria (those with high abundance and/or persistence) from the South Atlantic Microbial Observatory (SAMO) to assess their interactions with environmental and biological factors. Bimodality was hardly observable, while the community structuring reflected the influence of large scale environmental processes in a recurrent pattern that clusters winter-spring and summer-autumn communities, reinforcing previous findings of intense influence of marine currents in this region. Among the key organisms, SAR11 and Flavobacteriaceae stand out. Flavobacteriaceae is known for its adaptive capacity and large genomes, while SAR11 has a reduced genome, making it dependent on compounds produced by other organisms. We identified an annual fluctuation in its abundance linked to Synechococcus blooms. The significant co-occurrence of ASVs from both groups reinforces the evidence of a biological interaction that sustains SAR11 through the exchange of these compounds. Through this study, we contribute to clarifying the factors that locally influence the geographic patterns of marine bacteria and identifying the pathways that promote their dynamics and functions in the ecosystem.

ecology↗

bioRxiv2025

Seasonal dynamics and thermal vulnerability of marine microbial communities in Uruguayan coastal waters

In an era of accelerating ocean warming, understanding the drivers of microbial community composition is more urgent than ever. Long-term time-series studies are particularly powerful for this purpose. However, most previous work remains confined to the Northern Hemisphere and focuses solely on taxonomy, leaving the Southern Hemisphere underrepresented and microorganisms functional dimensions underexplored. Moreover, phenomena such as current-driven community shifts and microbial vulnerability to warming remain poorly understood. Here, we address these gaps by analyzing microbial communities over two years at the South Atlantic Microbial Observatory (SAMO), located off the coast of Uruguay within a Marine Protected Area. SAMO, situated under the contrasting influences of the Brazil and Malvinas currents and recognized as a global-warming hotspot, provides an ideal setting to study the temporal dynamics of marine microbial communities. Using shotgun metagenomics and 16S rRNA amplicon sequencing, we revealed strong seasonal shifts driven by environmental variability and the influence of these two ocean currents, resulting in two community states (summer-autumn vs winter-spring). These states exhibited contrasting composition, diversity, and assembly mechanisms. Further, our results show that taxonomic and functional diversity trends diverge between these periods. Crucially, our work suggests that the predicted tropicalization of Uruguays coast is likely to boost taxonomic richness but erode functional repertoires, while also increasing vulnerability to ocean warming, potentially undermining biogeochemical functioning.

microbiology↗

bioRxiv2024

Resource partitioning in organosulfonate utilization by free-living heterotrophic bacteria during a North Sea microalgal bloom

Blooming microalgae (phytoplankton) release diverse organic molecules that fuel the marine pools of dissolved and particulate organic matter. A highly specialized community of heterotrophic bacteria rapidly remineralizes substantial parts of this organic matter in the sun-lit upper ocean. In particular, microalgae produce large quantities of various organosulfur compounds that can serve as carbon and sulfur sources for bacteria. Here, we report on the analyses of a time series of previously generated 30 long-read metagenomes, 30 corresponding deeply sequenced short-read metatranscriptomes and 15 metaproteomes from 0.2-3 {micro}m size fractions that we sampled in 2020 during a biphasic phytoplankton bloom in the German Bight (Southern North Sea). We analyzed the assembled contigs as well as 70 bacterial metagenome-assembled genomes that recruited the highest transcript numbers with respect to the utilization of methyl sulfur compounds (dimethylsulfoniopropionate (DMSP), dimethyl sulfide (DMS), dimethyl sulfone (DMSO2)), C3-sulfonates (2,3-dihydroxypropane-1-sulfonate (DHPS), 3-sulfolactate, 3-sulfopyruvate) and 2-aminoethanesulfonic acid (taurine). We observed a pronounced resource partitioning among bacterial clades that utilize distinct organosulfur compounds, which may explain successions of these clades during the studied bloom. Alphaproteobacteria were the most active and degraded a variety of organosulfonates via various metabolic routes. However, we also found previously underreported roles of members of the Bacteroidota and Gammaproteobacteria as efficient degraders of DMSP, DMS, and DMSO2. One striking observation was a strong preference for DMSP cleavage in Bacteroidota as opposed to DMSP demethylation in Alphaproteobacteria and indications for a particular proficiency for taurine utilization in Ilumatobacter_A and Acidimicrobiia. ImportanceSulfur-containing low-molecular-weight algal metabolites play an important role in overall marine carbon and sulfur fluxes. This study highlights that such compounds may play a crucial role in governing the succession of distinct bacterioplankton clades in response to phytoplankton blooms in coastal shelf areas of the temperate zone, such as the German Bight of the North Sea. While Alphaproteobacteria are the most versatile and competitive degraders of dissolved organosulfur compounds during such blooms, this study repositions clades previously thought to play only a more limited role in dissolved organosulfur metabolism in situ, such as Gammaproteobacteria, Bacteroidota, and Acidimicrobiia, as crucial contributors to the remineralization of organosulfur compounds in the upper ocean. This study also highlights the high level of interconnectedness of bacterial carbon and sulfur cycling during phytoplankton blooms.

ecology↗

bioRxiv2024

Deep long-read metagenomic sequencing reveals niche differentiation in carbon cycling potential between benthic and planktonic microbial populations

Coastal marine sediments function as large-scale natural biocatalytic filters, remineralizing and transforming organic matter. Benthic microbiomes exhibit remarkable temporal stability, contrasting with the dynamic, substrate-driven successions of bacterioplankton. Nonetheless, understanding their role in carbon cycling and interactions between these microbial groups is limited due to the complexity of benthic microbiomes. Here, we used a seasonally resolved, deep short- and long-read metagenomic approach to examine distinctive genomic features of microbiomes recovered from sediment, the overlaying water column, and particle-attached bacteria and archaea in the North Sea. We recovered 115 benthic metagenome-assembled genomes (MAGs) that belonged to Woeseiales, Rhizobiales, Planctomycetia, Gemmatimonadota, and Desulfobacterota species. While Proteobacteria and Actinobacteriota were characteristic phyla of sediments, Acidimicrobiia and Desulfocapsaceae species were shared between sediments and particle-attached fractions of the water column, indicative of significant bentho-pelagic coupling. Predominant members of the family Woeseiaceae carried polysaccharide utilization loci (PULs) predicted to target laminarin, alginate, and -glucan in sediments. In contrast, water column species lacked PULs and encoded a significantly higher fraction of sulfatases and peptidases, indicating a role in the degradation of protein-rich and sulfated organic matter. Our findings disentangle family-level adaptations and niche differentiation between globally significant benthic and water column populations involved in marine organic matter degradation and carbon storage.

microbiology↗

bioRxiv2024

Microbial species exist and are maintained by ecological cohesiveness coupled to high homologous recombination.

Recent analyses of metagenomes and genomes have revealed that microbial communities are predominantly composed of persistent, sequence-discrete species and intraspecies units (genomovars). To advance the species concept the underlying genetic or ecological mechanisms that maintain these discrete units need to be elucidated. By analyzing closely related isolate genomes from the same or related samples we show that high ecological cohesiveness coupled to frequent-enough and unbiased (i.e., not selection driven) horizontal gene flow, mediated by homologous recombination, often underlie these diversity patterns. Ecological cohesiveness was inferred based on higher similarity in abundance patterns of genomes of the same vs. different units, while recombination frequency was shown to have two times or more impact on sequence evolution than point mutation. Therefore, our results represent a departure compared to previous models of microbial speciation that invoke either ecology or selection-driven recombination, but not their synergistic effect, as the mechanism of unit cohesion. These results were observed in both Salinibacter ruber, an environmental halophilic organism, and Escherichia coli, the model gut-associated organism and an opportunistic pathogen, indicating that they may be more broadly applicable to the microbial world. Therefore, our results have strong implications for how to identify and regulate microbial species and genomovars of clinical or environmental importance and answer an important question for microbiology: what a species is. SIGNIFICANCEA highly pressing issue to resolve toward advancing the species concept for microbes (i.e., "what a species is") is to elucidate the underlying mechanisms for creating and maintaining species- and intraspecies-level gaps in diversity, or simply "clusters". In this study, we provide a novel methodology and the appropriate data to elucidate these mechanisms, and thus provide a mechanistic explanation of how the evolution of species- and strain-level clusters takes place. Specifically, our results show that several bacteria may be evolving and speciating much more sexually than previously thought, even under conditions of no strong positive selection for DNA exchange (i.e., neutral conditions). These results have major implications for better understanding and modeling microbial diversity on the planet.

microbiology↗

bioRxiv2024

Seasonal recurrence and modular assembly of an Arctic pelagic marine microbiome

Deciphering how microbial communities are shaped by environmental variability is fundamental for understanding the structure and function of ocean ecosystems. Thus far, we know little about the structuring of community functionality and the coupling between taxonomy and function over seasonal environmental gradients. To address this, we employed autonomous sampling devices and in situ sensors to investigate the taxonomic and functional dynamics of a pelagic Arctic Ocean microbiome over a four-year period. We demonstrate that the dominant prokaryotic and microeukaryotic populations exhibit recurrent, unimodal fluctuations each year, with community gene content following the same trend. The recurrent dynamics within the prokaryotic microbiome are structured into five temporal modules that represent distinct ecological states, characterised by unique taxonomic and metabolic signatures and connections to specific microeukaryotic populations and oceanographic conditions. For instance, Cand. Nitrosopumilus and the machinery to oxidise ammonia and reduce nitrite are signatures of early polar night, along with Radiolarians. In contrast, late summer is characterised by Amylibacter, sulfur compound metabolism and diverse Haptophyta lineages. Exploring the composition of modules further along with their degree of functional redundancy and the structuring of genetic diversity within functions over time revealed seasonal heterogeneity in environmental selection processes. In particular, we observe strong selection pressure on a functional level in spring while late polar night features weaker selection pressure that likely acts on an organismal level. By integrating taxonomic, functional, and environmental information, our study provides fundamental insights into how microbiomes are structured under pronounced environmental variability in understudied, yet rapidly changing polar marine ecosystems.

microbiology↗

bioRxiv2023

Visualizing and identifying selfish bacteria: a hunting guide

Polysaccharides are dominant components of plant biomass, whose degradation is typically mediated by heterotrophic bacteria. These bacteria use extracellular enzymes to hydrolyze polysaccharides to oligosaccharides that are then also available to other bacteria. Recently, a new mechanism of polysaccharide processing - selfish uptake - has been recognized, initially among gut-derived bacteria. In selfish uptake, polysaccharides are bound at the outer membrane, partially hydrolyzed, and transported into the periplasmic space without loss of hydrolysis products, thus limiting the availability of smaller sugars to the surrounding environment. Selfish uptake is widespread in environments ranging from the oceans cool, oxygen-rich, organic carbon-poor waters to the warm, carbon-rich, anoxic environment of the human gut. We provide a detailed guide of how to hunt for selfish bacteria, including how to rapidly visualize selfish uptake in complex bacterial communities, identify selfish organisms, and distinguish the activity of selfish organisms from other members of the community.

microbiology↗

bioRxiv2023

Spatial heterogeneity in carbohydrates and their utilisation by microbial communities in the high North Atlantic

Carbohydrates are chemically and structurally diverse, represent a substantial fraction of marine organic matter and are key substrates for heterotrophic microbes. Studies on carbohydrate utilisation by marine microbes have been centred on phytoplankton blooms in temperate regions, while far less is known from high-latitude waters and during later seasonal stages. Here, we combine glycan microarrays and analytical chromatography with metagenomics and metatranscriptomics to show the spatial heterogeneity in glycan distribution and their utilisation by microbes in Atlantic waters of the Arctic during late summer. The composition and abundance of monomers and glycan structures in POM varied with location and depth. Complex fucose-containing sulfated polysaccharides, known to accumulate in the ocean, were consistently detected, suggesting limited degradation by microbes. In contrast, the more labile {beta}-1,3-glucan exhibited a patchy distribution, indicating local variations in primary productivity and rapid utilisation. Metatranscriptomics showed active and dynamic microbial populations that targeted specific glycans. Gene transcription of carbohydrate-active enzymes revealed narrow substrate niches for specialists, involving compounds such as -mannans and alginate, along with the targeting of communal substrates, such as laminarin, by multiple populations. The observed spatial heterogeneity indicates that local biological and physical processes continue to shape the carbohydrate pool during late summer in high latitude waters and microbial populations are active and responsive to such changes.

microbiology↗

bioRxiv2023

Towards estimating the number of strains that make up a natural bacterial population

What a strain is and how many strains make up a natural bacterial population remain elusive concepts despite their apparent importance for assessing the role of intra-population diversity in disease emergence or response to environmental perturbations. To advance these concepts, we sequenced 138 randomly selected Salinibacter ruber isolates from two solar salterns and assessed these genomes against companion short-read metagenomes from the same samples. The distribution of genome-aggregate average nucleotide identity (ANI) values among these isolates revealed a bimodal distribution, with significantly lower occurrence of values between 99.2% and 99.8% relative to ANI >99.8% or <99.2%, revealing a natural "gap" in the sequence space within species. Accordingly, we used this ANI gap to define genomovars and a higher ANI value of >99.99% and shared gene-content >99.0% to define strains. Using these thresholds and extrapolating from how many metagenomic reads each genomovar uniquely recruited, we estimated that -although our 138 isolates represented about 80% of the Sal. ruber population- the total population in one pond is composed of 5,500 to 11,000 genomovars, the great majority of which appear to be rare in situ. These data also revealed that the most frequently recovered isolate in lab media was often not the most abundant genomovar in situ, suggesting that cultivation biases are significant, even in cases that cultivation procedures are thought to be robust. Preliminary analyses of available genomes revealed that the thresholds used for defining strains and distinct intra-species units (genomovars) may be broadly applicable to additional bacterial species. Significance StatementStrains are the smallest distinguishable units within a microbial species. Strains that carry unique gene content often underly the emergence of disease outbreaks and the response of the species to environmental perturbations. Therefore, a major challenge in microbiome research across environmental and clinical settings is to evaluate how many strains of the same species coexist in nature and how dominant strains emerge from this diversity. Unfortunately, the available theoretical concept of strain is not directly applicable to culture-independent surveys. Here, we provide such a practical definition for strain and use it to show that that the number of strains making up a natural bacterial population may be large, in the order of a few thousands, but not infinite.

microbiology↗

bioRxiv2022

Variations in Atlantic water influx and sea-ice cover drive taxonomic and functional shifts in Arctic marine bacterial communities

The Arctic Ocean is experiencing unprecedented changes as a result of climate warming, necessitating detailed analyses on the ecology and dynamics of biological communities to understand current and future ecosystem shifts. Here we show the pronounced impact that variations in Atlantic water influx and sea-ice cover have on bacterial communities in the East Greenland Current (Fram Strait) using two, 2-year high-resolution amplicon datasets and an annual cycle of long-read metagenomes. Densely ice-covered polar waters harboured a temporally stable, resident microbiome. In contrast, low-ice cover and Atlantic water influx shifted community dominance to seasonally fluctuating populations enriched in genes for phytoplankton-derived organic matter degradation. We identified signature populations associated with distinct oceanographic conditions and predicted their ecological niches. Our study indicates progressing "Biological Atlantification" in the Arctic Ocean, where the niche space of Arctic bacterial populations will diminish, while communities that taxonomically and functionally resemble those in temperate oceans will become more widespread.

microbiology↗

bioRxiv2021

Selfish bacteria are active throughout the water column of the ocean

Heterotrophic bacteria use extracellular enzymes to hydrolyze high molecular weight (HMW) organic matter to low molecular weight (LMW) hydrolysis products that can be taken into the cell. These enzymes represent a considerable investment of carbon, nitrogen, and energy, yet the return on this investment is uncertain, since hydrolysis of a HMW substrate outside a cell yields LMW products that can be lost to diffusion and taken up by scavengers that do not produce extracellular enzymes1. However, an additional strategy of HMW organic matter utilization, selfish uptake2, is used for polysaccharide degradation, and has recently been found to be widespread among bacterial communities in surface ocean waters3. During selfish uptake, polysaccharides are bound at the cell surface, initially hydrolyzed, and transported into the periplasmic space without loss of hydrolysis products2, thereby retaining hydrolysate for the selfish bacteria and reducing availability of LMW substrates to scavenging bacteria. Here we show that selfish bacteria are common not only in the sunlit upper ocean, where polysaccharides are freshly produced by phytoplankton, but also deeper in the oceanic water column, including in bottom waters at depths of more than 5,500 meters. Thus, the return on investment, and therefore also the supply of suitable polysaccharides, must be sufficient to maintain these organisms.

microbiology↗

bioRxiv2021

Highly diverse flavobacterial phages isolated from North Sea spring blooms

It is generally recognized that phages have a modulating role in the marine environment. Therefore, we hypothesized that phages can be a mortality factor for the dense heterotrophic bacterial population succeeding in phytoplankton blooms. For the marine carbon cycle, spring phytoplankton blooms are important recurring events. In this study, we focused on Flavobacteriia, because they are main responders during these blooms and have an important role in the degradation of polysaccharides. A cultivation-based approach was used, obtaining 44 lytic flavobacterial phages (flavophages), representing twelve new species from two viral realms - Duplodnaviria and Monodnaviria. Taxonomic analysis allowed us to delineate ten new phage genera and seven new families, from which nine and four, respectively, had no previously cultivated representatives. Genomic analysis predicted various life styles and genomic replication strategies. A likely eukaryote-associated host habitat was reflected in the gene content of some of the flavophages. Detection in cellular metagenomes and by direct-plating indicated that part of these phages were actively replicating in the environment during the 2018 spring bloom. Furthermore, CRISPR/Cas spacers and re-isolation during two consecutive years indicated that, at least part of the new flavophages are stable components of the microbial community in the North Sea. Together, our results indicate that these diverse flavophages have the potential to modulate their respective host populations.

microbiology↗

bioRxiv2020

Quantifying fluorescent glycan uptake to elucidate strain-level variability in foraging behaviors of rumen bacteria

Gut microbiomes have vast catabolic potential and are essential to host health and nutrition. An in-depth understanding of the metabolic pathways in these ecosystems will enable us to design treatments (i.e. prebiotics) that influence microbiome structure and enhance host physiology. Currently, the investigation of metabolic pathways relies on inferences derived from metagenomics or in vitro cultivations, however, novel approaches targeting specific cell physiologies can illuminate the functional potential encoded within microbial (meta)genomes to accurately assess metabolic abilities. Here, we present a multi-faceted study using complimentary next-generation physiology and omics approaches to characterize the microbial adaptation to a prebiotic in the rumen ecosystem. Using fluorescently labeled polysaccharides, we identified bacteria that actively metabolize a glycan prebiotic in the rumen microbiome ex vivo. Subsequently, we characterized strain-level variability in carbohydrate utilization systems and predict metabolic strategies of isolated bovine-adapted strains of Bacteroides thetaiotaomicron using comparative whole genome sequencing, RNA-Seq, and carbohydrate-active enzyme fingerprinting.

microbiology↗

bioRxiv2020

Candidatus Ethanoperedens, a thermophilic genus of archaea mediating the anaerobic oxidation of ethane

Cold seeps and hydrothermal vents deliver large amounts of methane and other gaseous alkanes into marine surface sediments. Consortia of archaea and partner bacteria thrive on the oxidation of these alkanes and its coupling to sulfate reduction. The inherently slow growth of the involved organisms and the lack of pure cultures have impeded the understanding of the molecular mechanisms of archaeal alkane degradation. Here, using hydrothermal sediments of the Guaymas Basin (Gulf of California) and ethane as substrate we cultured microbial consortia of a novel anaerobic ethane oxidizer Candidatus Ethanoperedens thermophilum (GoM-Arc1 clade) and its partner bacterium Candidatus Desulfofervidus auxilii previously known from methane-oxidizing consortia. The sulfate reduction activity of the culture doubled within one week, indicating a much faster growth than in any other alkane-oxidizing archaea described before. The dominance of a single archaeal phylotype in this culture allowed retrieving a closed genome of Ca. Ethanoperedens, a sister genus of the recently reported ethane oxidizer Candidatus Argoarchaeum. The metagenome-assembled genome of Ca. Ethanoperedens encoded for a complete methanogenesis pathway including a methyl-coenzyme M reductase (MCR) that is highly divergent from those of methanogens and methanotrophs. Combined substrate and metabolite analysis showed ethane as sole growth substrate and production of ethyl-coenzyme M as activation product. Stable isotope probing showed that the enzymatic mechanisms of ethane oxidation in Ca. Ethanoperedens is fully reversible, thus its enzymatic machinery has potential for the biotechnological development of microbial ethane production from carbon dioxide. IMPORTANCEIn the seabed gaseous alkanes are oxidized by syntrophic microbial consortia that thereby reduce fluxes of these compounds into the water column. Because of the immense quantities of seabed alkane fluxes, these consortia are key catalysts of the global carbon cycle. Due to their obligate syntrophic lifestyle, the physiology of alkane-degrading archaea remains poorly understood. We have now cultivated a thermophilic, relatively fast-growing ethane oxidizer in partnership with a sulfate-reducing bacterium known to aid in methane oxidation, and have retrieved the first complete genome of a short-chain alkane-degrading archaeon. This will greatly enhance the understanding of non-methane alkane activation by non-canonical methyl-coenzyme M reductase enzymes, and provide insights into additional metabolic steps and the mechanisms underlying syntrophic partnerships. Ultimately, this knowledge could lead to the biotechnological development of alkanogenic microorganisms to support the carbon neutrality of industrial processes. EtymologyEthanoperedens. ethano, (new Latin): pertaining to ethane; peredens (Latin): consuming, devouring; thermophilum. (Greek): heat-loving. The name implies an organism capable of ethane oxidation at elevated temperatures. LocalityEnriched from hydrothermally heated, hydrocarbon-rich marine sediment of the Guaymas Basin at 2000 m water depth, Gulf of California, Mexico. DiagnosisAnaerobic, ethane-oxidizing archaeon, mostly coccoid, about 0.7 m in diameter, forms large irregular cluster in large dual-species consortia with the sulfate-reducing partner bacterium Candidatus Desulfofervidus auxilii.

microbiology↗

bioRxiv2020

Short-term changes in polysaccharide utilization mechanisms of marine bacterioplankton during a spring phytoplankton bloom

Spring phytoplankton blooms in temperate environments contribute disproportionately to global marine productivity. Bloom-derived organic matter, much of it occurring as polysaccharides, fuels biogeochemical cycles driven by interacting autotrophic and heterotrophic communities. We tracked changes in the mode of polysaccharide utilization by heterotrophic bacteria during the course of a diatom-dominated bloom in the German Bight, North Sea. Polysaccharides can be taken up in a selfish mode, where initial hydrolysis is coupled to transport into the periplasm, such that little to no low molecular weight (LMW) products are externally released to the environment. Alternatively, polysaccharides hydrolyzed by cell-surface attached or free extracellular enzymes (external hydrolysis) yield LMW products available to the wider bacterioplankton community. In the early bloom phase, selfish activity was accompanied by low extracellular hydrolysis rates of a few polysaccharides. As the bloom progressed, selfish uptake increased markedly, and external hydrolysis rates increased, but only for a limited range of substrates. The late bloom phase was characterized by high external hydrolysis rates of a broad range of polysaccharides, and reduced selfish uptake of polysaccharides, except for laminarin. Substrate utilization mode is related both to substrate structural complexity and to the bloom-stage dependent composition of the heterotrophic bacterial community. Originality statementThe means by which heterotrophic bacteria cooperate and compete to obtain substrates is a key factor determining the rate and location at which organic matter is cycled in the ocean. Much of this organic matter is high molecular weight (HMW), and must be enzymatically hydrolyzed to smaller pieces to be processed by bacterial communities. Some of these enzyme-producing bacteria are selfish, processing HMW organic matter without releasing low molecular weight (LMW) products to the environment. Other bacteria hydrolyze HMW substrates in a manner that releases LMW products to the wider bacterial community. How these mechanisms of substrate hydrolysis work against a changing background of organic matter supply is unclear. Here, we measured changing rates and mechanisms of substrate processing during the course of a natural phytoplankton bloom in the North Sea. Selfish bacteria generally dominate in the initial bloom stages, but a greater supply of increasingly complex substrates in later bloom stages leads to external hydrolysis of a wider range of substrates, increasing the supply of LMW hydrolysis products to the wider bacterial community.

microbiology↗

bioRxiv2019

In situ abundance and carbon fixation activity of distinct anoxygenic phototrophs in the stratified seawater lake Rogoznica

Sulfide-driven anoxygenic photosynthesis is an ancient microbial metabolism that contributes significantly to inorganic carbon fixation in stratified, sulfidic water bodies. Methods commonly applied to quantify inorganic carbon fixation by anoxygenic phototrophs, however, cannot resolve the contributions of distinct microbial populations to the overall process. We implemented a straightforward workflow, consisting of radioisotope labeling and flow cytometric cell sorting based on the distinct autofluorescence of bacterial photo pigments, to discriminate and quantify contributions of co-occurring anoxygenic phototrophic populations to in situ inorganic carbon fixation in environmental samples. This allowed us to assign 89.3 {+/-}7.6% of daytime inorganic carbon fixation by anoxygenic phototrophs in Lake Rogoznica (Croatia) to an abundant chemocline-dwelling population of green sulfur bacteria (dominated by Chlorobium phaeobacteroides), whereas the co-occurring purple sulfur bacteria (Halochromatium sp.) contributed only 1.8 {+/-}1.4%. Furthermore, we obtained two metagenome assembled genomes of green sulfur bacteria and one of a purple sulfur bacterium which provides the first genomic insights into the genus Halochromatium, confirming its high metabolic flexibility and physiological potential for mixo-and heterotrophic growth.

microbiology↗