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Chromium stable isotope composition (δ53Cr) is a promising tracer for redox conditions throughout Earth's history; however, the geochemical controls of δ53Cr have not been assessed in modern redox-stratified basins. We present new chromium (Cr) concentration and δ53Cr data in dissolved, sinking particulate, and sediment samples from the redox-stratified Lake Cadagno (Switzerland), a modern Proterozoic ocean analog. These data demonstrate isotope fractionation during incomplete (non-quantitative) reduction and removal of Cr above the chemocline, driving isotopically light Cr accumulation in euxinic deep waters. Sediment authigenic Cr is isotopically distinct from overlying waters but comparable to average continental crust. New and published data from other redox-stratified basins show analogous patterns. This challenges assumptions from δ53Cr paleoredox applications that quantitative Cr reduction and removal limits isotope fractionation. Instead, fractionation from non-quantitative Cr removal leads to sedimentary records offset from overlying waters and not reflecting high δ53Cr from oxidative continental weathering.
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Bacterial and archaeal communities inhabiting the subsurface seabed live under strong energy limitation and have growth rates that are orders of magnitude slower than laboratory-grown cultures. It is not understood how subsurface microbial communities are assembled and whether populations undergo adaptive evolution or accumulate mutations as a result of impaired DNA repair under such energy-limited conditions. Here we use amplicon sequencing to explore changes of microbial communities during burial and isolation from the surface to the >5,000-y-old subsurface of marine sediment and identify a small core set of mostly uncultured bacteria and archaea that is present throughout the sediment column. These persisting populations constitute a small fraction of the entire community at the surface but become predominant in the subsurface. We followed patterns of genome diversity with depth in four dominant lineages of the persisting populations by mapping metagenomic sequence reads onto single-cell genomes. Nucleotide sequence diversity was uniformly low and did not change with age and depth of the sediment. Likewise, there was no detectable change in mutation rates and efficacy of selection. Our results indicate that subsurface microbial communities predominantly assemble by selective survival of taxa able to persist under extreme energy limitation.
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Evolução Biológica , Sedimentos Geológicos/microbiologia , Metagenômica , Microbiota , Archaea/classificação , Archaea/genética , Bactérias/classificação , Bactérias/genética , Biodiversidade , Biomassa , Variação Genética , Metagenômica/métodos , MutaçãoRESUMO
Double-stranded RNA (dsRNA) pesticides are a new generation of crop protectants that interfere with protein expression in targeted pest insects by a cellular mechanism called RNA interference (RNAi). The ecological risk assessment of these emerging pesticides necessitates an understanding of the fate of dsRNA molecules in receiving environments, among which agricultural soils are most important. We herein present an experimental approach using phosphorus-32 (32P)-radiolabeled dsRNA that allows studying key fate processes of dsRNA in soils with unprecedented sensitivity. This approach resolves previous analytical challenges in quantifying unlabeled dsRNA and its degradation products in soils. We demonstrate that 32P-dsRNA and its degradation products are quantifiable at concentrations as low as a few nanograms of dsRNA per gram of soil by both Cerenkov counting (to quantify total 32P-activity) and by polyacrylamide gel electrophoresis followed by phosphorimaging (to detect intact 32P-dsRNA and its 32P-containing degradation products). We show that dsRNA molecules added to soil suspensions undergo adsorption to soil particle surfaces, degradation in solution, and potential uptake by soil microorganisms. The results of this work on dsRNA adsorption and degradation advance a process-based understanding of the fate of dsRNA in soils and will inform ecological risk assessments of emerging dsRNA pesticides.
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Praguicidas , RNA de Cadeia Dupla , Adsorção , Animais , Interferência de RNA , SoloRESUMO
Half of the microbial cells in the Earth's oceans are found in sediments. Many of these cells are members of the Archaea, single-celled prokaryotes in a domain of life separate from Bacteria and Eukaryota. However, most of these archaea lack cultured representatives, leaving their physiologies and placement on the tree of life uncertain. Here we show that the uncultured miscellaneous crenarchaeotal group (MCG) and marine benthic group-D (MBG-D) are among the most numerous archaea in the marine sub-sea floor. Single-cell genomic sequencing of one cell of MCG and three cells of MBG-D indicated that they form new branches basal to the archaeal phyla Thaumarchaeota and Aigarchaeota, for MCG, and the order Thermoplasmatales, for MBG-D. All four cells encoded extracellular protein-degrading enzymes such as gingipain and clostripain that are known to be effective in environments chemically similar to marine sediments. Furthermore, we found these two types of peptidase to be abundant and active in marine sediments, indicating that uncultured archaea may have a previously undiscovered role in protein remineralization in anoxic marine sediments.
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Archaea/metabolismo , Sedimentos Geológicos/microbiologia , Peptídeo Hidrolases/metabolismo , Adesinas Bacterianas/metabolismo , Archaea/classificação , Archaea/enzimologia , Archaea/genética , Cisteína Endopeptidases/metabolismo , Sedimentos Geológicos/química , Cisteína Endopeptidases Gingipaínas , Biologia Marinha , Dados de Sequência Molecular , Filogenia , Proteólise , RNA Ribossômico 16S/genética , Análise de Célula ÚnicaRESUMO
Peat particulate organic matter (POM) is an important terminal electron acceptor for anaerobic respiration in northern peatlands provided that the electron-accepting capacity of POM is periodically restored by oxidation with O2 during peat oxygenation events. We employed push-pull tests with dissolved O2 as reactant to determine pseudo-first-order rate constants of O2 consumption ( kobs) in anoxic peat soil of an unperturbed Swedish ombrotrophic bog. Dissolved O2 was rapidly consumed in anoxic peat with a mean kobs of 2.91 ± 0.60 h-1, corresponding to an O2 half-life of â¼14 min. POM dominated O2 consumption, as evidenced from approximately 50-fold smaller kobs in POM-free control tests. Inhibiting microbial activity with formaldehyde did not appreciably slow O2 consumption, supporting abiotic O2 reduction by POM moieties, not aerobic respiration, as the primary route of O2 consumption. Peat preoxygenation with dissolved O2 lowered kobs in subsequent oxygen consumption tests, consistent with depletion of reduced moieties in POM. Finally, repeated oxygen consumption tests demonstrated that anoxic peat POM has a high reduction capacity, in excess to 20 µmol electrons donated per gram POM. This work demonstrates rapid abiotic oxidation of reduced POM by O2, supporting that short-term oxygenation events can restore the capacity of POM to accept electrons from anaerobic respiration in temporarily anoxic parts of peatlands.
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Oxigênio , Solo , Oxirredução , Consumo de Oxigênio , Material ParticuladoRESUMO
Although subseafloor sediments are known to harbour a vast number of microbial cells, the distribution, diversity, and origins of fungal populations remain largely unexplored. In this study, we cultivated fungi from 34 of 47 deep coal-associated sediment samples collected at depths ranging from 1289 to 2457 m below the seafloor (mbsf) off the Shimokita Peninsula, Japan (1118 m water depth). We obtained a total of 69 fungal isolates under strict contamination controls, representing 61 Ascomycota (14 genera, 23 species) and 8 Basidiomycota (4 genera, 4 species). Penicillium and Aspergillus relatives were the most dominant genera within the Ascomycetes, followed by the members of genera Cladosporium, Hamigera, Chaetomium, Eutypella, Acremonium, Aureobasidium, Candida, Eurotium, Exophiala, Nigrospora, Bionectria and Pseudocercosporella. Four Basidiomycota species were identified as genera Schizophyllum, Irpex, Bjerkandera and Termitomyces. Among these isolates, Cladosporium sphaerospermum and Aspergillus sydowii relatives were isolated from a thin lignite coal-sandstone formation at 2457 mbsf. Our results indicate that these cultivable fungal populations are indigenous, originating from past terrigenous environments, which have persisted, possibly as spores, through â¼20 million years of depositional history.
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Carvão Mineral/microbiologia , Fungos/isolamento & purificação , Sedimentos Geológicos/microbiologia , Água do Mar/microbiologia , Fungos/classificação , Fungos/genética , Fungos/crescimento & desenvolvimento , Japão , Oceanos e Mares , FilogeniaRESUMO
Most sulfate-reducing microorganisms (SRMs) present in subsurface marine sediments belong to uncultured groups only distantly related to known SRMs, and it remains unclear how changing geochemical zones and sediment depth influence their community structure. We mapped the community composition and abundance of SRMs by amplicon sequencing and quantifying the dsrB gene, which encodes dissimilatory sulfite reductase subunit beta, in sediment samples covering different vertical geochemical zones ranging from the surface sediment to the deep sulfate-depleted subsurface at four locations in Aarhus Bay, Denmark. SRMs were present in all geochemical zones, including sulfate-depleted methanogenic sediment. The biggest shift in SRM community composition and abundance occurred across the transition from bioturbated surface sediments to nonbioturbated sediments below, where redox fluctuations and the input of fresh organic matter due to macrofaunal activity are absent. SRM abundance correlated with sulfate reduction rates determined for the same sediments. Sulfate availability showed a weaker correlation with SRM abundances and no significant correlation with the composition of the SRM community. The overall SRM species diversity decreased with depth, yet we identified a subset of highly abundant community members that persists across all vertical geochemical zones of all stations. We conclude that subsurface SRM communities assemble by the persistence of members of the surface community and that the transition from the bioturbated surface sediment to the unmixed sediment below is a main site of assembly of the subsurface SRM community.IMPORTANCE Sulfate-reducing microorganisms (SRMs) are key players in the marine carbon and sulfur cycles, especially in coastal sediments, yet little is understood about the environmental factors controlling their depth distribution. Our results suggest that macrofaunal activity is a key driver of SRM abundance and community structure in marine sediments and that a small subset of SRM species of high relative abundance in the subsurface SRM community persists from the sulfate-rich surface sediment to sulfate-depleted methanogenic subsurface sediment. More generally, we conclude that SRM communities inhabiting the subsurface seabed assemble by the selective survival of members of the surface community.
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Bactérias/isolamento & purificação , Bactérias/metabolismo , Sedimentos Geológicos/microbiologia , Sulfatos/metabolismo , Bactérias/classificação , Bactérias/genética , Baías/microbiologia , DNA Bacteriano/genética , Dinamarca , Oxirredução , Filogenia , RNA Ribossômico 16S/genética , Água do Mar/microbiologiaRESUMO
The zonation of anaerobic methane-cycling Archaea in hydrothermal sediment of Guaymas Basin was studied by general primerpairs (mcrI, ME1/ME2, mcrIRD) targeting the alpha subunit of methyl coenzyme M reductase gene (mcrA) and by new group specific mcrA and 16S rRNA gene primer pairs. The mcrIRD primer pair outperformed the other general mcrA primer pairs indetection sensitivity and phylogenetic coverage. Methanotrophic ANME-1 Archaea were the only group detected with group specific primers only. The detection of 14 mcrA lineages surpasses the diversity previously found in this location. Most phylotypes have high sequence similarities to hydrogenotrophs, methylotrophs, and anaerobic methanotrophs previously detected at Guaymas Basin or at hydrothermal vents, cold seeps, and oil reservoirs worldwide. Additionally, five mcrA phylotypes belonging to newly defined lineages are detected. Two of these belong to deeply branching new orders, while the others are new species or genera of Methanopyraceae and Methermicoccaceae. Downcore diversity decreases from all groups detected in the upper 6 cm(2 to 40 °C, sulfate measurable to 4 cm) to only two groups below 6 cm (>40 °C). Despite the presence of hyperthermophilic genera (Methanopyrus, Methanocaldococcus) in cooler surface strata, no genes were detected below 10 cm (>60 °C). While mcrAbased and 16S rRNA gene-based community compositions are generally congruent, the deeply branching mcrA cannot be assigned to specific 16S rRNA gene lineages. Our study indicates that even among well-studied metabolic groups and in previously characterized model environments, major evolutionary branches are overlooked. Detecting these groups by improved molecular biological methods is a crucial first step toward understanding their roles in nature.
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Archaea/isolamento & purificação , Archaea/metabolismo , Sedimentos Geológicos/microbiologia , Metano/metabolismo , Archaea/classificação , Archaea/genética , Biodiversidade , Primers do DNA/genética , Sedimentos Geológicos/química , Dados de Sequência Molecular , Filogenia , Reação em Cadeia da Polimerase , Especificidade da EspécieRESUMO
Emissions of microbially produced methane (CH4) from lake sediments are a major source of this potent greenhouse gas to the atmosphere. The rates of CH4 production and emission are believed to be influenced by electron acceptor distributions and organic carbon contents, which in turn are affected by anthropogenic inputs of nutrients leading to eutrophication. Here, we investigate how eutrophication influences the abundance and community structure of CH4 producing Archaea and methanogenesis pathways across time-resolved sedimentary records of five Swiss lakes with well-characterized trophic histories. Despite higher CH4 concentrations which suggest higher methanogenic activity in sediments of eutrophic lakes, abundances of methanogens were highest in oligotrophic lake sediments. Moreover, while the methanogenic community composition differed significantly at the lowest taxonomic levels (OTU), depending on whether sediment layers had been deposited under oligotrophic or eutrophic conditions, it showed no clear trend in relation to in situ distributions of electron acceptors. Remarkably, even though methanogenesis from CO2-reduction was the dominant pathway in all sediments based on carbon isotope fractionation values, taxonomic identities, and genomes of resident methanogens, CO2-reduction with hydrogen (H2) was thermodynamically unfavorable based on measured reactant and product concentrations. Instead, strong correlations between genomic abundances of CO2-reducing methanogens and anaerobic bacteria with potential for extracellular electron transfer suggest that methanogenic CO2-reduction in lake sediments is largely powered by direct electron transfer from syntrophic bacteria without involvement of H2 as an electron shuttle.
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The production and anaerobic oxidation of methane (AOM) by microorganisms is widespread in organic-rich deep subseafloor sediments. Yet, the organisms that carry out these processes remain largely unknown. Here we identify members of the methane-cycling microbial community in deep subsurface, hydrate-containing sediments of the Peru Trench by targeting functional genes of the alpha subunit of methyl coenzyme M reductase (mcrA). The mcrA profile reveals a distinct community zonation that partially matches the zonation of methane oxidizing and -producing activity inferred from sulfate and methane concentrations and carbon-isotopic compositions of methane and dissolved inorganic carbon (DIC). McrA appears absent from sulfate-rich sediments that are devoid of methane, but mcrA sequences belonging to putatively methane-oxidizing ANME-1a-b occur from the zone of methane oxidation to several meters into the methanogenesis zone. A sister group of ANME-1a-b, referred to as ANME-1d, and members of putatively aceticlastic Methanothrix (formerly Methanosaeta) occur throughout the remaining methanogenesis zone. Analyses of 16S rRNA and mcrA-mRNA indicate that the methane-cycling community is alive throughout (rRNA to 230 mbsf) and active in at least parts of the sediment column (mRNA at 44 mbsf). Carbon-isotopic depletions of methane relative to DIC (-80 to -86) suggest mostly methane production by CO2 reduction and thus seem at odds with the widespread detection of ANME-1 and Methanothrix. We explain this apparent contradiction based on recent insights into the metabolisms of both ANME-1 and Methanothricaceae, which indicate the potential for methanogenetic growth by CO2 reduction in both groups.
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Intertidal sands are global hotspots of terrestrial and marine carbon cycling with strong hydrodynamic forcing by waves and tides and high macrofaunal activity. Yet, the relative importance of hydrodynamics and macrofauna in controlling these ecosystems remains unclear. Here, we compare geochemical gradients and bacterial, archaeal, and eukaryotic gene sequences in intertidal sands dominated by subsurface deposit-feeding worms (Abarenicola pacifica) to adjacent worm-free areas. We show that hydrodynamic forcing controls organismal assemblages in surface sediments, while in deeper layers selective feeding by worms on fine, algae-rich particles strongly decreases the abundance and richness of all three domains. In these deeper layers, bacterial and eukaryotic network connectivity decreases, while percentages of clades involved in degradation of refractory organic matter, oxidative nitrogen, and sulfur cycling increase. Our findings reveal macrofaunal activity as the key driver of biological community structure and functioning, that in turn influence carbon cycling in intertidal sands below the mainly physically controlled surface layer.
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The greenhouse gas methane (CH4) is of pivotal importance for Earth's climate system and as a human energy source. A significant fraction of this CH4 is produced by anaerobic Archaea. Here, we describe the first CH4 production by facultative anaerobic wood-rot fungi during growth on hydroxylated/carboxylated aromatic compounds, including lignin and lignite. The amount of CH4 produced by fungi is positively correlated with the amount of CH3Cl produced during the rapid growth period of the fungus. Biochemical, genetic, and stable isotopic tracer analyses reveal the existence of a novel halomethane-dependent fungal CH4 production pathway during the degradation of phenol and benzoic acid monomers and polymers and utilization of cyclic sugars. Even though this halomethane-dependent pathway may only play a side role in anaerobic fungal activity, it could represent a globally significant, previously overlooked source of biogenic CH4 in natural ecosystems. IMPORTANCE Here, we demonstrate that wood-rot fungi produce methane anaerobically without the involvement of methanogenic archaea via a new, halomethane-dependent pathway. These findings of an anaerobic fungal methane formation pathway open another avenue in methane research and will further assist with current efforts in the identification of the processes involved and their ecological implications.
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Gases de Efeito Estufa , Madeira , Humanos , Madeira/química , Madeira/metabolismo , Madeira/microbiologia , Lignina/metabolismo , Anaerobiose , Ecossistema , Gases de Efeito Estufa/análise , Gases de Efeito Estufa/metabolismo , Metano/análise , Metano/metabolismo , Archaea/metabolismo , Fungos/genética , Fungos/metabolismo , Carvão Mineral/análise , Açúcares/metabolismo , Fenóis/análise , Fenóis/metabolismo , Ácido Benzoico/análise , Ácido Benzoico/metabolismoRESUMO
The nitrogen (N) cycle is of global importance, as N is an essential element and a limiting nutrient in terrestrial and aquatic ecosystems. Excessive anthropogenic N fertilizer usage threatens sensitive downstream aquatic ecosystems. Although freshwater lake sediments remove N through various microbially mediated processes, few studies have investigated the microbial communities involved. In an integrated biogeochemical and microbiological study on a eutrophic and oligotrophic lake, we estimated N removal rates from pore water concentration gradients in sediments. Simultaneously, the abundance of different microbial N transformation genes was investigated using metagenomics on a seasonal and spatial scale. We observed that contrasting nutrient concentrations in sediments were associated with distinct microbial community compositions and significant differences in abundances of various N transformation genes. For both characteristics, we observed a more pronounced spatial than seasonal variability within each lake. The eutrophic Lake Baldegg showed a higher denitrification potential with higher nosZ gene (N2O reductase) abundances and higher nirS:nirK (nitrite reductase) ratios, indicating a greater capacity for complete denitrification. Correspondingly, this lake had a higher N removal efficiency. The oligotrophic Lake Sarnen, in contrast, had a higher potential for nitrification. Specifically, it harbored a high abundance of Nitrospira, including some with the potential for comammox. Our results demonstrate that knowledge of the genomic N transformation potential is important for interpreting N process rates and understanding how the lacustrine sedimentary N cycle responds to variations in trophic conditions. IMPORTANCE Anthropogenic nitrogen (N) inputs can lead to eutrophication in surface waters, especially in N-limited coastal ecosystems. Lakes effectively remove reactive N by transforming it to N2 through microbial denitrification or anammox. The rates and distributions of these microbial processes are affected by factors such as the amount and quality of settling organic material and nitrate concentrations. However, the microbial communities mediating these N transformation processes in freshwater lake sediments remain largely unknown. We provide the first seasonally and spatially resolved metagenomic analysis of the N cycle in sediments of two lakes with different trophic states. We show that lakes with different trophic states select for distinct communities of N-cycling microorganisms with contrasting functional potentials for N transformation.
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Lagos , Microbiota , Eutrofização , Lagos/microbiologia , Nitratos/análise , NitrogênioRESUMO
A novel thermophilic and lithoautotrophic sulfate-reducing archaeon was isolated from black rust formed on the steel surface of a borehole observatory (CORK 1026B) retrieved during IODP Expedition 301 on the eastern flank of Juan de Fuca Ridge, eastern Pacific Ocean. Cells of the strain were lobe-shaped or triangular. The optimum temperature, pH and NaCl concentration for growth were 75°C, pH 7 and 2â% (w/v), respectively. The isolate was strictly anaerobic, growing lithoautotrophically on H(2) and CO(2) using sulfate, sulfite or thiosulfate as electron acceptors. Lactate and pyruvate could serve as alternative energy and carbon sources. The G+C content of the genomic DNA was 42 mol%. Phylogenetic analyses of the 16S rRNA gene indicated that the isolate was closely related to members of the family Archaeoglobaceae, with sequence similarities of 90.3-94.4â%. Physiological and molecular properties showed that the isolate represents a novel species of the genus Archaeoglobus. The name Archaeoglobus sulfaticallidus sp. nov. is proposed; the type strain is PM70-1(T) (=DSM 19444(T)=JCM 14716(T)).
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Archaeoglobus/classificação , Filogenia , Água do Mar/microbiologia , Archaeoglobus/genética , Archaeoglobus/isolamento & purificação , Técnicas de Tipagem Bacteriana , Composição de Bases , DNA Arqueal/genética , Temperatura Alta , Dados de Sequência Molecular , Oxirredução , Oceano Pacífico , RNA Ribossômico 16S/genética , Análise de Sequência de DNA , Sulfatos/metabolismoRESUMO
We investigated the impact of temperature on the microbial turnover of organic matter (OM) in a hydrothermal vent system in Guaymas Basin, by calculating microbial bio- and necromass turnover times based on the culture-independent D:L-amino acid model. Sediments were recovered from two stations near hydrothermal mounds (<74°C) and from one cold station (<9°C). Cell abundance at the two hydrothermal stations dropped from 108 to 106 cells cm-3 within â¼5 m of sediment depth resulting in a 100-fold lower cell number at this depth than at the cold site where numbers remained constant at 108 cells cm-3 throughout the recovered sediment. There were strong indications that the drop in cell abundance was controlled by decreasing OM quality. The quality of the sedimentary OM was determined by the diagenetic indicators %TAAC (percentage of total organic carbon present as amino acid carbon), %TAAN (percentage of total nitrogen present as amino acid nitrogen), aspartic acid:ß-alanine ratios, and glutamic acid:γ-amino butyric acid ratios. All parameters indicated that the OM became progressively degraded with increasing sediment depth, and the OM in the hydrothermal sediment was more degraded than in the uniformly cold sediment. Nonetheless, the small community of microorganisms in the hydrothermal sediment demonstrated short turnover times. The modeled turnover times of microbial bio- and necromass in the hydrothermal sediments were notably faster (biomass: days to months; necromass: up to a few hundred years) than in the cold sediments (biomass: tens of years; necromass: thousands of years), suggesting that temperature has a significant influence on the microbial turnover rates. We suggest that short biomass turnover times are necessary for maintance of essential cell funtions and to overcome potential damage caused by the increased temperature.The reduced OM quality at the hyrothemal sites might thus only allow for a small population size of microorganisms.
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Microbial life inhabiting subseafloor sediments plays an important role in Earth's carbon cycle. However, the impact of geodynamic processes on the distributions and carbon-cycling activities of subseafloor life remains poorly constrained. We explore a submarine mud volcano of the Nankai accretionary complex by drilling down to 200 m below the summit. Stable isotopic compositions of water and carbon compounds, including clumped methane isotopologues, suggest that ~90% of methane is microbially produced at 16° to 30°C and 300 to 900 m below seafloor, corresponding to the basin bottom, where fluids in the accretionary prism are supplied via megasplay faults. Radiotracer experiments showed that relatively small microbial populations in deep mud volcano sediments (102 to 103 cells cm-3) include highly active hydrogenotrophic methanogens and acetogens. Our findings indicate that subduction-associated fluid migration has stimulated microbial activity in the mud reservoir and that mud volcanoes may contribute more substantially to the methane budget than previously estimated.
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Microbial communities living in deeply buried sediment may be adapted to long-term energy limitation as they are removed from new detrital energy inputs for thousands to millions of years. However, sediment layers near the underlying oceanic crust may receive inputs from below that influence microbial community structure and/or activity. As part of the Census of Deep Life, we used 16S rRNA gene tag pyrosequencing on DNA extracted from a spectrum of deep sediment-basement interface samples from the subsurface of the Juan de Fuca Ridge flank (collected on IODP Expedition 327) to examine this possible basement influence on deep sediment communities. This area experiences rapid sedimentation, with an underlying basaltic crust that hosts a dynamic flux of hydrothermal fluids that diffuse into the sediment. Chloroflexi sequences dominated tag libraries in all sediment samples, with variation in the abundance of other bacterial groups (e.g., Actinobacteria, Aerophobetes, Atribacteria, Planctomycetes, and Nitrospirae). These variations occur in relation to the type of sediment (clays versus carbonate-rich) and the depth of sample origin, and show no clear connection to the distance from the discharge outcrop or to basement fluid microbial communities. Actinobacteria-related sequences dominated the basalt libraries, but these should be viewed cautiously due to possibilities for imprinting from contamination. Our results indicate that proximity to basement or areas of seawater recharge is not a primary driver of microbial community composition in basal sediment, even though fluids diffusing from basement into sediment may stimulate microbial activity.
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The hydrothermal mats, mounds, and chimneys of the southern Guaymas Basin are the surface expression of complex subsurface hydrothermal circulation patterns. In this overview, we document the most frequently visited features of this hydrothermal area with photographs, temperature measurements, and selected geochemical data; many of these distinct habitats await characterization of their microbial communities and activities. Microprofiler deployments on microbial mats and hydrothermal sediments show their steep geochemical and thermal gradients at millimeter-scale vertical resolution. Mapping these hydrothermal features and sampling locations within the southern Guaymas Basin suggest linkages to underlying shallow sills and heat flow gradients. Recognizing the inherent spatial limitations of much current Guaymas Basin sampling calls for comprehensive surveys of the wider spreading region.
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Acetate is a major product of fermentation processes and an important substrate for sulfate reducing bacteria and methanogenic archaea. Most studies on acetate catabolism by sulfate reducers and methanogens have used pure cultures. Less is known about acetate conversion by mixed pure cultures and the interactions between both groups. We tested interspecies hydrogen transfer and coexistence between marine methanogens and sulfate reducers using mixed pure cultures of two types of microorganisms. First, Desulfovibrio vulgaris subsp. vulgaris (DSM 1744), a hydrogenotrophic sulfate reducer, was cocultured together with the obligate aceticlastic methanogen Methanosaeta concilii using acetate as carbon and energy source. Next, Methanococcus maripaludis S2, an obligate H2- and formate-utilizing methanogen, was used as a partner organism to M. concilii in the presence of acetate. Finally, we performed a coexistence experiment between M. concilii and an acetotrophic sulfate reducer Desulfobacter latus AcSR2. Our results showed that D. vulgaris was able to reduce sulfate and grow from hydrogen leaked by M. concilii. In the other coculture, M. maripaludis was sustained by hydrogen leaked by M. concilii as revealed by qPCR. The growth of the two aceticlastic microbes indicated co-existence rather than competition. Altogether, our results indicate that H2 leaking from M. concilii could be used by efficient H2-scavengers. This metabolic trait, revealed from coculture studies, brings new insight to the metabolic flexibility of methanogens and sulfate reducers residing in marine environments in response to changing environmental conditions and community compositions. Using dedicated physiological studies we were able to unravel the occurrence of less obvious interactions between marine methanogens and sulfate-reducing bacteria.