Critically testing olivine‐hosted putative martian biosignatures in the Yamato 000593 meteorite—Geobiological implications

On rocky planets such as Earth and Mars the serpentinization of olivine in ultramafic crust produces hydrogen that can act as a potential energy source for life. Direct evidence of fluid–rock interaction on Mars comes from iddingsite alteration veins found in martian meteorites. In the Yamato 000593 meteorite, putative biosignatures have been reported from altered olivines in the form of microtextures and associated organic material that have been compared to tubular bioalteration textures found in terrestrial sub‐seafloor volcanic rocks. Here, we use a suite of correlative, high‐sensitivity, in situ chemical, and morphological analyses to characterize and re‐evaluate these microalteration textures in Yamato 000593, a clinopyroxenite from the shallow subsurface of Mars. We show that the altered olivine crystals have angular and micro‐brecciated margins and are also highly strained due to impact‐induced fracturing. The shape of the olivine microalteration textures is in no way comparable to microtunnels of inferred biological origin found in terrestrial volcanic glasses and dunites, and rather we argue that the Yamato 000593 microtextures are abiotic in origin. Vein filling iddingsite extends into the olivine microalteration textures and contains amorphous organic carbon occurring as bands and sub‐spherical concentrations <300 nm across. We propose that a martian impact event produced the micro‐brecciated olivine crystal margins that reacted with subsurface hydrothermal fluids to form iddingsite containing organic carbon derived from abiotic sources. These new data have implications for how we might seek potential biosignatures in ultramafic rocks and impact craters on both Mars and Earth.     

Sources of organic nitrogen at the serpentinite‐hosted Lost City hydrothermal field

The reaction of ultramafic rocks with water during serpentinization at moderate temperatures results in alkaline fluids with high concentrations of reduced chemical compounds such as hydrogen and methane. Such environments provide unique habitats for microbial communities capable of utilizing these reduced compounds in present‐day and, possibly, early Earth environments. However, these systems present challenges to microbial communities as well, particularly due to high fluid pH and possibly the availability of essential nutrients such as nitrogen. Here we investigate the source and cycling of organic nitrogen at an oceanic serpentinizing environment, the Lost City hydrothermal field (30°N, Mid‐Atlantic Ridge). Total hydrolizable amino acid (THAA) concentrations in the fluids range from 736 to 2300 nm and constitute a large fraction of the dissolved organic carbon (2.5–15.1%). The amino acid distributions, and the relative concentrations of these compounds across the hydrothermal field, indicate they most likely derived from chemolithoautotrophic production. Previous studies have identified the presence of numerous nitrogen fixation genes in the fluids and the chimneys. Organic nitrogen in actively venting chimneys has δ¹⁵N values as low as 0.1‰ which is compatible with biological nitrogen fixation. Total hydrolizable amino acids in the chimneys are enriched in ¹³C by 2–7‰ compared to bulk organic matter. The distribution and absolute δ¹³C_THAA values are compatible with a chemolithoautotrophic source, an attribution also supported by molar organic C/N ratios in most active chimneys (4.1–5.5) which are similar to those expected for microbial communities. In total, these data indicate nitrogen is readily available to microbial communities at Lost City.     

Microbial life associated with low‐temperature alteration of ultramafic rocks in the Leka ophiolite complex

Water–rock interactions in ultramafic lithosphere generate reduced chemical species such as hydrogen that can fuel subsurface microbial communities. Sampling of this environment is expensive and technically demanding. However, highly accessible, uplifted oceanic lithospheres emplaced onto continental margins (ophiolites) are potential model systems for studies of the subsurface biosphere in ultramafic rocks. Here, we describe a microbiological investigation of partially serpentinized dunite from the Leka ophiolite (Norway). We analysed samples of mineral coatings on subsurface fracture surfaces from different depths (10–160 cm) and groundwater from a 50‐m‐deep borehole that penetrates several major fracture zones in the rock. The samples are suggested to represent subsurface habitats ranging from highly anaerobic to aerobic conditions. Water from a surface pond was analysed for comparison. To explore the microbial diversity and to make assessments about potential metabolisms, the samples were analysed by microscopy, construction of small subunit ribosomal RNA gene clone libraries, culturing and quantitative‐PCR. Different microbial communities were observed in the groundwater, the fracture‐coating material and the surface water, indicating that distinct microbial ecosystems exist in the rock. Close relatives of hydrogen‐oxidizing Hydrogenophaga dominated (30% of the bacterial clones) in the oxic groundwater, indicating that microbial communities in ultramafic rocks at Leka could partially be driven by H₂ produced by low‐temperature water–rock reactions. Heterotrophic organisms, including close relatives of hydrocarbon degraders possibly feeding on products from Fischer–Tropsch‐type reactions, dominated in the fracture‐coating material. Putative hydrogen‐, ammonia‐, manganese‐ and iron‐oxidizers were also detected in fracture coatings and the groundwater. The microbial communities reflect the existence of different subsurface redox conditions generated by differences in fracture size and distribution, and mixing of fluids. The particularly dense microbial communities in the shallow fracture coatings seem to be fuelled by both photosynthesis and oxidation of reduced chemical species produced by water–rock reactions.     

Serpentinization as a source of energy at the origin of life

For life to have emerged from CO₂, rocks, and water on the early Earth, a sustained source of chemically transducible energy was essential. The serpentinization process is emerging as an increasingly likely source of that energy. Serpentinization of ultramafic crust would have continuously supplied hydrogen, methane, minor formate, and ammonia, as well as calcium and traces of acetate, molybdenum and tungsten, to off‐ridge alkaline hydrothermal springs that interfaced with the metal‐rich carbonic Hadean Ocean. Silica and bisulfide were also delivered to these springs where cherts and sulfides were intersected by the alkaline solutions. The proton and redox gradients so generated represent a rich source of naturally produced chemiosmotic energy, stemming from geochemistry that merely had to be tapped, rather than induced, by the earliest biochemical systems. Hydrothermal mounds accumulating at similar sites in today’s oceans offer conceptual and experimental models for the chemistry germane to the emergence of life, although the ubiquity of microbial communities at such sites in addition to our oxygenated atmosphere preclude an exact analogy.     

Comparison of microbial communities associated with phase-separation-induced hydrothermal fluids at the Yonaguni Knoll IV hydrothermal field, the Southern Okinawa Trough

Microbial communities associated with a variety of hydrothermal emissions at the Yonaguni Knoll IV hydrothermal field, the southernmost Okinawa Trough, were analyzed by culture-dependent and -independent techniques. In this hydrothermal field, dozens of vent sites hosting physically and chemically distinct hydrothermal fluids were observed. Variability in the gas content and formation in the hydrothermal fluids was observed and could be controlled by the potential subseafloor phase-separation and -partition processes. The hydrogen concentration in the hydrothermal fluids was also variable (0.8–3.6 mmol kg⁻¹) among the chimney sites, but was unusually high as compared with those in other Okinawa Trough hydrothermal fields. Despite the physical and chemical variabilities of the hydrothermal fluids, the microbial communities were relatively similar among the habitats. Based on both culture-dependent and -independent analyses of the microbial community structures, members of Thermococcales, Methanococcales and Desulfurococcales likely represent the predominant archaeal components, while members of Nautiliaceae and Thioreductoraceae are considered to dominate the bacterial population. Most of the abundant microbial components appear to be chemolithotrophs sustained by hydrogen oxidation. The relatively consistent microbial communities found in this study could have been because of the sufficient input of hydrogen from the hydrothermal fluids rather than other chemical properties.     

Barite encrustation of benthic sulfur‐oxidizing bacteria at a marine cold seep

Crusts and chimneys composed of authigenic barite are found at methane seeps and hydrothermal vents that expel fluids rich in barium. Microbial processes have not previously been associated with barite precipitation in marine cold seep settings. Here, we report on the precipitation of barite on filaments of sulfide‐oxidizing bacteria at a brine seep in the Gulf of Mexico. Barite‐mineralized bacterial filaments in the interiors of authigenic barite crusts resemble filamentous sulfide‐oxidizing bacteria of the genus Beggiatoa. Clone library and iTag amplicon sequencing of the 16S rRNA gene show that the barite crusts that host these filaments also preserve DNA of Candidatus Maribeggiatoa, as well as sulfate‐reducing bacteria. Isotopic analyses show that the sulfur and oxygen isotope compositions of barite have lower δ³⁴S and δ¹⁸O values than many other marine barite crusts, which is consistent with barite precipitation in an environment in which sulfide oxidation was occurring. Laboratory experiments employing isolates of sulfide‐oxidizing bacteria from Gulf of Mexico seep sediments showed that under low sulfate conditions, such as those encountered in brine fluids, sulfate generated by sulfide‐oxidizing bacteria fosters rapid barite precipitation localized on cell biomass, leading to the encrustation of bacteria in a manner reminiscent of our observations of barite‐mineralized Beggiatoa in the Gulf of Mexico. The precipitation of barite directly on filaments of sulfide‐oxidizing bacteria, and not on other benthic substrates, suggests that sulfide oxidation plays a role in barite formation at certain marine brine seeps where sulfide is oxidized to sulfate in contact with barium‐rich fluids, either prior to, or during, the mixing of those fluids with sulfate‐containing seawater in the vicinity of the sediment/water interface. As with many other geochemical interfaces that foster mineral precipitation, both biological and abiological processes likely contribute to the precipitation of barite at marine brine seeps such as the one studied here.     

Record of archaeal activity at the serpentinite‐hosted Lost City Hydrothermal Field

Samples of young, outer surfaces of brucite–carbonate deposits from the ultramafic‐hosted Lost City hydrothermal field were analyzed for DNA and lipid biomarker distributions and for carbon and hydrogen stable isotope compositions of the lipids. Methane‐cycling archaeal communities, notably the Lost City Methanosarcinales (LCMS) phylotype, are specifically addressed. Lost City is unlike all other hydrothermal systems known to date and is characterized by metal‐ and CO₂‐poor, high pH fluids with high H₂ and CH₄ contents resulting from serpentinization processes at depth. The archaeal fraction of the microbial community varies widely within the Lost City chimneys, from 1–81% and covaries with concentrations of hydrogen within the fluids. Archaeal lipids include isoprenoid glycerol di‐ and tetraethers and C₂₅ and C₃₀ isoprenoid hydrocarbons (pentamethylicosane derivatives – PMIs – and squalenoids). In particular, unsaturated PMIs and squalenoids, attributed to the LCMS archaea, were identified for the first time in the carbonate deposits at Lost City and probably record processes exclusively occurring at the surface of the chimneys. The carbon isotope compositions of PMIs and squalenoids are remarkably heterogeneous across samples and show highly ¹³C‐enriched signatures reaching δ¹³C values of up to +24.6‰. Unlike other environments in which similar structural and isotopic lipid heterogeneity has been observed and attributed to diversity in the archaeal assemblage, the lipids here appear to be synthesized solely by the LCMS. Some of the variations in lipid isotope signatures may, in part, be due to unusual isotopic fractionation during biosynthesis under extreme conditions. However, we argue that the diversity in archaeal abundances, lipid structure and carbon isotope composition rather reflects the ability of the LCMS archaeal biofilms to adapt to chemical gradients in the hydrothermal chimneys and possibly to perform either methanotrophy or methanogenesis using dissolved inorganic carbon, methane or formate as a function of the prevailing environmental conditions.     

Metagenomic resolution of microbial functions in deep-sea hydrothermal plumes across the Eastern Lau Spreading Center

Microbial processes within deep-sea hydrothermal plumes affect ocean biogeochemistry on global scales. In rising hydrothermal plumes, a combination of microbial metabolism and particle formation processes initiate the transformation of reduced chemicals like hydrogen sulfide, hydrogen, methane, iron, manganese and ammonia that are abundant in hydrothermal vent fluids. Despite the biogeochemical importance of this rising portion of plumes, it is understudied in comparison to neutrally buoyant plumes. Here we use metagenomics and bioenergetic modeling to describe the abundance and genetic potential of microorganisms in relation to available electron donors in five different hydrothermal plumes and three associated background deep-sea waters from the Eastern Lau Spreading Center located in the Western Pacific Ocean. Three hundred and thirty one distinct genomic ‘bins'' were identified, comprising an estimated 951 genomes of archaea, bacteria, eukarya and viruses. A significant proportion of these genomes is from novel microorganisms and thus reveals insights into the energy metabolism of heretofore unknown microbial groups. Community-wide analyses of genes encoding enzymes that oxidize inorganic energy sources showed that sulfur oxidation was the most abundant and diverse chemolithotrophic microbial metabolism in the community. Genes for sulfur oxidation were commonly present in genomic bins that also contained genes for oxidation of hydrogen and methane, suggesting metabolic versatility in these microbial groups. The relative diversity and abundance of genes encoding hydrogen oxidation was moderate, whereas that of genes for methane and ammonia oxidation was low in comparison to sulfur oxidation. Bioenergetic-thermodynamic modeling supports the metagenomic analyses, showing that oxidation of elemental sulfur with oxygen is the most dominant catabolic reaction in the hydrothermal plumes. We conclude that the energy metabolism of microbial communities inhabiting rising hydrothermal plumes is dictated by the underlying plume chemistry, with a dominant role for sulfur-based chemolithoautotrophy.     

Low archaeal diversity linked to subseafloor geochemical processes at the Lost City Hydrothermal Field, Mid-Atlantic Ridge

The recently discovered Lost City Hydrothermal Field (LCHF) represents a new type of submarine hydrothermal system driven primarily by exothermic serpentinization reactions in ultramafic oceanic crust. Highly reducing, alkaline hydrothermal environments at the LCHF produce considerable quantities of hydrogen, methane and organic molecules through chemo- and biosynthetic reactions. Here, we report the first analyses of microbial communities inhabiting carbonate chimneys awash in warm, high pH fluids at the LCHF and the predominance of a single group of methane-metabolizing Archaea. The predominant phylotype, related to the Methanosarcinales, formed tens of micrometre-thick biofilms in regions adjacent to hydrothermal flow. Exterior portions of active structures harboured a diverse microbial community composed primarily of filamentous Eubacteria that resembled sulphide-oxidizing species. Inactive samples, away from regions of hydrothermal flow, contained phylotypes related to pelagic microorganisms. The abundance of organisms linked to the volatile chemistry at the LCHF hints that similar metabolic processes may operate in the subseafloor. These results expand the range of known geological settings that support biological activity to include submarine hydrothermal systems that are not dependent upon magmatic heat sources.     

Making and working with hydrogen sulfide: The chemistry and generation of hydrogen sulfide in vitro and its measurement in vivo: A review

Hydrogen sulfide is rapidly emerging as an important vasoactive mediator formed in health and disease. Its biological action is centered on its reactivity with heme-proteins and its ability to activate K_ATP channels. Hydrogen sulfide is a signalling molecule of the inflammatory and nervous systems, and in particular the cardiovascular system where it regulates vascular tone, cardiac work, and exerts cardioprotection.This has led to an explosion of papers in which the role of hydrogen sulfide generated in vitro has been used to stimulate biological responses, and where a variety of methods have been used to measure the concentration of this compound in biological fluids. Understanding the chemistry and the inherent problems in the analytical techniques used to measure hydrogen sulfide concentrations is critical to our expanding knowledge on the biology of hydrogen sulfide. In this brief review we will cover the chemistry of hydrogen sulfide, including sources of hydrogen sulfide, its speciation at physiological pH, the susceptibility of sulfide to aerobic oxidation, and the methods used to measure hydrogen sulfide concentrations in solution, including biological fluids. We also give a brief overview of knockout animals and inhibition of the enzymes involved in the formation of hydrogen sulfide in vivo.     

Die vielen Seiten des Sulfids. Tödlich und doch lebensnotwendig

Hydrogen sulfide is highly toxic, but nevertheless it has several physiological functions. Animals from sulfide containing habitats are able to protect themselves from sulfide poisoning and furthermore use this reduced sulfur compound for ATP production. Life at the deep‐sea hydrothermal vents entirely depends on the oxidation of inorganic substrates, mainly sulfide. In humans sulfide acts as a gaseous signalling molecule. It is produced in many tissues and takes part in a number of important metabolic processes such as the regulation of blood pressure and insulin secretion. Several severe diseases are caused by dysfunctions in sulfur metabolism. Thus, a detailed knowledge of the reactions and effects of hydrogen sulfide is of considerable clinical relevance.     

Methane- and sulfur-metabolizing microbial communities dominate the Lost City hydrothermal field ecosystem

Hydrothermal venting and the formation of carbonate chimneys in the Lost City hydrothermal field (LCHF) are driven predominantly by serpentinization reactions and cooling of mantle rocks, resulting in a highly reducing, high-pH environment with abundant dissolved hydrogen and methane. Phylogenetic and terminal restriction fragment length polymorphism analyses of 16S rRNA genes in fluids and carbonate material from this site indicate the presence of organisms similar to sulfur-oxidizing, sulfate-reducing, and methane-oxidizing Bacteria as well as methanogenic and anaerobic methane-oxidizing Archaea. The presence of these metabolic groups indicates that microbial cycling of sulfur and methane may be the dominant biogeochemical processes active within this ultramafic rock-hosted environment. 16S rRNA gene sequences grouping within the Methylobacter and Thiomicrospira clades were recovered from a chemically diverse suite of carbonate chimney and fluid samples. In contrast, 16S rRNA genes corresponding to the Lost City Methanosarcinales phylotype were found exclusively in high-temperature chimneys, while a phylotype of anaerobic methanotrophic Archaea (ANME-1) was restricted to lower-temperature, less vigorously venting sites. A hyperthermophilic habitat beneath the LCHF may be reflected by 16S rRNA gene sequences belonging to Thermococcales and uncultured Crenarchaeota identified in vent fluids. The finding of a diverse microbial ecosystem supported by the interaction of high-temperature, high-pH fluids resulting from serpentinization reactions in the subsurface provides insight into the biogeochemistry of what may be a pervasive process in ultramafic subseafloor environments.     

Formation of micro‐spherulitic barite in association with organic matter within sulfidized stromatolites of the 3.48 billion‐year‐old Dresser Formation,Pilbara Craton

The shallow marine and subaerial sedimentary and hydrothermal rocks of the ~3.48 billion‐year‐old Dresser Formation are host to some of Earth's oldest stromatolites and microbial remains. This study reports on texturally distinctive, spherulitic barite micro‐mineralization that occur in association with primary, autochthonous organic matter within exceptionally preserved, strongly sulfidized stromatolite samples obtained from drill cores. Spherulitic barite micro‐mineralization within the sulfidized stromatolites generally forms submicron‐scale aggregates that show gradations from hollow to densely crystallized, irregular to partially radiating crystalline interiors. Several barite micro‐spherulites show thin outer shells. Within stromatolites, barite micro‐spherulites are intimately associated with petrographically earliest dolomite and nano‐porous pyrite enriched in organic matter, the latter of which is a possible biosignature assemblage that hosts microbial remains. Barite spherulites are also observed within layered barite in proximity to stromatolite layers, where they are overgrown by compositionally distinct (Sr‐rich), coarsely crystalline barite that may have been sourced from hydrothermal veins at depth. Micro‐spherulitic barite, such as reported here, is not known from hydrothermal systems that exceed the upper temperature limit for life. Rather, barite with near‐identical morphology and micro‐texture is known from zones of high bio‐productivity under low‐temperature conditions in the modern oceans, where microbial activity and/or organic matter of degrading biomass controls the formation of spherulitic aggregates. Hence, the presence of micro‐spherulitic barite in the organic matter‐bearing Dresser Formation sulfidized stromatolites lend further support for a biogenic origin of these unusual, exceptionally well‐preserved, and very ancient microbialites.     

Bacterial Communities Associated with Subsurface Geochemical Processes in Continental Serpentinite Springs

Reactions associated with the geochemical process of serpentinization can generate copious quantities of hydrogen and low-molecular-weight organic carbon compounds, which may provide energy and nutrients to sustain subsurface microbial communities independently of the photosynthetically supported surface biosphere. Previous microbial ecology studies have tested this hypothesis in deep sea hydrothermal vents, such as the Lost City hydrothermal field. This study applied similar methods, including molecular fingerprinting and tag sequencing of the 16S rRNA gene, to ultrabasic continental springs emanating from serpentinizing ultramafic rocks. These molecular surveys were linked with geochemical measurements of the fluids in an interdisciplinary approach designed to distinguish potential subsurface organisms from those derived from surface habitats. The betaproteobacterial genus Hydrogenophaga was identified as a likely inhabitant of transition zones where hydrogen-enriched subsurface fluids mix with oxygenated surface water. The Firmicutes genus Erysipelothrix was most strongly correlated with geochemical factors indicative of subsurface fluids and was identified as the most likely inhabitant of a serpentinization-powered subsurface biosphere. Both of these taxa have been identified in multiple hydrogen-enriched subsurface habitats worldwide, and the results of this study contribute to an emerging biogeographic pattern in which Betaproteobacteria occur in near-surface mixing zones and Firmicutes are present in deeper, anoxic subsurface habitats.     

Investigating flexible feeding effects on the biogas quality in full‐scale anaerobic digestion by high resolution,photoacoustic‐based NDIR sensing

In future energy systems based on renewable energies, biogas plants can make a significant contribution to stabilizing the electricity grids. However, this requires load‐flexible and demand‐oriented electricity production by means of flexible feed management. However, these flexible feeding strategies using greatly oscillating, temporally varying high mass loads may lead to critical process failures of the anaerobic digestion process. Currently there is no online, high resolution gas quality measurement technique to detect and prevent biological process failures available. In this contribution, we present a miniaturized, low‐cost biogas quality measurement system providing data with high precision and high temporal resolution to overcome this technology gap. To highlight the capabilities of the system we have installed it using a bypass to the main biogas duct after hydrogen sulfide removal at a full‐scale research biogas plant. During a three‐month field trial, the effect of flexible feeding on the biogas quality has been monitored. The results demonstrate long‐term stability of the sensor solution and reveal the effects of changing feeding frequency and composition on gas quantity and quality, which cannot be detected with commercially available state‐of‐the‐art sensing systems.     

Bacterial sulfur cycling shapes microbial communities in surface sediments of an ultramafic hydrothermal vent field

The ultramafic-hosted Logatchev hydrothermal field (LHF) is characterized by vent fluids, which are enriched in dissolved hydrogen and methane compared with fluids from basalt-hosted systems. Thick sediment layers in LHF are partly covered by characteristic white mats. In this study, these sediments were investigated in order to determine biogeochemical processes and key organisms relevant for primary production. Temperature profiling at two mat-covered sites showed a conductive heating of the sediments. Elemental sulfur was detected in the overlying mat and metal-sulfides in the upper sediment layer. Microprofiles revealed an intensive hydrogen sulfide flux from deeper sediment layers. Fluorescence in situ hybridization showed that filamentous and vibrioid, Arcobacter-related Epsilonproteobacteria dominated the overlying mats. This is in contrast to sulfidic sediments in basalt-hosted fields where mats of similar appearance are composed of large sulfur-oxidizing Gammaproteobacteria. Epsilonproteobacteria (7-21%) and Deltaproteobacteria (20-21%) were highly abundant in the surface sediment layer. The physiology of the closest cultivated relatives, revealed by comparative 16S rRNA sequence analysis, was characterized by the capability to metabolize sulfur components. High sulfate reduction rates as well as sulfide depleted in (34)S further confirmed the importance of the biogeochemical sulfur cycle. In contrast, methane was found to be of minor relevance for microbial life in mat-covered surface sediments. Our data indicate that in conductively heated surface sediments microbial sulfur cycling is the driving force for bacterial biomass production although ultramafic-hosted systems are characterized by fluids with high levels of dissolved methane and hydrogen.     

Microbial colonization of metal sulfide minerals at a diffuse‐flow deep‐sea hydrothermal vent at 9°50′N on the East Pacific Rise

Metal sulfide minerals, including mercury sulfides (HgS), are widespread in hydrothermal vent systems where sulfur‐oxidizing microbes are prevalent. Questions remain as to the impact of mineral composition and structure on sulfur‐oxidizing microbial populations at deep‐sea hydrothermal vents, including the possible role of microbial activity in remobilizing elemental Hg from HgS. In the present study, metal sulfides varying in metal composition, structure, and surface area were incubated for 13 days on and near a diffuse‐flow hydrothermal vent at 9°50′N on the East Pacific Rise. Upon retrieval, incubated minerals were examined by scanning electron microscopy with energy‐dispersive X‐ray spectroscopy (SEM‐EDS), X‐ray diffraction (XRD), and epifluorescence microscopy (EFM). DNA was extracted from mineral samples, and the 16S ribosomal RNA gene sequenced to characterize colonizing microbes. Sulfur‐oxidizing genera common to newly exposed surfaces (Sulfurimonas, Sulfurovum, and Arcobacter) were present on all samples. Differences in their relative abundance between and within incubation sites point to constraining effects of the immediate environment and the minerals themselves. Greater variability in colonizing community composition on off‐vent samples suggests that the bioavailability of mineral‐derived sulfide (as influenced by surface area, crystal structure, and reactivity) exerted greater control on microbial colonization in the ambient environment than in the vent environment, where dissolved sulfide is more abundant. The availability of mineral‐derived sulfide as an electron donor may thus be a key control on the activity and proliferation of deep‐sea chemosynthetic communities, and this interpretation supports the potential for microbial dissolution of HgS at hydrothermal vents.     

Methane- and Sulfur-Metabolizing Microbial Communities Dominate the Lost City Hydrothermal Field Ecosystem

Hydrothermal venting and the formation of carbonate chimneys in the Lost City hydrothermal field (LCHF) are driven predominantly by serpentinization reactions and cooling of mantle rocks, resulting in a highly reducing, high-pH environment with abundant dissolved hydrogen and methane. Phylogenetic and terminal restriction fragment length polymorphism analyses of 16S rRNA genes in fluids and carbonate material from this site indicate the presence of organisms similar to sulfur-oxidizing, sulfate-reducing, and methane-oxidizing Bacteria as well as methanogenic and anaerobic methane-oxidizing Archaea. The presence of these metabolic groups indicates that microbial cycling of sulfur and methane may be the dominant biogeochemical processes active within this ultramafic rock-hosted environment. 16S rRNA gene sequences grouping within the Methylobacter and Thiomicrospira clades were recovered from a chemically diverse suite of carbonate chimney and fluid samples. In contrast, 16S rRNA genes corresponding to the Lost City Methanosarcinales phylotype were found exclusively in high-temperature chimneys, while a phylotype of anaerobic methanotrophic Archaea (ANME-1) was restricted to lower-temperature, less vigorously venting sites. A hyperthermophilic habitat beneath the LCHF may be reflected by 16S rRNA gene sequences belonging to Thermococcales and uncultured Crenarchaeota identified in vent fluids. The finding of a diverse microbial ecosystem supported by the interaction of high-temperature, high-pH fluids resulting from serpentinization reactions in the subsurface provides insight into the biogeochemistry of what may be a pervasive process in ultramafic subseafloor environments.     

Energy landscapes shape microbial communities in hydrothermal systems on the Arctic Mid-Ocean Ridge

Methods developed in geochemical modelling combined with recent advances in molecular microbial ecology provide new opportunities to explore how microbial communities are shaped by their chemical surroundings. Here, we present a framework for analyses of how chemical energy availability shape chemotrophic microbial communities in hydrothermal systems through an investigation of two geochemically different basalt-hosted hydrothermal systems on the Arctic Mid-Ocean Ridge: the Soria Moria Vent field (SMVF) and the Loki''s Castle Vent Field (LCVF). Chemical energy landscapes were evaluated through modelling of the Gibbs energy from selected redox reactions under different mixing ratios between seawater and hydrothermal fluids. Our models indicate that the sediment-influenced LCVF has a much higher potential for both anaerobic and aerobic methane oxidation, as well as aerobic ammonium and hydrogen oxidation, than the SMVF. The modelled energy landscapes were used to develop microbial community composition models, which were compared with community compositions in environmental samples inside or on the exterior of hydrothermal chimneys, as assessed by pyrosequencing of partial 16S rRNA genes. We show that modelled microbial communities based solely on thermodynamic considerations can have a high predictive power and provide a framework for analyses of the link between energy availability and microbial community composition.