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A hydrogen-based subsurface microbial community dominated by methanogens
Abstract and Figures
The search for extraterrestrial life may be facilitated if ecosystems can be found on Earth that exist under conditions analogous to those present on other planets or moons. It has been proposed, on the basis of geochemical and thermodynamic considerations, that geologically derived hydrogen might support subsurface microbial communities on Mars and Europa in which methanogens form the base of the ecosystem. Here we describe a unique subsurface microbial community in which hydrogen-consuming, methane-producing Archaea far outnumber the Bacteria. More than 90% of the 16S ribosomal DNA sequences recovered from hydrothermal waters circulating through deeply buried igneous rocks in Idaho are related to hydrogen-using methanogenic microorganisms. Geochemical characterization indicates that geothermal hydrogen, not organic carbon, is the primary energy source for this methanogen-dominated microbial community. These results demonstrate that hydrogen-based methanogenic communities do occur in Earth's subsurface, providing an analogue for possible subsurface microbial ecosystems on other planets.
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Supplementary resources (4)
Nucleotide Sequence
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Nucleotide Sequence
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Nucleotide Sequence
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Nucleotide Sequence
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... For metabolic pathways, oxidation process was explored. Surface and ground water systems harbor viable microorganisms, which employ redox reaction to sustain life functions, such as respiration, growth, and reproduction (Chapelle et al. 2002(Chapelle et al. ). al. 2022. ...
Iron (Fe) and Manganese (Mn) oxidizing bacteria (IMnOB) play a crucial role in the oxidation and removal of Fe and
Mn in aquatic ecosystems. Marshlands shelter a broad spectrum of biota, sequester CO2, and influence climate resilience.
Additionally, they are hotspots of wastewater pollution; thus, they may shelter various bioremediators. Previous works
have explored the isolation and characterization of IMnOB in soil, water, wastewater, and medical settings. However,
IMnBO have not been isolated in urban marshlands so far. This study aimed to isolate, identify, and phylogenetically
characterize the IMnOB from the surface water of Gikondo marshland in Rwanda. Bacterial culturing showed rusty
(yellow) and brown colonies of iron-oxidizing bacteria (IOB) and manganese-oxidizing bacteria (MnOB), respectively.
Furthermore, Sanger sequencing data revealed that bacterial colonies were similar to two distinct Betaproteobacterial species. One group clustered with Ralstonia pickettii, while the other clustered with Pseudomonas sp. with high homology
to Pseudomonas seleniipraecipitans. Taken together, bacteria culturing and Sanger sequencing revealed that the isolates
were assigned to the versatile metabolic genera Ralstonia and Pseudomonas. These results suggested that isolated bacterial
species could serve as inocula for the improved simultaneous removal of Fe and Mn in water treatment systems.
... For metabolic pathways, oxidation process was explored. Surface and ground water systems harbor viable microorganisms, which employ redox reaction to sustain life functions, such as respiration, growth, and reproduction (Chapelle et al. 2002(Chapelle et al. ). al. 2022. ...
Iron (Fe) and Manganese (Mn) oxidizing bacteria (IMnOB) play a crucial role in the oxidation and removal of Fe and Mn in aquatic ecosystems. Marshlands shelter a broad spectrum of biota, sequester CO2, and influence climate resilience. Additionally, they are hotspots of wastewater pollution; thus, they may shelter various bioremediators. Previous works have explored the isolation and characterization of IMnOB in soil, water, wastewater, and medical settings. However, IMnBO have not been isolated in urban marshlands so far. This study aimed to isolate, identify, and phylogenetically characterize the IMnOB from the surface water of Gikondo marshland in Rwanda. Bacterial culturing showed rusty (yellow) and brown colonies of iron-oxidizing bacteria (IOB) and manganese-oxidizing bacteria (MnOB), respectively. Furthermore, Sanger sequencing data revealed that bacterial colonies were similar to two distinct Betaproteobacterial species. One group clustered with Ralstonia pickettii, while the other clustered with Pseudomonas sp. with high homology to Pseudomonas seleniipraecipitans. Taken together, bacteria culturing and Sanger sequencing revealed that the isolates were assigned to the versatile metabolic genera Ralstonia and Pseudomonas. These results suggested that isolated bacterial species could serve as inocula for the improved simultaneous removal of Fe and Mn in water treatment systems.
... This poses a concern about the validity and applicability of the results for long-term storage in natural conditions (St Clair et al. 2019). Under typical reservoir conditions (50-100 °C, circumneutral to near acidic pH), sulfate-reducing and methaneproducing bacteria may potentially consume hydrogen within months (Berta et al. 2018;Chapelle et al. 2002;Thaysen et al. 2021). Methanogenesis or sulfate reduction lowers the partial pressures for injected hydrogen. ...
Advancing underground hydrogen storage (UHS) is essential for a sustainable, emission-free future, with its success highly contingent on the unique properties of each subsurface reservoir. To ensure optimal storage, detailed site assessments are required. One of the critical gaps in knowledge necessary for ensuring safe storage is geochemical redox reactions, especially those involving iron. These redox reactions are crucial as they influence hydrogen retention or loss in the subsurface environments. In this study, we have theoretically addressed hydrogen consumption via abiotic reduction of a Fe³⁺ oxide under different Fe²⁺ activities. Simulations indicate that in scenarios, where the initial hydrogen partial pressure is extremely low (around 10⁻⁵ bars), decreasing the activity of Fe²⁺ by a factor of 10 can lead to a marked decrease in the initial hydrogen pressure by a maximum factor of 1000 within a few years. Variations in Fe²⁺ activity can significantly influence abiotic hydrogen consumption only under very low hydrogen partial pressures. This is primarily due to enhanced dissolution of Fe³⁺ oxides. In comparison, in conditions where hydrogen partial pressure is higher (> 10⁻² bars), reduction of Fe³⁺ oxide can yield magnetite, resulting in a muted loss of hydrogen over time. The transition in the reduction behavior of Fe³⁺ oxide from a ‘dissolution-driven’ process to ‘magnetite crystallization,’ which also determines the fate of stored hydrogen, depends on initial hydrogen partial pressure. Our results demonstrate that low quantities of hydrogen can be maintained within typical storage cycles spanning less than a year, depending upon aqueous Fe content.
... Man y deep envir onments ar e home to micr obial comm unities that use H 2 as an energy source . T hese communities are referred to by the acr on ym SLiME, standing for SubLithoautotrophic Microbial Ecosystem (Stevens and McKinley 1995, Fry et al. 1997, Chapelle et al. 2002, Takai et al. 2003, Lin et al. 2005, Crespo-Medina et al. 2014. Se v er al works have demonstrated, directly or indir ectl y, that UHS in por ous r eservoirs, particularl y in deep aquifers, could lead under certain conditions to in situ biomethanation by hydr ogenotr ophic methanogenic arc haea that natur all y e volv e in these ecosystems (Amigáň et al. 1990, Buzek et al. 1994, P anfilov 2010, Liebsc her et al. 2016, Gr egory et al. 2019, Str obel et al. 2020, Haddad et al. 2022a, Molíková et al. 2022 ). ...
The dihydrogen (H2) sector is undergoing development and will require massive storage solutions. To minimize costs, the conversion of underground geological storage sites, such as deep aquifers, used for natural gas storage into future underground hydrogen storage sites is the favored scenario. However, these sites contain microorganisms capable of consuming H2, mainly sulfate reducers and methanogens. Methanogenesis is, therefore expected but its intensity must be evaluated. Here, in a deep aquifer used for underground geological storage, 17 sites were sampled, with low sulfate concentrations ranging from 21.9 to 197.8 µM and a slow renewal of formation water. H2-selected communities mainly were composed of the families Methanobacteriaceae and Methanothermobacteriaceae and the genera Desulfovibrio, Thermodesulfovibrio, and Desulforamulus. Experiments were done under different conditions, and sulfate reduction, as well as methanogenesis, were demonstrated in the presence of a H2 or H2/CO2 (80/20) gas phase, with or without calcite/site rock. These metabolisms led to an increase in pH up to 10.2 under certain conditions (without CO2). The results suggest competition for CO2 between lithoautotrophs and carbonate mineral precipitation, which could limit microbial H2 consumption.
... When methanogens and SRB are present, the latter can maintain H 2 at a low steady-state concentration, at which methanogenesis is endergonic (Hoehler et al., 2001). In the subsurface, methanogen-, or methanogen/homoacetogendominated communities were detected in water with sulfate concentration around 1 mM (i.e., 15-fold lower than in Opalinus Clay porewater) Pedersen, 1997, 1998;Chapelle et al., 2002). In the sand/bentonite backfill, methanogens could be indigenous to bentonite, as evidenced by the detection of methane during bentonite incubation (Maanoja et al., 2020). ...
The activity of subsurface microorganisms can be harnessed for engineering projects. For instance, the Swiss radioactive waste repository design can take advantage of indigenous microorganisms to tackle the issue of a hydrogen gas (H2) phase pressure build-up. After repository closure, it is expected that anoxic steel corrosion of waste canisters will lead to an H2 accumulation. This occurrence should be avoided to preclude damage to the structural integrity of the host rock. In the Swiss design, the repository access galleries will be back-filled, and the choice of this material provides an opportunity to select conditions for the microbially-mediated removal of excess gas. Here, we investigate the microbial sinks for H2. Four reactors containing an 80/20 (w/w) mixture of quartz sand and Wyoming bentonite were supplied with natural sulfate-rich Opalinus Clay rock porewater and with pure H2 gas for up to 108 days. Within 14 days, a decrease in the sulfate concentration was observed, indicating the activity of the sulfate-reducing bacteria detected in the reactor, e.g., from Desulfocurvibacter genus. Additionally, starting at day 28, methane was detected in the gas phase, suggesting the activity of methanogens present in the solid phase, such as the Methanosarcina genus. This work evidences the development, under in-situ relevant conditions, of a backfill microbiome capable of consuming H2 and demonstrates its potential to contribute positively to the long-term safety of a radioactive waste repository.
... Together with infiltration from the photosynthesis-driven surface (Osterholz et al., 2022), geogenic hydrogen and carbon dioxide from the deep subsurface are the main energy sources for subsurface life (Pedersen, 2000). Research on the subsurface has been enabled and accelerated by the use of springs (Suzuki et al., 2014), artesian wells (Chapelle et al., 2002), boreholes (Bomberg et al., 2016), mines (Magnabosco et al., 2016), and dedicated underground laboratories (Boylan et al., 2019;Wu et al., 2016). Genomic approaches have greatly increased understanding of microbial diversity in the continental subsurface and revealed a large proportion of diversity for which the ecology is not fully understood (Hug et al., 2016). ...
Scientific drilling expeditions offer a unique opportunity to characterize microbial communities in the subsurface that have long been isolated from the surface. With subsurface microbial biomass being low in general, biological contamination from the drilling fluid, sample processing, or molecular work is a major concern. To address this, characterization of the contaminant populations in the drilling fluid and negative extraction controls are essential for assessing and evaluating such sequencing data. Here, rock cores down to 2250 m depth, groundwater-bearing fractures, and the drilling fluid were sampled for DNA to characterize the microbial communities using a broad genomic approach. However, even after removing potential contaminant populations present in the drilling fluid, notorious contaminants were abundant and mainly affiliated with the bacterial order Burkholderiales. These contaminant microorganisms likely originated from the reagents used for isolating DNA despite stringent quality standards during the molecular work. The detection of strictly anaerobic sulfate reducers such as Candidatus Desulforudis audaxviator suggested the presence of autochthonous deep biosphere taxa in the sequenced libraries, yet these clades represented only a minor fraction of the sequence counts (< 0.1 %), hindering further ecological interpretations. The described methods and findings emphasize the importance of sequencing extraction controls and can support experimental design for future microbiological studies in conjunction with continental drilling operations.
... Biostimulation can improve the nutritional conditions of microorganisms by adding specific nutrients or biostimulants to the water (Maldonado et al., 2022). This approach not only facilitates the colonisation of the applied microorganisms, but also selectively expands the population of beneficial indigenous bacteria, thereby improving the environmental remediation capacity of the microorganisms (Chapelle et al., 2002;Darmayati et al., 2015;Pavlova et al., 2019;Chen et al., 2023). ...
Globally, various types of pollution affect coastal waters as a result of human activities. Bioaugmentation and biostimu-lation are effective methods for treating water pollution. However, few studies have explored the response of coastal prokaryotic and eukaryotic communities to bioaugmentation and biostimulation. Here, a 28-day outdoor mesocosm experiment with two treatments (bioaugmentation-A and combined treatment of bioaugmentation and biostimulation-AS) and a control (untreated-C) were carried out. The experiment was conducted in Meishan Bay to explore the composition, dynamics, and co-occurrence patterns of prokaryotic and eukaryotic communities in response to the A and AS using 16S rRNA and 18S rRNA gene amplicon sequencing. After treatment, Gammaproteobacteria and Epsilonproteobacteria were significantly increased in group AS compared to group C, while Flavobac-teriia and Saprospirae were significantly reduced. Dinoflagellata was significantly reduced in AS compared to C, while Chrysophyta was significantly reduced in both AS and A. Compared to C, the principal response curve analyses of the prokaryotic and eukaryotic communities both showed an increasing trend followed by a decreasing trend for AS. Furthermore, the trends of prokaryotic and eu-karyotic communities in group A were similar to those in group AS compared with group C, but AS changed them more than A did. According to the species weight table on principal response curves, a significant increase was observed in beneficial bacteria in pro-karyotic communities, such as Rhodobacterales and Oceanospirillales, along with a decrease in autotrophs in eukaryotic communities, such as Chrysophyta and Diatom. Topological properties of network analysis reveal that A and AS complicate the interactions between the prokaryotic and eukaryotic communities. Overall, these findings expand our understanding of the response pattern of the bioaug-mentation and biostimulation on coastal prokaryotic and eukaryotic communities.
The terrestrial subsurface hosts microbial communities that, collectively, are predicted to comprise as many microbial cells as global surface soils. Although initially thought to be associated with deposited organic matter, deep subsurface microbial communities are supported by chemolithoautotrophic primary production, with hydrogen serving as an important source of electrons. Despite recent progress, relatively little is known about the deep terrestrial subsurface compared to more commonly studied environments. Understanding the composition of deep terrestrial subsurface microbial communities and the factors that influence them is of importance because of human-associated activities including long-term storage of used nuclear fuel, carbon capture, and storage of hydrogen for use as an energy vector. In addition to identifying deep subsurface microorganisms, recent research focuses on identifying the roles of microorganisms in subsurface communities, as well as elucidating myriad interactions—syntrophic, episymbiotic, and viral—that occur among community members. In recent years, entirely new groups of microorganisms (i.e. candidate phyla radiation bacteria and Diapherotrites, Parvarchaeota, Aenigmarchaeota, Nanoloarchaeota, Nanoarchaeota archaea) have been discovered in deep terrestrial subsurface environments, suggesting that much remains unknown about this biosphere. This review explores the historical context for deep terrestrial subsurface microbial ecology and highlights recent discoveries that shape current ecological understanding of this poorly explored microbial habitat. Additionally, we highlight the need for multifaceted experimental approaches to observe phenomena such as cryptic cycles, complex interactions, and episymbiosis, which may not be apparent when using single approaches in isolation, but are nonetheless critical to advancing our understanding of this deep biosphere.
Organic matter is the material basis of shale gas. The Qaidam Basin is a key exploration and development area for shallow biogenic shale gas in China. In this study, we have focused on Quaternary Pleistocene shale in the Qaidam Basin, and the mechanism of organic matter enrichment was investigated in terms of water column stratification and paleoclimate. The results show that the K9–K7 section has greater biological productivity than the K5–K4 section. During the early–middle Pleistocene (K9–K7 deposition period), due to a warm and humid climate, the water column was strongly stratified and herbaceous plants developed, resulting in increased biological productivity. Stronger stratification also led to a reducing environment in the lower layer, which was conducive to the preservation of organic matter from the upper layer. During the late Pleistocene (K5–K4 deposition period), with a dry and hot climate, stratification became weaker and the vegetation evolved into woody plants, reducing biological productivity. Weaker stratification led to destruction of the reducing environment, which was not conducive to enrichment of the sedimentary organic matter. Moreover, the increased temperatures increased the activity of methanogenic bacteria, which consumed a lot of the organic matter.
Background
Biofilms in sulfide-rich springs present intricate microbial communities that play pivotal roles in biogeochemical cycling. We studied chemoautotrophically based biofilms that host diverse CPR bacteria and grow in sulfide-rich springs to investigate microbial controls on biogeochemical cycling.
Results
Sulfide springs biofilms were investigated using bulk geochemical analysis, genome-resolved metagenomics, and scanning transmission X-ray microscopy (STXM) at room temperature and 87 K. Chemolithotrophic sulfur-oxidizing bacteria, including Thiothrix and Beggiatoa, dominate the biofilms, which also contain CPR Gracilibacteria, Absconditabacteria, Saccharibacteria, Peregrinibacteria, Berkelbacteria, Microgenomates, and Parcubacteria. STXM imaging revealed ultra-small cells near the surfaces of filamentous bacteria that may be CPR bacterial episymbionts. STXM and NEXAFS spectroscopy at carbon K and sulfur L2,3 edges show that filamentous bacteria contain protein-encapsulated spherical elemental sulfur granules, indicating that they are sulfur oxidizers, likely Thiothrix. Berkelbacteria and Moranbacteria in the same biofilm sample are predicted to have a novel electron bifurcating group 3b [NiFe]-hydrogenase, putatively a sulfhydrogenase, potentially linked to sulfur metabolism via redox cofactors. This complex could potentially contribute to symbioses, for example, with sulfur-oxidizing bacteria such as Thiothrix that is based on cryptic sulfur cycling. One Doudnabacteria genome encodes adjacent sulfur dioxygenase and rhodanese genes that may convert thiosulfate to sulfite. We find similar conserved genomic architecture associated with CPR bacteria from other sulfur-rich subsurface ecosystems.
Conclusions
Our combined metagenomic, geochemical, spectromicroscopic, and structural bioinformatics analyses of biofilms growing in sulfide-rich springs revealed consortia that contain CPR bacteria and sulfur-oxidizing Proteobacteria, including Thiothrix, and bacteria from a new family within Beggiatoales. We infer roles for CPR bacteria in sulfur and hydrogen cycling.
The track of the Yellowstone hot spot is represented by a systematic northeast-trending linear belt of silicic, caldera-forming volcanism that arrived at Yel- lowstone 2 Ma, was near American Falls, Idaho about 10 Ma, and started about 16 Ma near the Nevada-Oregon-Idaho border. From 16 to 10 Ma, particularly 16 to 14 Ma, volcanism was widely dispersed around the inferred hot-spot track in a region that now forms a moderately high volcanic plateau. From 10 to 2 Ma, silicic volcanism migrated N54OE toward Yellowstone at about 3 cm/year, leaving in its wake the topographic and structural depression of the eastern Snake River Plain (SRP). This 700 m high. Belt I1 has two arms forming a V that joins at Yellowstone: One arm of Belt I1 trends south to the Wasatch front; the other arm trends west and includes the sites of the 1959 Hebgen Lake and 1983 Borah Peak earthquakes. Fault Belt I is farthest away from the SRP and contains relatively new and reactivated faults that have not produced new bedrock escarpments higher than 200 m during the present episode of faulting. Belt I11 is the innermost active belt near the SRP. It contains faults that have moved since 15 to 120 ka and that have been active long enough to produce range fronts more than 500 m high. A belt with inactive faults, belt IV, occurs only south of the SRP and contains range-front faults that experienced high rates of activity coincident with hot-spot volcan- ism in the late Tertiary on the adjacent SRP. Comparison of these belts of fault activity with historic seismic activity reveals similarities but differences in detail. That uplift migrated outward from the hot-spot track is suggested by (I) the Yellowstone crescent of high terrain that is about 0.5 km higher than the surrounding terrain, is about 350 km across at Yellowstone, wraps around Yellowstone like a bow wave, and has arms that extend 400 km southerly and westerly from its apex; (2) readily erodible rocks forming young, high mountains in parts of this crescent; (3) geodetic surveys and paleotopographic reconstructions that indicate young uplift near the axis of the Yellowstone crescent; (4) the fact that on the outer slope of this
The frequent discrepancy between direct microscopic counts and numbers of culturable bacteria from environmental samples is just one of several indications that we currently know only a minor part of the diversity of microorganisms in nature. A combination of direct retrieval of rRNA sequences and whole-cell oligonucleotide probing can be used to detect specific rRNA sequences of uncultured bacteria in natural samples and to microscopically identify individual cells. Studies have been performed with microbial assemblages of various complexities ranging from simple two-component bacterial endosymbiotic associations to multispecies enrichments containing magnetotactic bacteria to highly complex marine and soil communities. Phylogenetic analysis of the retrieved rRNA sequence of an uncultured microorganism reveals its closest culturable relatives and may, together with information on the physicochemical conditions of its natural habitat, facilitate more directed cultivation attempts. For the analysis of complex communities such as multispecies biofilms and activated-sludge flocs, a different approach has proven advantageous. Sets of probes specific to different taxonomic levels are applied consecutively beginning with the more general and ending with the more specific (a hierarchical top-to-bottom approach), thereby generating increasingly precise information on the structure of the community. Not only do rRNA-targeted whole-cell hybridizations yield data on cell morphology, specific cell counts, and in situ distributions of defined phylogenetic groups, but also the strength of the hybridization signal reflects the cellular rRNA content of individual cells. From the signal strength conferred by a specific probe, in situ growth rates and activities of individual cells might be estimated for known species. In many ecosystems, low cellular rRNA content and/or limited cell permeability, combined with background fluorescence, hinders in situ identification of autochthonous populations. Approaches to circumvent these problems are discussed in detail.
It has been proposed that hydrogen produced from basalt–ground-water interactions may serve as an energy source that supports the existence of microorganisms in the deep subsurface on Earth and possibly on other planets. However, experiments demonstrated that hydrogen is not produced from basalt at an environmentally relevant, alkaline pH. Small amounts of hydrogen were produced at a lower pH in laboratory incubations, but even this hydrogen production was transitory. Furthermore, geochemical considerations suggest that previously reported rates of hydrogen production cannot be sustained over geologically significant time frames. These findings indicate that hydrogen production from basalt–ground-water interactions may not support microbial metabolism in the subsurface.
Of the three primary phylogenetic domains--Archaea (archaebacteria), Bacteria (eubacteria), and Eucarya (eukaryotes)--Archaea is the least understood in terms of its diversity, physiologies, and ecological panorama. Although many species of Crenarchaeota (one of the two recognized archaeal kingdoms sensu Woese [Woese, C. R., Kandler, O. & Wheelis, M. L. (1990) Proc. Natl. Acad. Sci. USA 87, 4576-4579]) have been isolated, they constitute a relatively tight-knit cluster of lineages in phylogenetic analyses of rRNA sequences. It seemed possible that this limited diversity is merely apparent and reflects only a failure to culture organisms, not their absence. We report here phylogenetic characterization of many archaeal small subunit rRNA gene sequences obtained by polymerase chain reaction amplification of mixed population DNA extracted directly from sediment of a hot spring in Yellowstone National Park. This approach obviates the need for cultivation to identify organisms. The analyses document the existence not only of species belonging to well-characterized crenarchaeal genera or families but also of crenarchaeal species for which no close relatives have so far been found. The large number of distinct archaeal sequence types retrieved from this single hot spring was unexpected and demonstrates that Crenarchaeota is a much more diverse group than was previously suspected. The results have impact on our concepts of the phylogenetic organization of Archaea.
Bacterial communities were detected in deep crystalline rock aquifers within the Columbia River Basalt Group (CRB). CRB ground waters contained up to 60 {mu}M dissolved H{sub 2} and autotrophic microorganisms outnumbered heterotrophs. Stable carbon isotope measurements implied that autotrophic methanogenesis dominated this ecosystem and was coupled to the depletion of dissolved inorganic carbon. In laboratory experiments, H{sub 2} a potential energy source for bacteria, was produced by reactions between crushed basalt and anaerobic water. Microcosms containing only crushed basalt and ground water supported microbial growth. These results suggest that the CRB contains a lithoautotrophic microbial ecosystem that is independent of photosynthetic primary production. 38 refs., 4 figs., 3 tabs.
Conspicuous anomalies in gas compositions were observed at a fumarole and three mineral springs about 1-3 months prior to an inland earthquake of 6.8 magnitude in central Japan, on September 14, 1984. The epicentral distances are 9 km for the fumarole and 50, 71, and 95 km for the springs. The anomalies in He/Ar, N2/Ar, and CH4/Ar ratios of bubble gases from the springs can be attributed to fluctuations in the emission rate of some deep-seated gas due to change in pore pressure, which is subjected to crustal stress, resulting in the earthquake. The anomaly at the fumarole showed an increase in discharge rate of deep volcanic gas under a compressional stress near the focal region. Since the behavior of subsurface fluids represented by He/Ar and other ratios is presumably controlled by the change of pore pressure, temporal variations in the fluids can be used as a strain gauge for the crust. On the other hand, the precursory emissions of H2, which were observed after changes in He/Ar and other ratios had occurred, may have resulted from a reaction between groundwater and the surface of newly formed cracks. The timing and amplitude of the geochemical anomalies are compatible with the result predicted by a dilatancy model without fluid diffusion, although H2 emission may be related to goundwater permeation into newly formed cracks. Monitoring of several gases with automatic chromatographs at several stations may be useful for unraveling earthquake mechanisms and for predicting earthquakes.
Hydrothermal systems may have provided favorable environments for the prebiotic synthesis of organic compounds necessary for life and may also have been a site for life's origin. They could also have provided a refuge for thermophilic (heat-loving) microorganisms during late, giant-impact events. Phylogenetic information encoded in the genomes of extant thermophiles provides important clues about this early period of biosphere development that are broadly consistent with geological evidence for Archean environments. Hydrothermal environments often exhibit high rates of mineralization, which favors microbial fossilization. Thus, hydrothermal deposits are often rich storehouses of paleobiologic information. This is illustrated by studies of the microbial biosedimentology of hot springs in Yellowstone National Park that provide important constraints for interpreting the fossil record of thermophilic ecosystems. Hydrothermal processes appear to be inextricably linked to planetary formation and evolution and are likely to have existed on other bodies in the solar system. Such environments may have sustained an independent, extraterrestrial origin of life. Thus, hydrothermal systems and their deposits are considered primary targets in the search for fossil evidence of life elsewhere in the solar system.
Geochemical models are used to explore the possibility that lithoautotrophic methanogenesis (the conversion of CO2 plus H2 to methane) could be a source of metabolically useful chemical energy for the production of biomass at putative Europan hydrothermal systems. Two cases are explored: a relatively reduced methane-rich ocean and a relatively oxidized sulfate- and bicarbonate-rich ocean. In the case of a methane-rich ocean, a source of CO2 for methanogenesis is provided by conversion of dissolved methane to CO2 during reaction of ocean water with igneous rocks at high temperatures in the subsurface. Fluid-rock reactions also provide a source of dissolved H2 in the hydrothermal fluid. When this fluid circulates back to the ocean floor and mixes with seawater, conversion of the dissolved CO2 and H2 to methane provides a potential source of chemical energy that can be used to drive metabolic processes. For the case of a sulfate- and carbonate-rich ocean, reaction with reduced igneous rocks at high temperatures will also produce hydrothermal fluids with high H2 concentrations (as occurs in hydrothermal systems on Earth). Mixing of the resulting hydrothermal fluid with seawater in a relatively oxidized ocean could supply energy from either methanogenesis or sulfate reduction. For plausible compositions of a Europan ocean, methanogenesis can supply similar amounts of energy to that which supports the prolific ecosystems surrounding submarine hydrothermal vents on Earth. Even in the most optimistic case, however, the total amount of biomass that could be supported globally by lithoautotrophic microbes on Europa is extremely small compared to the biomass produced photosynthetically on Earth. Nevertheless, sufficient metabolic energy could apparently be available at hydrothermal systems on Europa to support an origin of life and localized ecosystems.