Search

The Oman Drilling Project established an "Active Alteration" multi‐borehole observatory in peridotites undergoing low‐temperature serpentinization in the Samail Ophiolite. The highly serpentinized rocks are in contact with strongly reducing fluids. Distinct hydrological regimes, governed by differences in rock porosity and fracture density, give rise to steep redox (Eh +200 to −750 mV) and pH (pH range 8.5–11.2) gradients within the 300–400 m deep boreholes. The serpentinites and fluids host an active subsurface ecosystem. Microbial cell abundances in serpentinite vary at least six orders of magnitude, from ≤3.5 × 101 to 2.9 × 107 cells/g. Low levels of biological sulfate reduction (2–1,000 fmol/cm3/day) can be detected in rock cores, particularly in rocks in contact with reduced groundwaters with pH < 10.5. Thermodesulfovibrio is the predominant sulfate reducer identified via metagenomic sequencing of adjacent groundwater communities. We infer that transport and reaction of microbially generated sulfide with the serpentine and brucite assemblages gives rise to optical darkening and sulfide overprinting, including the formation of tochilinite‐vallerite group minerals, potentially serving as an indicator that this system is inhabited by microbial life. Olivine mesh‐cores replaced with ferroan brucite and minor awaruite, abundant veins containing hydroandradite garnet and polyhedral serpentine, and late‐stage carbonate veins are suggested as targets for future spatially resolved life‐detection investigations. The high‐quality whole‐round core samples that have been preserved can be further probed to define how life distributes itself and functions within a system where chemical disequilibria are sustained by low‐temperature water/rock interaction, and how biosignatures of in situ microbial activity are generated. Plain Language Summary: Ultramafic rocks undergoing water/rock interaction, and storing fluids that are far from chemical equilibrium, may be one of the most common habitats in our solar system. Through the Oman Drilling Project we collected >1 km of intact serpentinite in contact with groundwaters. These cores capture parts of the rock‐hosted biosphere and show how cells are distributed within serpentinites that vary in their mineralogical, physical and chemical properties. The cores are also biologically active, enabling us to detect specific metabolisms, such as when microorganisms combine hydrogen as reductant and sulfate as an oxidant to fuel their metabolism. Although the distribution of microbial cells in the rock cores is very heterogeneous, there are many intervals where the abundance of cells constitutes robust biomass. In the deeper cores, slow, albeit detectable, microbial sulfate reduction proceeds. We suggest that this pervasive biological activity releases byproducts such as sulfide that can react with the serpentinite and change the optical and chemical properties of the rocks. The feedbacks between the rock alteration and microbial activity produce markers that enable us to focus our search for rock‐hosted life and any specific biosignatures it may produce on Earth and perhaps on other planetary bodies. Key Points: Highly serpentinized subsurface rocks exhibit steep redox gradients and host microbial cell abundances that vary >6 orders of magnitudeLow rates of microbial sulfate reduction in rock cores are inferred to result in optical darkening and sulfide overprinting of the mineralogyWidespread andradite garnet, abundant ferroan brucite, and rare carbonate are targets for future spatially resolved life‐detection efforts [ABSTRACT FROM AUTHOR]

Serpentinization involves the hydration and alteration of ultramafic rocks, which produces hydrogen (H2) and methane (CH4) and results in distinctive groundwater chemistries. As reacted fluids mix with recharging surface water, gradients in chemistry and microbiology develop in the subsurface. We present a comprehensive analysis of biogeochemical gradients in the water column of a serpentinite‐hosted well, CSW1.1, at the Coast Range Ophiolite Microbial Observatory (CROMO) in California, USA. Samples for geochemistry, 16S rRNA gene sequencing, and metagenomics were collected at four discrete depths from the top of the well corresponding to 100%, 50%, 15%, and 0% of atmospheric oxygen (O2) levels, and from the well base at 19.5 m depth. Gibbs energy calculations assessed the energy available for a suite of reactions coupled to O2, sulfate (SO42−), and nitrate (NO3−). Metagenomic data from the profile was used to construct metagenome assembled genomes (MAGs) to evaluate the completeness of biochemical pathways and compare the relative abundance of key diagnostic genes. Bioenergetic data point to the favorability of CH4 oxidation reactions despite little genetic evidence for this. Amplicon sequencing results highlight the abundance of key taxa affiliated with the genera Truepera, Serpentinomonas, and Dethiobacter. Although concentrations of NO3− and H2 are low, genes for NO3− reduction and oxidation of H2 and carbon monoxide (CO) were found in high abundance. Conceptual modeling results demonstrate the net depletion of H2 and CO in the groundwater, the consumption of CO2 and O2, and the potential for CH4 emission into the atmosphere at this terrestrial site of serpentinization. Plain Language Summary: Microorganisms living in extreme habitats on Earth generate energy by catalyzing chemical reactions using the compounds available. A water‐rock interaction known as serpentinization creates high pH and low‐oxygen waters, and a limited number of compounds for energy generation. Hydrogen, carbon monoxide, and methane are gases produced through serpentinization that may be used by microorganisms. To unravel small‐scale subsurface processes, we studied a depth profile from a serpentinization‐influenced groundwater well, and sampled the water at five depths. We measured chemical components of the groundwater (anions, dissolved gases), and calculated the energy available to organisms using a set of common metabolic reactions. We also assessed the microbial communities present, and determined their potential contributions to biogeochemical cycles. We found that three taxa, Serpentinomonas, Truepera, and Dethiobacter dominated. Despite high methane concentrations, genes for methane oxidation were absent throughout the profile. Rather, genes associated with the use of low abundance compounds, like carbon monoxide, hydrogen, and nitrate, were prevalent. This implies that carbon monoxide and hydrogen consumption can deplete these compounds before they reach the surface, and the lack of methane consumption may enable its release into the atmosphere. This work provides insight into the biogeochemical processes mediated by subsurface microorganisms in serpentinization‐influenced groundwater. Key Points: Chemical and bioenergetic gradients in ultrabasic groundwater change with depth in well CSW1.1Bioenergetic reactions yielding the most energy are incongruent with the functional capabilities encoded within metagenomic data16S rRNA gene sequence data and metagenome assembled genomes reveal that CSW1.1 is dominated by Serpentinomonas, Truepera, and Dethiobacter [ABSTRACT FROM AUTHOR]

Summary: Serpentinization is the hydration and oxidation of ultramafic rock, which occurs as oceanic lithosphere is emplaced onto continental margins (ophiolites), and along the seafloor as faulting exposes this mantle‐derived material to circulating hydrothermal fluids. This process leads to distinctive fluid chemistries as molecular hydrogen (H2) and hydroxyl ions (OH−) are produced and reduced carbon compounds are mobilized. Serpentinizing ophiolites also serve as a vector to transport sulfur compounds from the seafloor onto the continents. We investigated hyperalkaline, sulfur‐rich, brackish groundwater in a serpentinizing continental ophiolite to elucidate the role of sulfur compounds in fuelling in situ microbial activities. Here we illustrate that key sulfur‐cycling taxa, including Dethiobacter, Desulfitispora and 'Desulforudis', persist throughout this extreme environment. Biologically catalysed redox reactions involving sulfate, sulfide and intermediate sulfur compounds are thermodynamically favourable in the groundwater, which indicates they may be vital to sustaining life in these characteristically oxidant‐ and energy‐limited systems. Furthermore, metagenomic and metatranscriptomic analyses reveal a complex network involving sulfate reduction, sulfide oxidation and thiosulfate reactions. Our findings highlight the importance of the complete inorganic sulfur cycle in serpentinizing fluids and suggest sulfur biogeochemistry provides a key link between terrestrial serpentinizing ecosystems and their submarine heritage. [ABSTRACT FROM AUTHOR]

Summary: Hydrogen (H2) is enriched in hot springs and can support microbial primary production. Using a series of geochemical proxies, a model to describe variable H2 concentrations in Yellowstone National Park (YNP) hot springs is presented. Interaction between water and crustal iron minerals yields H2 that partition into the vapour phase during decompressional boiling of ascending hydrothermal fluids. Variable vapour input leads to differences in H2 concentration among springs. Analysis of 50 metagenomes from a variety of YNP springs reveals that genes encoding oxidative hydrogenases are enriched in communities inhabiting springs sourced with vapour‐phase gas. Three springs in the Smokejumper (SJ) area of YNP that are sourced with vapour‐phase gas and with the most H2 in YNP were examined to determine the fate of H2. SJ3 had the most H2, the most 16S rRNA gene templates and the greatest abundance of culturable hydrogenotrophic and autotrophic cells of the three springs. Metagenomics and transcriptomics of SJ3 reveal a diverse community comprised of abundant populations expressing genes involved in H2 oxidation and carbon dioxide fixation. These observations suggest a link between geologic processes that generate and source H2 to hot springs and the distribution of organisms that use H2 to generate energy. [ABSTRACT FROM AUTHOR]

The geochemistry of hot springs and the availability of oxidants capable of supporting microbial metabolisms are influenced by subsurface processes including the separation of hydrothermal fluids into vapor and liquid phases. Here, we characterized the influence of geochemical variation and oxidant availability on the abundance, composition, and activity of hydrogen (H2)‐dependent chemoautotrophs along the outflow channels of two‐paired hot springs in Yellowstone National Park. The hydrothermal fluid at Roadside East (RSE; 82.4°C, pH 3.0) is acidic due to vapor‐phase input while the fluid at Roadside West (RSW; 68.1°C, pH 7.0) is circumneutral due to liquid‐phase input. Most chemotrophic communities exhibited net rates of H2 oxidation, consistent with H2 support of primary productivity, with one chemotrophic community exhibiting a net rate of H2 production. Abundant H2‐oxidizing chemoautotrophs were supported by reduction in oxygen, elemental sulfur, sulfate, and nitrate in RSW and oxygen and ferric iron in RSE; O2 utilizing hydrogenotrophs increased in abundance down both outflow channels. Sequencing of 16S rRNA transcripts or genes from native sediments and dilution series incubations, respectively, suggests that members of the archaeal orders Sulfolobales, Desulfurococcales, and Thermoproteales are likely responsible for H2 oxidation in RSE, whereas members of the bacterial order Thermoflexales and the archaeal order Thermoproteales are likely responsible for H2 oxidation in RSW. These observations suggest that subsurface processes strongly influence spring chemistry and oxidant availability, which in turn select for unique assemblages of H2 oxidizing microorganisms. Therefore, these data point to the role of oxidant availability in shaping the ecology and evolution of hydrogenotrophic organisms. [ABSTRACT FROM AUTHOR]

Abstract: Geophysical remote sensing both on land and at sea has emerged as a powerful approach to characterize in situ water‐rock interaction processes in time and space. We conducted 2‐D Electrical Resistivity Tomography (ERT) surveys to investigate in situ hydrogeological architecture within the Jurassic age tectonic mélange portion of the Coast Range Ophiolite Microbial Observatory (CROMO) during wet and dry seasons, where water‐rock interactive processes are thought to facilitate a subsurface biosphere. Integrating survey tracks traversing two previously drilled wells, QV1,1 and CSW1,1 at the CROMO site with wireline and core data, and the Serpentine Valley site, we successfully documented changes in hydrogeologic properties in the CROMO formation, i.e., lateral and vertical distribution of conductive zones and their temporal behavior that are dependent upon seasonal hydrology. Based on the core‐log‐ERT integration, we propose a hydrogeological architectural model, in which the formation is composed of three distinct aquifer systems: perched serpentinite aquifer without seasonal dependency (shallow system), well‐cemented serpentine confining beds with seasonal dependency (intermediate system), serpentinite aquifer (deep system), and the ultramafic basement that acts as a quasi‐aquiclude (below the deep system). The stunning contrast between the seasonality in the surface water availability and groundwater storativity in the formation allowed us to locate zones where serpentinite weathering and possibly deeper serpentinization processes might have taken place. We based our findings primarily on lithological composition and the distribution of the conductive formation, our work highlights the link between serpentinite weathering processes and possible sources of water in time and space. [ABSTRACT FROM AUTHOR]

While most oxygenic phototrophs harvest light only in the visible range (400-700 nm, VIS), anoxygenic phototrophs can harvest near infrared light (> 700 nm, NIR). To study interactions between the photosynthetic guilds we used microsensors to measure oxygen and gross oxygenic photosynthesis ( gOP) in a hypersaline microbial mat under full ( VIS + NIR) and VIS illumination. Under normal dissolved inorganic carbon ( DIC) concentrations (2 mM), volumetric rates of gOP were reduced up to 65% and areal rates by 16-31% at full compared with VIS illumination. This effect was enhanced (reduction up to 100% in volumetric, 50% in areal rates of gOP) when DIC was lowered to 1 mM, but diminished at 10 mM DIC or lowered pH. In conclusion, under full-light illumination anoxygenic phototrophs are able to reduce the activity of oxygenic phototrophs by efficiently competing for inorganic carbon within the highly oxygenated layer. Anoxygenic photosynthesis, calculated from the difference in gOP under full and VIS illumination, represented between 10% and 40% of the C-fixation. The DIC depletion in the euphotic zone as well as the significant C-fixation by anoxygenic phototrophs in the oxic layer influences the carbon isotopic composition of the mat, which needs to be taken into account when interpreting isotopic biosignals in geological records. [ABSTRACT FROM AUTHOR]

Cyanobacterial mats collected in hypersaline salterns were incubated in a greenhouse under low sulphate concentrations ([ ]) and examined for their primary productivity and emissions of methane and other major carbon species. Atmospheric greenhouse warming by gases such as carbon dioxide and methane must have been greater during the Archean than today in order to account for a record of moderate to warm palaeoclimates, despite a less luminous early sun. It has been suggested that decreased levels of oxygen and sulphate in Archean oceans could have significantly stimulated microbial methanogenesis relative to present marine rates, with a resultant increase in the relative importance of methane in maintaining the early greenhouse. We maintained modern microbial mats, models of ancient coastal marine communities, in artificial brine mixtures containing both modern [ ] ( c. 70 m m) and ‘Archean’[ ] (<0.2 m m). At low [ ], primary production in the mats was essentially unaffected, while rates of sulphate reduction decreased by a factor of three, and methane fluxes increased by up to 10-fold. However, remineralization by methanogenesis still amounted to less than 0.4% of the total carbon released by the mats. The relatively low efficiency of conversion of photosynthate to methane is suggested to reflect the particular geometry and chemical microenvironment of hypersaline cyanobacterial mats. Therefore, such mats were probably relatively weak net sources of methane throughout their 3.5 Ga history, even during periods of low environmental levels oxygen and sulphate. [ABSTRACT FROM AUTHOR]