Evidence for Seismogenic Hydrogen Gas, a Potential Microbial Energy Source on Earth and Mars

Potential Habitability of Present-Day Martian Subsurface for Earth-Like Methanogens

The intense debate about the presence of methane in the martian atmosphere has stimulated the study of methanogenic species that are adapted to terrestrial habitats that resemble martian environments. We examined the environmental conditions, energy sources, and ecology of terrestrial methanogens that thrive in deep crystalline fractures, subsea hypersaline lakes, and subglacial water bodies, considered analogs of a hypothetical habitable martian subsurface. We combined this information with recent data on the distribution of buried water/ice and radiogenic elements on Mars, and with models of the subsurface thermal regime of this planet, we identified a 4.3-8.8 km-deep regolith habitat at the midlatitude location of Acidalia Planitia that might fit the requirements for hosting putative martian methanogens analogous to the methanogenic families, Methanosarcinaceae and Methanomicrobiaceae.

Hydrogenotrophic methanogenesis at 7-12 mbar by Methanosarcina barkeri under simulated martian atmospheric conditions

Mars, with its ancient history of long-lived habitable environments, continues to captivate researchers exploring the potential for extant life. This study investigates the biosignature potential of Martian methane by assessing the viability of hydrogenotrophic methanogenesis in Methanosarcina barkeri MS under simulated Martian surface conditions. We expose M. barkeri to sustained hypobaria (7-12 mbar), low temperature (0˚C), and a CO2-dominated gas mixture mimicking the Martian atmosphere. The results demonstrate statistically quantifiable CH4 production under all tested conditions, including at 7-12 mbar. Transcriptomics reveal that low total pressure and temperature did not significantly impact gene expression, highlighting the resilience of M. barkeri. However, atmospheric gas composition, specifically Mars gas with 2.9% pH2, led to significant down-regulation of methanogenesis genes, hindering growth over 14 days. Notably, CH4 production scaled with the partial pressure of H2, revealing that hydrogen uptake affinity is a stronger predictor of habitability and methanogenic potential than favorable Gibbs free energy of reaction. Our findings suggest that Mars' subsurface could harbor habitable refugia capable of supporting methanogenesis, sustaining microbial life at low metabolic steady states. These insights challenge assumptions about Martian habitability and have implications for astrobiological exploration, planetary protection, and in situ resource utilization for future human missions.

Radiolytic Water Splitting Sensitized by Nanoscale Metal-Organic Frameworks

High-energy radiation that is compatible with renewable energy sources enables direct H2 production from water for fuels; however, the challenge is to convert it as efficiently as possible, and the existing strategies have limited success. Herein, we report the use of Zr/Hf-based nanoscale UiO-66 metal-organic frameworks as highly effective and stable radiation sensitizers for purified and natural water splitting under γ-ray irradiation. Scavenging and pulse radiolysis experiments with Monte Carlo simulations show that the combination of 3D arrays of ultrasmall metal-oxo clusters and high porosity affords unprecedented effective scattering between secondary electrons and confined water, generating increased precursors of solvated electrons and excited states of water, which are the main species responsible for H2 production enhancement. The use of a small quantity (<80 mmol/L) of UiO-66-Hf-OH can achieve a γ-rays-to-hydrogen conversion efficiency exceeding 10% that significantly outperforms Zr-/Hf-oxide nanoparticles and the existing radiolytic H2 promoters. Our work highlights the feasibility and merit of MOF-assisted radiolytic water splitting and promises a competitive method for creating a green H2 economy.

Tectonically-driven oxidant production in the hot biosphere

Genomic reconstructions of the common ancestor to all life have identified genes involved in H₂O₂ and O₂ cycling. Commonly dismissed as an artefact of lateral gene transfer after oxygenic photosynthesis evolved, an alternative is a geological source of H₂O₂ and O₂ on the early Earth. Here, we show that under oxygen-free conditions high concentrations of H₂O₂ can be released from defects on crushed silicate rocks when water is added and heated to temperatures close to boiling point, but little is released at temperatures <80 °C. This temperature window overlaps the growth ranges of evolutionary ancient heat-loving and oxygen-respiring Bacteria and Archaea near the root of the Universal Tree of Life. We propose that the thermal activation of mineral surface defects during geological fault movements and associated stresses in the Earth’s crust was a source of oxidants that helped drive the (bio)geochemistry of hot fractures where life first evolved.

Researchers at Newcastle University have discovered a mechanism by which earthquakes create bursts of hydrogen peroxide and oxygen in hot underground fractures. These may have played a vital role in the early evolution and origin of life on Earth.

Geological activity shapes the microbiome in deep-subsurface aquifers by advection

Sheltered from radiation and asteroid strikes in Earth’s early history and home to a significant portion of today’s prokaryotic biomass, the deep subsurface could hold keys to understanding the early and continuing evolution of life. However, the processes that shape the distribution of subsurface microbial communities remain poorly understood due to sample inaccessibility. Here, at a deep-underground fractured hard-rock aquifer, we show that fracture activity leads to altered groundwater flow that drives profound changes in fluid-associated microbial communities by physical transport instead of environmental selection. We thereby identify advection induced by geological activity (a notable trigger for fracture activity) as a prominent yet overlooked mechanism shaping subsurface biogeography with potentially profound implications for life’s evolutionary history.

Subsurface environments host diverse microorganisms in fluid-filled fractures; however, little is known about how geological and hydrological processes shape the subterranean biosphere. Here, we sampled three flowing boreholes weekly for 10 mo in a 1478-m-deep fractured rock aquifer to study the role of fracture activity (defined as seismically or aseismically induced fracture aperture change) and advection on fluid-associated microbial community composition. We found that despite a largely stable deep-subsurface fluid microbiome, drastic community-level shifts occurred after events signifying physical changes in the permeable fracture network. The community-level shifts include the emergence of microbial families from undetected to over 50% relative abundance, as well as the replacement of the community in one borehole by the earlier community from a different borehole. Null-model analysis indicates that the observed spatial and temporal community turnover was primarily driven by stochastic processes (as opposed to deterministic processes). We, therefore, conclude that the observed community-level shifts resulted from the physical transport of distinct microbial communities from other fracture(s) that outpaced environmental selection. Given that geological activity is a major cause of fracture activity and that geological activity is ubiquitous across space and time on Earth, our findings suggest that advection induced by geological activity is a general mechanism shaping the microbial biogeography and diversity in deep-subsurface habitats across the globe.

Mineral Matrix Effects on Pyrolysis Products of Kerogens Infer Difficulties in Determining Biological Provenance of Macromolecular Organic Matter at Mars

Ancient martian organic matter is likely to take the form of kerogen-like recalcitrant macromolecular organic matter (MOM), existing in close association with reactive mineral surfaces, especially iron oxides. Detecting and identifying a biological origin for martian MOM will therefore be of utmost importance for life-detection efforts at Mars. We show that Type I and Type IV kerogens provide effective analogues for putative martian MOM of biological and abiological (meteoric) provenances, respectively. We analyze the pyrolytic breakdown products when these kerogens are mixed with mineral matrices highly relevant for the search for life on Mars. We demonstrate that, using traditional thermal techniques as generally used by the Sample Analysis at Mars and Mars Organic Molecule Analyser instruments, even the breakdown products of highly recalcitrant MOM are transformed during analysis in the presence of reactive mineral surfaces, particularly iron. Analytical transformation reduces the diagnostic ability of this technique, as detected transformation products of both biological and abiological MOM may be identical (low molecular weight gas phases and benzene) and indistinguishable. The severity of transformational effects increased through calcite < kaolinite < hematite < nontronite < magnetite < goethite. Due to their representation of various habitable aqueous environments and the preservation potential of organic matter by iron, it is not advisable to completely avoid iron-rich strata. We conclude that hematite-rich localities, with evidence of extensive aqueous alteration of originally reducing phases, such as the Vera Rubin Ridge, may be relatively promising targets for identifying martian biologically sourced MOM.

Methanogenesis from Mineral Carbonates, a Potential Indicator for Life on Mars

Priorities for the exploration of Mars involve the identification and observation of biosignatures that indicate the existence of life on the planet. The atmosphere and composition of the sediments on Mars suggest suitability for anaerobic chemolithotrophic metabolism. Carbonates are often considered as morphological biosignatures, such as stromatolites, but have not been considered as potential electron acceptors. Within the present study, hydrogenotrophic methanogen enrichments were generated from sediments that had received significant quantities of lime from industrial processes (lime kiln/steel production). These enrichments were then supplemented with calcium carbonate powder or marble chips as a sole source of carbon. These microcosms saw a release of inorganic carbon into the liquid phase, which was subsequently removed, resulting in the generation of methane, with 0.37 ± 0.09 mmoles of methane observed in the steel sediment enrichments supplemented with calcium carbonate powder. The steel sediment microcosms and lime sediments with carbonate powder enrichments were dominated by Methanobacterium sp., whilst the lime/marble enrichments were more diverse, containing varying proportions of Methanomassiliicoccus, Methanoculleus and Methanosarcina sp. In all microcosm experiments, acetic acid was detected in the liquid phase. Our results indicate that chemolithotrophic methanogenesis should be considered when determining biosignatures for life on Mars.

Transformation of Cyanobacterial Biomolecules by Iron Oxides During Flash Pyrolysis: Implications for Mars Life-Detection Missions

Answering the question of whether life ever existed on Mars is a key goal of both NASA's and ESA's imminent Mars rover missions. The obfuscatory effects of oxidizing salts, such as perchlorates and sulfates, on organic matter during thermal decomposition analysis techniques are well established. Less well studied are the transformative effects of iron oxides and (oxy)hydroxides, which are present in great abundances in the martian regolith. We examined the products of flash pyrolysis-gas chromatography-mass spectrometry (a technique analogous to the thermal techniques employed by past, current, and future landed Mars missions) which form when the cyanobacteria Arthrospira platensis are heated in the presence of a variety of Mars-relevant iron-bearing minerals. We found that iron oxides/(oxy)hydroxides have transformative effects on the pyrolytic products of cyanobacterial biomolecules. Both the abundance and variety of molecular species detected were decreased as iron substrates transformed biomolecules, by both oxidative and reductive processes, into lower fidelity alkanes, aromatic and aryl-bonded hydrocarbons. Despite the loss of fidelity, a suite that contains mid-length alkanes and polyaromatic hydrocarbons and/or aryl-bonded molecules in iron-rich samples subjected to pyrolysis may allude to the transformation of cyanobacterially derived mid-long chain length fatty acids (particularly unsaturated fatty acids) originally present in the sample. Hematite was found to be the iron oxide with the lowest transformation potential, and because this iron oxide has a high affinity for codeposition of organic matter and preservation over geological timescales, sampling at Mars should target sediments/strata that have undergone a diagenetic history encouraging the dehydration, dihydroxylation, and oxidation of more reactive iron-bearing phases to hematite by looking for (mineralogical) evidence of the activity of oxidizing, acidic/neutral, and either hot or long-lived fluids.

Active microbial ecosystem in glacier basal ice fuelled by iron and silicate comminution-derived hydrogen

The basal zone of glaciers is characterized by physicochemical properties that are distinct from firnified ice due to strong interactions with underlying substrate and bedrock. Basal ice (BI) ecology and the roles that the microbiota play in biogeochemical cycling, weathering, and proglacial soil formation remain poorly described. We report on basal ice geochemistry, bacterial diversity (16S rRNA gene phylogeny), and inferred ecological roles at three temperate Icelandic glaciers. We sampled three physically distinct basal ice facies (stratified, dispersed, and debris bands) and found facies dependent on biological similarities and differences; basal ice character is therefore an important sampling consideration in future studies. Based on a high abundance of silicates and Fe-containing minerals and, compared to earlier BI literature, total C was detected that could sustain the basal ice ecosystem. It was hypothesized that C-fixing chemolithotrophic bacteria, especially Fe-oxidisers and hydrogenotrophs, mutualistically support associated heterotrophic communities. Basal ice-derived rRNA gene sequences corresponding to genera known to harbor hydrogenotrophic methanogens suggest that silicate comminution-derived hydrogen can also be utilized for methanogenesis. PICRUSt-predicted metabolism suggests that methane metabolism and C-fixation pathways could be highly relevant in BI, indicating the importance of these metabolic routes. The nutrients and microbial communities release from melting basal ice may play an important role in promoting pioneering communities establishment and soil development in deglaciating forelands.

Mars-Analog Calcium Sulfate Veins Record Evidence of Ancient Subsurface Life

Ancient veins of calcium sulfate minerals (anhydrite, bassanite, and gypsum) deposited by subsurface aqueous fluids crosscut fluviolacustrine sedimentary rocks at multiple localities on Mars. Although these veins have been considered an attractive target for astrobiological investigation, their potential to preserve biosignatures is poorly understood. Here, we report the presence of biogenic authigenic pyrite in a fibrous gypsum vein of probable Cenozoic emplacement age from Permian lacustrine rocks in Northwest England. Pyrite occurs at the vein margins and displays a complex interfingering boundary with the surrounding gypsum suggestive of replacive authigenic growth. Gypsum-entombed carbonaceous material of probable organic origin was also identified by Raman spectroscopic microscopy in close proximity to the pyrite. Spatially resolved ion microprobe (SIMS) measurements reveal that the pyrite sulfur isotope composition is consistently very light (δ³⁴S_VCDT = -30.7‰). Comparison with the sulfate in the vein gypsum (δ³⁴S_VCDT = +8.5‰) indicates a fractionation too large to be explained by nonbiological (thermochemical) sulfate reduction. We infer that the pyrite was precipitated by microorganisms coupling the reduction of vein-derived sulfate with the oxidation of wall-derived organic matter. This is the first evidence that such veins can incorporate biosignatures that remain stable over geological time, which could be detected in samples returned from Mars.

Paleo-Rock-Hosted Life on Earth and the Search on Mars: A Review and Strategy for Exploration

Here we review published studies on the abundance and diversity of terrestrial rock-hosted life, the environments it inhabits, the evolution of its metabolisms, and its fossil biomarkers to provide guidance in the search for life on Mars. Key findings are (1) much terrestrial deep subsurface metabolic activity relies on abiotic energy-yielding fluxes and in situ abiotic and biotic recycling of metabolic waste products rather than on buried organic products of photosynthesis; (2) subsurface microbial cell concentrations are highest at interfaces with pronounced chemical redox gradients or permeability variations and do not correlate with bulk host rock organic carbon; (3) metabolic pathways for chemolithoautotrophic microorganisms evolved earlier in Earth's history than those of surface-dwelling phototrophic microorganisms; (4) the emergence of the former occurred at a time when Mars was habitable, whereas the emergence of the latter occurred at a time when the martian surface was not continually habitable; (5) the terrestrial rock record has biomarkers of subsurface life at least back hundreds of millions of years and likely to 3.45 Ga with several examples of excellent preservation in rock types that are quite different from those preserving the photosphere-supported biosphere. These findings suggest that rock-hosted life would have been more likely to emerge and be preserved in a martian context. Consequently, we outline a Mars exploration strategy that targets subsurface life and scales spatially, focusing initially on identifying rocks with evidence for groundwater flow and low-temperature mineralization, then identifying redox and permeability interfaces preserved within rock outcrops, and finally focusing on finding minerals associated with redox reactions and associated traces of carbon and diagnostic chemical and isotopic biosignatures. Using this strategy on Earth yields ancient rock-hosted life, preserved in the fossil record and confirmable via a suite of morphologic, organic, mineralogical, and isotopic fingerprints at micrometer scale. We expect an emphasis on rock-hosted life and this scale-dependent strategy to be crucial in the search for life on Mars.

Occurrence Forms, Composition, Distribution, Origin and Potential Hazard of Natural Hydrogen–Hydrocarbon Gases in Ore Deposits of the Khibiny and Lovozero Massifs: A Review

The Khibiny and Lovozero massifs—the world’s largest alkaline massifs—contain deposits with unique reserves of phosphorus and rare metals, respectively. The reduced gas content in the rocks and, especially, the ore deposits of these massifs is unusually high for igneous complexes, thus representing both geochemical and practical interests. There are three morphological types (or occurrence forms) of the gas phase in these deposits: occluded (predominantly in vacuoles of micro-inclusions in minerals), diffusely dispersed, and free. All three morphological types have the same qualitative chemical gas composition. Methane is the main component, and molecular hydrogen (which sometimes dominates) and ethane are the subordinate constituents. Heavier methane homologs (up to and including pentanes), alkenes, helium, and rarely carbon oxide and dioxide are present in minor or trace amounts. All three morphological types of gases are irregularly distributed in space to various degrees. Free gases also show a release intensity that varies in time. The majority of researchers recognize that the origin of these gases is abiogenic and mostly related to the formation of the massifs and deposits. However, the relative time and mechanism of their generation are still debated. Emissions of combustible and explosive hydrogen–hydrocarbon gases pose hazards during the underground mining of ore deposits. Therefore, the distinctive features of gas-bearing capacity are an essential part of the mining and geological characterization of such deposits because they provide a basis for establishing and implementing special measures of the gas regime during mining operations.

A Maximum Subsurface Biomass on Mars from Untapped Free Energy: CO and H2 as Potential Antibiosignatures

Whether extant life exists in the martian subsurface is an open question. High concentrations of photochemically produced CO and H₂ in the otherwise oxidizing martian atmosphere represent untapped sources of biologically useful free energy. These out-of-equilibrium species diffuse into the regolith, so subsurface microbes could use them as a source of energy and carbon. Indeed, CO oxidation and methanogenesis are relatively simple and evolutionarily ancient metabolisms on Earth. Consequently, assuming CO- or H₂-consuming metabolisms would evolve on Mars, the persistence of CO and H₂ in the martian atmosphere sets limits on subsurface metabolic activity. In this study, we constrain such maximum subsurface metabolic activity on Mars using a one-dimensional photochemical model with a hypothetical global biological sink on atmospheric CO and H₂. We increase the biological sink until the modeled atmospheric composition diverges from observed abundances. We find maximum biological downward subsurface sinks of 1.5 × 10⁸ molecules/(cm²·s) for CO and 1.9 × 10⁸ molecules/(cm²·s¹) for H₂. These convert to a maximum metabolizing biomass of ≲10²⁷ cells or ≤2 × 10¹¹ kg, equivalent to ≤10^-4-10^-5 of Earth's biomass, depending on the terrestrial estimate. Diffusion calculations suggest that this upper biomass limit applies to the top few kilometers of the martian crust in communication with the atmosphere at low to mid-latitudes. This biomass limit is more robust than previous estimates because we test multiple possible chemoautotrophic ecosystems over a broad parameter space of tunable model variables using an updated photochemical model with precise atmospheric concentrations and uncertainties from Curiosity. Our results of sparse or absent life in the martian subsurface also demonstrate how the atmospheric redox pairs of CO-O₂ and H₂-O₂ may constitute antibiosignatures, which may be relevant to excluding life on exoplanets.

Rock-crushing derived hydrogen directly supports a methanogenic community: significance for the deep biosphere

Microbial populations exist to great depths on Earth, but with apparently insufficient energy supply. Earthquake rock fracturing produces H₂ from mechanochemical water splitting, however, microbial utilization of this widespread potential energy source has not been directly demonstrated. Here, we show experimentally that mechanochemically generated H₂ from granite can be directly, long-term, utilized by a CH₄ producing microbial community. This is consistent with CH₄ formation in subsurface rock fracturing in the environment. Our results not only support water splitting H₂ generation as a potential deep biosphere energy source, but as an oxidant must also be produced, they suggest that there is also a respiratory oxidant supply in the subsurface which is independent of photosynthesis. This may explain the widespread distribution of facultative aerobes in subsurface environments. A range of common rocks were shown to produce mechanochemical H₂ , and hence, this process should be widespread in the subsurface, with the potential for considerable mineral fuelled CH₄ production.

New ecosystems in the deep subsurface follow the flow of water driven by geological activity

Eukarya have been discovered in the deep subsurface at several locations in South Africa, but how organisms reach the subsurface remains unknown. We studied river-subsurface fissure water systems and identified Eukarya from a river that are genetically identical for 18S rDNA. To further confirm that these are identical species one metazoan species recovered from the overlying river interbred successfully with specimen recovered from an underlying mine at −1.4 km. In situ seismic simulation experiments were carried out and show seismic activity to be a major force increasing the hydraulic conductivity in faults allowing organisms to create ecosystems in the deep subsurface. As seismic activity is a non-selective force we recovered specimen of algae and Insecta that defy any obvious other explanation at a depth of −3.4 km. Our results show there is a steady flow of surface organisms to the deep subsurface where some survive and adapt and others perish. As seismic activity is also present on other planets and moons in our solar system the mechanism elucidated here may be relevant for future search and selection of landing sites in planetary exploration.

Subsurface Microbial Hydrogen Cycling: Natural Occurrence and Implications for Industry

Hydrogen is a key energy source for subsurface microbial processes, particularly in subsurface environments with limited alternative electron donors, and environments that are not well connected to the surface. In addition to consumption of hydrogen, microbial processes such as fermentation and nitrogen fixation produce hydrogen. Hydrogen is also produced by a number of abiotic processes including radiolysis, serpentinization, graphitization, and cataclasis of silicate minerals. Both biotic and abiotically generated hydrogen may become available for consumption by microorganisms, but biotic production and consumption are usually tightly coupled. Understanding the microbiology of hydrogen cycling is relevant to subsurface engineered environments where hydrogen-cycling microorganisms are implicated in gas consumption and production and corrosion in a number of industries including carbon capture and storage, energy gas storage, and radioactive waste disposal. The same hydrogen-cycling microorganisms and processes are important in natural sites with elevated hydrogen and can provide insights into early life on Earth and life on other planets. This review draws together what is known about microbiology in natural environments with elevated hydrogen, and highlights where similar microbial populations could be of relevance to subsurface industry.