ABSTRACT Evidence from the Cassini mission confirmed that Saturn’s moon Enceladus hosts a subsurface alkaline ocean where rock-water reactions may generate redox disequilibria capable of supporting microbial metabolisms. To investigate potential microbial survival under simulated Enceladus ocean conditions, we used thermodynamic modeling to develop a salt formulation consistent with one possible Enceladus ocean composition and supplemented it with putative microbial energy sources to create a growth medium. The medium was inoculated with samples from diverse ocean world analog environments on Earth to determine which microorganisms could persist under Enceladus-like conditions. The microorganisms persisting in this geochemically bounded medium were heterotrophic, metabolically versatile bacteria with low carbon requirements. Genomic and physiological analyses further showed the presence of multiple stress-response pathways, sodium-based bioenergetic systems, osmoregulation strategies, and other adaptations consistent with survival in alkaline, low-nutrient settings. These results suggest that some stress-tolerant heterotrophic bacteria may serve as useful model organisms for life in Enceladus’ subsurface ocean. These findings demonstrate the value of geochemically modeled media as a framework for constraining habitability, identifying relevant biosignatures, and probing potential microbial survival strategies beyond Earth.

Several moons in the outer solar system host liquid water oceans. A key next step in assessing the habitability of these ocean worlds is to determine whether life’s elemental and energy requirements are also met. Phosphorus is required by all known life and is often limited to biological productivity in Earth’s oceans. This raises the possibility that its availability may limit the abundance or productivity of Earth-like life on ocean worlds. To address this potential problem, here we calculate the equilibrium dissolved phosphate concentrations associated with the reaction of water and rocks—a key driver of ocean chemical evolution—across a broad range of compositional inputs and reaction conditions. Equilibrium dissolved phosphate concentrations range from 10 −11 to 10 −1 mol/kg across the full range of carbonaceous chondrite compositions and reaction conditions considered, but are generally > 10 −5 mol/kg for most plausible scenarios. Relative to the phosphate requirements and uptake kinetics of microorganisms in Earth’s oceans, such concentrations would be sufficient to support initially rapid cell growth and construction of global ocean cell populations larger than those observed in Earth’s deep oceans.

Putative alkaline hydrothermal systems on Noachian Mars were potentially habitable environments for microorganisms. However, the types of reactions that could have fueled microbial life in such systems and the amount of energy available from them have not been quantitatively constrained. In this study, we use thermodynamic modeling to calculate which catabolic reactions could have supported ancient life in a saponite-precipitating hydrothermal vent system in the Eridania basin on Mars. To further evaluate what this could mean for microbial life, we evaluated the energy potential of an analog site in Iceland, the Strytan Hydrothermal Field. Results show that, of the 84 relevant redox reactions that were considered, the highest energy-yielding reactions in the Eridania hydrothermal system were dominated by methane formation. By contrast, Gibbs energy calculations carried out for Strytan indicate that the most energetically favorable reactions are CO2 and O2 reduction coupled to H2 oxidation. In particular, our calculations indicate that an ancient hydrothermal system within the Eridania basin could have been a habitable environment for methanogens using NH4+ as an electron acceptor. Differences in Gibbs energies between the two systems were largely determined by oxygen-its presence on Earth and absence on Mars. However, Strytan can serve as a useful analog for Eridania when studying methane-producing reactions that do not involve O2.

Abstract Water‐rock reactions liberate bioavailable energy, a necessary condition for chemotrophic habitability and origins of life. The major minerals of ultramafic (UM) rocks: olivine (ol), orthopyroxene (opx), and clinopyroxene (cpx), are of particular astrobiological interest as they are widespread in the solar system and known to produce significant quantities of H2 during aqueous alteration (serpentinization). H2 yields energy to life when oxidized by chemical species common to planetary fluids and is the most deeply rooted metabolite in Earth's phylogenetic tree. However, while field observations and calculations have corroborated the H2‐generating potential of UM rocks, specific formation pathways remain elusive, as the alteration assemblages in natural samples contain substantial heterogeneity. Here we show that variable UM compositions, temperature (T), and the mass ratio of Earth's seawater to rock (w/r) conspire to segregate alteration systems into those that produce supra‐mmol H2, and those that produce sub‐µmol H2. The oxidation of UM Fe(II) to Fe(III) provides the electrons necessary to reduce H2O to H2. However, the Fe(III)‐bearing phases that facilitate this process form together with more abundant phases competing for the same elements, including Ca, Si, and Fe(II). As a result, H2 abundance is determined by non‐redox‐active elements. Maximum H2 at high T requires low Si and is generally supported by Fe(III)‐serpentine and magnetite formation. Andradite formation, which stores Fe(III) but requires elevated Ca abundance, throttles up H2 production as T decreases in aging systems. These conditions are achieved in ol‐ and cpx‐rich rocks while opx‐rich rocks sequester less Fe(III), producing less H2.

A geochemical gradient established by mixing between reduced, hyperalkaline (pH > 11), H 2 ‐rich fluids generated through the process of serpentinization and surrounding surface water (pH ∼ 8) in the Samail Ophiolite of Oman provides an opportunity to characterize the geochemical and biological factors that influence the distribution of H 2 oxidizing chemotrophs, hydrogenotrophs. In this study, 16S rRNA gene amplicon sequencing was implemented to characterize hydrogenotrophs in sediments underlying surface expressed serpentinized fluids in Oman. Hydrogenotroph phylotype distribution was evaluated as functions of chemical energy supplies for their given metabolic redox reactions. Through this approach, it was discovered that hydrogenotrophic taxa are likely constrained to sediments with overlying fluids that have <∼60 μ m O 2 , including microorganisms of the genus, Hydrogenophaga . Sulfate reducers of the family, Thermodesulfovibrionaceae , likely require >∼10 μ m SO 4 −2 for survival. In sediments with fluids having >∼10 μ m SO 4 −2 , sulfate reducers likely outcompete microorganisms of the methanogen genus, Methanobacterium , for H 2 . Additionally, differences in distribution between Thermodesulfovibrionaceae and Methanobacterium may be driven by the availability of electron acceptors and the redox reaction that is most energy yielding in the fluid. Taken together, observations from the Oman geochemical gradient result in a hydrogenotroph niche model that can be used to evaluate global distribution patterns of hydrogenotrophs in continental serpentinized fluids. On a global scale, based on previous studies, Methanobacterium is constrained to fluids that have <∼10 μ m SO 4 −2 .

Microbial denitrification converts fixed nitrogen species into gases in extant oceans. However, it is unclear how such transformations occurred within the early nitrogen cycle of the Archean. Here we present experimental anoxic surface-catalyzed reduction of nitrite and nitrate via green rust and magnetite combined with diffusion and photochemical modeling. We find that in a Fe2+-rich marine environment, Fe minerals could have catalyzed abiotic denitrification reactions leading to the formation of nitric oxide (NO) and nitrous oxide (N2O). Nitrate did not exhibit reactivity in the presence of either mineral or aqueous Fe2+, however, both minerals induced rapid nitrite reduction to NO and N2O. While N2O escaped into the atmosphere (63% of nitrite-nitrogen, with green rust as catalyst), NO remained associated with precipitates (7%) serving as a potential shuttle to the benthic ocean. The modeling suggests that marine N2O emissions would have sustained 0.8-6 ppb of atmospheric N2O without a protective ozone layer. Our findings add detail to the as yet incompletely documented Archean nitrogen cycle, implying a globally distributed process driven by chemical kinetics similar to those of modern enzymatically mediated conversions.

At present, molecular hydrogen (H 2 ) produced through Fe(II) oxidation during serpentinization of ultramafic rocks represents a small fraction of the global sink for O 2 due to limited exposures of ultramafic rocks. In contrast, ultramafic rocks such as komatiites were much more common in the Early Earth and H 2 production via serpentinization was a likely factor in maintaining an O 2 -free atmosphere throughout most of the Archean. Using thermodynamic simulations, this work quantifies the global O 2 consumption attributed to serpentinization during the past 3.5 billion years. Results show that H 2 generation is strongly dependent on rock compositions where serpentinization of more magnesian lithologies generated substantially higher amounts of H 2 . Consumption of >2 Tmole O 2 yr −1 via low-temperature serpentinization of Archean continents and seafloor is possible. This O 2 sink diminished greatly towards the end of the Archean as ultramafic rocks became less common and helped set the stage for the Great Oxidation Event.

Maximizing scientific return is critical to the success of space exploration. During the SUBSEA project, which explored the Sea Cliff vent site aboard the E/V Nautilus as an analog for hydrothermal systems on Ocean Worlds, we used forward geochemical modeling to guide decision-making during the process of exploration. Before the expedition, we performed 1670 reaction-path calculations to simulate water-rock interactions during hydrothermal circulation covering wide ranges of reaction conditions to predict the diversity of possible chemical compositions and energy available for chemosynthetic microorganisms at the Sea Cliff vents. Calculation of the information entropy of predicted concentrations of major solutes and pH allowed us to identify dissolved silica as the chemical species capable of yielding the most information about reaction conditions; as such, the measurement of this parameter was implemented aboard ship for our field program. Using telepresence, results of onboard chemical analyses of fluid samples collected during seafloor Dive n were sent to our shore-based scientific team, who processed the data and used the outcomes to inform the design of Dive n + 2. Combining data processing with forward modeling revealed, within just two dives, that all the observed fluids venting from 10 degrees C to 300 degrees C most likely resulted from simple conservative mixing between seawater and a common hydrothermal fluid endmember: the result of reaction of seawater with basalt at >= 350 degrees C. Identification of these reaction conditions early within the cruise allowed additional calculations to be performed to quantify the energy available from redox disequilibria as a function of vent-fluid temperatures as they exited the seafloor. These calculations can help inform and optimize real-time microbiological sampling and culture experiments onboard the ship during field expeditions. The success of our approach coupling forward modeling and onboard ship analyses allowed improved efficiency in completing process studies at the Sea Cliff vent site, providing time for further exploration and sampling of a newly discovered vent site: Apollo. This study demonstrates a novel application of forward and real-time modeling for scientific exploration that allows the time required for result-informed decision making to be reduced from years to hours - an essential breakthrough for future space exploration missions.

Microbial metabolisms were limited by available terminal electron acceptors in the anoxic environment of the early Archean. However, iron mineral phases in Fe2+-rich (ferruginous) oceans could have catalyzed reactions with abiotically fixed nitrogen leading to the formation of nitrous oxide (N2O), a potentially favorable terminal electron acceptor. We experimentally simulated anoxic surface-catalyzed reduction of nitrite and nitrate via green rust and magnetite. Besides N2O, we detected and quantified the formation of substantial amounts of nitric oxide (NO). While N2O escaped into the gas phase (63% of nitrite-nitrogen, with green rust as catalyst), NO remained associated with precipitates (7% of nitrite-nitrogen). Using diffusion and photochemical modeling, we show that marine N2O emissions could have sustained atmospheric N2O pools of 1-7 ppb. Although this concentration was insufficient to cause significant warming, the seawater enriched in N2O and NO could have critically affected early benthic life by providing the opportunity to conserve energy.

Mantle derived basalts along the entirety of the Earth’s Mid-Ocean Ridge (MOR) spreading centers are continuously altered by seawater, allowing the hydrosphere to subsume energy and exchange mass with the deep, slowly cooling Earth. Compositional heterogeneities inherent to these basalts—the result of innumerable geophysical and geochemical processes in the mantel and crust—generate spatial variation in the equilibrium states toward which these water-rock environments cascade. This alteration results in a unique distribution of precipitate assemblages, hydrothermal fluid chemistries, and energetic landscapes among ecosystems rooted within and above the seafloor. The equilibrium states for the full range of basalt compositional heterogeneity present today are calculated over all appropriate temperatures and extents of reaction with seawater, along with the non-equilibrium mixtures generated when hydrothermal fluids mix back into seawater. These mixes support ancient and diverse ecosystems fed not by the energy of the sun, but by the geochemical energy of the Earth. Facilitated by novel, high throughout code, this effort has yielded a high-resolution compositional database that is mapped back onto all ridge systems. By resolving the chemical and energetic consequences of basalt-seawater interaction to sub-ridge scales, alteration features that are globally homogeneous can be distinguished from those that are locally unique, guiding future field observations with testable geochemical and biochemical predictions.