Investigating Europa's Habitability with the Europa Clipper.

The habitability of Europa is a property within a system, which is driven by a multitude of physical and chemical processes and is defined by many interdependent parameters, so that its full characterization requires collaborative investigation. To explore Europa as an integrated system to yield a complete picture of its habitability, the Europa Clipper mission has three primary science objectives: (1) characterize the ice shell and ocean including their heterogeneity, properties, and the nature of surface-ice-ocean exchange; (2) characterize Europa's composition including any non-ice materials on the surface and in the atmosphere, and any carbon-containing compounds; and (3) characterize Europa's geology including surface features and localities of high science interest. The mission will also address several cross-cutting science topics including the search for any current or recent activity in the form of thermal anomalies and plumes, performing geodetic and radiation measurements, and assessing high-resolution, co-located observations at select sites to provide reconnaissance for a potential future landed mission.

An examination of protist diversity in serpentinization-hosted ecosystems of the Samail Ophiolite of Oman.

In the Samail Ophiolite of Oman, the geological process of serpentinization produces reduced, hydrogen rich, hyperalkaline (pH > 11) fluids. These fluids are generated through water reacting with ultramafic rock from the upper mantle in the subsurface. On Earth's continents, serpentinized fluids can be expressed at the surface where they can mix with circumneutral surface water and subsequently generate a pH gradient (∼pH 8 to pH > 11) in addition to variations in other chemical parameters such as dissolved CO, O, and H.

Characterization of microbiomic and geochemical compositions across the photosynthetic fringe.

Hot spring outflow channels provide geochemical gradients that are reflected in microbial community compositions. In many hot spring outflows, there is a distinct visual demarcation as the community transitions from predominantly chemotrophs to having visible pigments from phototrophs. It has been hypothesized that this transition to phototrophy, known as the photosynthetic fringe, is a result of the pH, temperature, and/or sulfide concentration gradients in the hot spring outflows.

Phosphate availability and implications for life on ocean worlds.

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.

Using thermodynamics to obtain geochemical information from genomes.

Thermodynamic characterization of the relative stabilities of chemical compounds is a pillar of conceptual models in various fields of geosciences. Analogous models applied to genomes can yield new information about the relationship between genomes and their geochemical environments. In this perspective article, we present a chemical and thermodynamic analysis of prokaryotic lineages that have been the target of previous phylogenomic studies of evolutionary adaptation to varying redox conditions.

Kinetics and Mechanisms of Hydrothermal Dehydration of Cyclic 1,2- and 1,4-Diols.

Hydrothermal dehydration is an attractive method for deoxygenation and upgrading of biofuels because it requires no reagents or catalysts other than superheated water. Although mono-alcohols cleanly deoxygenate via dehydration under many conditions, polyols such as those derived from saccharides and related structures are known to be recalcitrant with respect to dehydration. Here, we describe detailed mechanistic and kinetic studies of hydrothermal dehydration of 1,2- and 1,4-cyclohexanediols as model compounds to investigate how interactions between the hydroxyls can control the reaction.

Decreasing extents of Archean serpentinization contributed to the rise of an oxidized atmosphere.

At present, molecular hydrogen (H) produced through Fe(II) oxidation during serpentinization of ultramafic rocks represents a small fraction of the global sink for O due to limited exposures of ultramafic rocks. In contrast, ultramafic rocks such as komatiites were much more common in the Early Earth and H production via serpentinization was a likely factor in maintaining an O-free atmosphere throughout most of the Archean. Using thermodynamic simulations, this work quantifies the global O consumption attributed to serpentinization during the past 3.

Ecological Dichotomies Arise in Microbial Communities Due to Mixing of Deep Hydrothermal Waters and Atmospheric Gas in a Circumneutral Hot Spring.

Little is known of how the confluence of subsurface and surface processes influences the assembly and habitability of hydrothermal ecosystems. To address this knowledge gap, the geochemical and microbial composition of a high-temperature, circumneutral hot spring in Yellowstone National Park was examined to identify the sources of solutes and their effect on the ecology of microbial inhabitants. Metagenomic analysis showed that populations comprising planktonic and sediment communities are archaeal dominated, are dependent on chemical energy (chemosynthetic), share little overlap in their taxonomic composition, and are differentiated by their inferred use of/tolerance to oxygen and mode of carbon metabolism.

Seasonal hydrologic and geologic forcing drive hot spring geochemistry and microbial biodiversity.

Hot springs integrate hydrologic and geologic processes that vary over short- and long-term time scales. However, the influence of temporal hydrologic and geologic change on hot spring biodiversity is unknown. Here, we coordinated near-weekly, cross-seasonal (~140 days) geochemical and microbial community analyses of three widely studied hot springs with local precipitation data in Yellowstone National Park.

The Molecular Basis for Life in Extreme Environments.

Sampling and genomic efforts over the past decade have revealed an enormous quantity and diversity of life in Earth's extreme environments. This new knowledge of life on Earth poses the challenge of understandingits molecular basis in such inhospitable conditions, given that such conditions lead to loss of structure and of function in biomolecules from mesophiles. In this review, we discuss the physicochemical properties of extreme environments.

Cooperative formation of porous silica and peptides on the prebiotic Earth.

Modern technology has perfected the synthesis of catalysts such as zeolites and mesoporous silicas using organic structure directing agents (SDA) and their industrial use to catalyze a large variety of organic reactions within their pores. We suggest that early in prebiotic evolution, synergistic interplay arose between organic species in aqueous solution and silica formed from rocks by dynamic dissolution-recrystallization. The natural organics, for example, amino acids, small peptides, and fatty acids, acted as SDA for assembly of functional porous silica structures that induced further polymerization of amino acids and peptides, as well as other organic reactions.

Lipid Biomarker Record of the Serpentinite-Hosted Ecosystem of the Samail Ophiolite, Oman and Implications for the Search for Biosignatures on Mars.

Serpentinization is a weathering process in which ultramafic rocks react with water, generating a range of products, including serpentine and other minerals, in addition to H and low-molecular-weight hydrocarbons that are capable of sustaining microbial life. Lipid biomarker analyses of serpentinite-hosted ecosystems hold promise as tools for investigating microbial activity in ancient Earth environments and other terrestrial planets such as Mars because lipids have the potential for longer term preservation relative to DNA, proteins, and other more labile organic molecules. Here, we report the first lipid biomarker record of microbial activity in the mantle section of the Samail Ophiolite, in the Sultanate of Oman, a site undergoing active serpentinization.

Carbon Oxidation State in Microbial Polar Lipids Suggests Adaptation to Hot Spring Temperature and Redox Gradients.

The influence of oxidation-reduction (redox) potential on the expression of biomolecules is a topic of ongoing exploration in geobiology. In this study, we investigate the novel possibility that structures and compositions of lipids produced by microbial communities are sensitive to environmental redox conditions. We extracted lipids from microbial biomass collected along the thermal and redox gradients of four alkaline hot springs in Yellowstone National Park (YNP) and investigated patterns in the average oxidation state of carbon (Z), a metric calculated from the chemical formulae of lipid structures.

Probing the geological source and biological fate of hydrogen in Yellowstone hot springs.

Hydrogen (H ) is enriched in hot springs and can support microbial primary production. Using a series of geochemical proxies, a model to describe variable H concentrations in Yellowstone National Park (YNP) hot springs is presented. Interaction between water and crustal iron minerals yields H that partition into the vapour phase during decompressional boiling of ascending hydrothermal fluids.

Geobiological feedbacks and the evolution of thermoacidophiles.

Oxygen-dependent microbial oxidation of sulfur compounds leads to the acidification of natural waters. How acidophiles and their acidic habitats evolved, however, is largely unknown. Using 16S rRNA gene abundance and composition data from 72 hot springs in Yellowstone National Park, Wyoming, we show that hyperacidic (pH<3.

Microbial substrate preference dictated by energy demand, not supply.

Growth substrates that maximize energy yield are widely thought to be utilized preferentially by microorganisms. However, observed distributions of microorganisms and their activities often deviate from predictions based solely on thermodynamic considerations of substrate energy supply. Here we present observations of the bioenergetics and growth yields of a metabolically flexible, thermophilic strain of the archaeon when grown autotrophically on minimal medium with hydrogen (H) or elemental sulfur (S°) as an electron donor, and S° or ferric iron (Fe) as an electron acceptor.

Carbonaceous Chondrite Meteorites: the Chronicle of a Potential Evolutionary Path between Stars and Life.

The biogenic elements, H, C, N, O, P and S, have a long cosmic history, whose evolution can still be observed in diverse locales of the known universe, from interstellar clouds of gas and dust, to pre-stellar cores, nebulas, protoplanetary discs, planets and planetesimals. The best analytical window into this cosmochemical evolution as it neared Earth has been provided so far by the small bodies of the Solar System, some of which were not significantly altered by the high gravitational pressures and temperatures that accompanied the formation of larger planets and may carry a pristine record of early nebular chemistry. Asteroids have delivered such records, as their fragments reach the Earth frequently and become available for laboratory analyses.

Ecological differentiation in planktonic and sediment-associated chemotrophic microbial populations in Yellowstone hot springs.

Chemosynthetic sediment and planktonic community composition and sizes, aqueous geochemistry and sediment mineralogy were determined in 15 non-photosynthetic hot springs in Yellowstone National Park (YNP). These data were used to evaluate the hypothesis that differences in the availability of dissolved or mineral substrates in the bulk fluids or sediments within springs coincides with ecologically differentiated microbial communities and their populations. Planktonic and sediment-associated communities exhibited differing ecological characteristics including community sizes, evenness and richness.

Organic Oxidations Using Geomimicry.

Oxidations of phenylacetic acid to benzaldehyde, benzyl alcohol to benzaldehyde, and benzaldehyde to benzoic acid have been observed, in water as the solvent and using only copper(II) chloride as the oxidant. The reactions are performed at 250 °C and 40 bar, conditions that mimic hydrothermal reactions that are geochemically relevant. Speciation calculations show that the oxidizing agent is not freely solvated copper(II) ions, but complexes of copper(II) with chloride and carboxylate anions.

Stable isotope labeling confirms mixotrophic nature of streamer biofilm communities at alkaline hot springs.

Streamer biofilm communities (SBC) are often observed within chemosynthetic zones of Yellowstone hot spring outflow channels, where temperatures exceed those conducive to photosynthesis. Nearest the hydrothermal source (75-88°C) SBC comprise thermophilic Archaea and Bacteria, often mixed communities including Desulfurococcales and uncultured Crenarchaeota, as well as Aquificae and Thermus, each carrying diagnostic membrane lipid biomarkers. We tested the hypothesis that SBC can alternate their metabolism between autotrophy and heterotrophy depending on substrate availability.

High pH microbial ecosystems in a newly discovered, ephemeral, serpentinizing fluid seep at Yanartaş (Chimera), Turkey.

Gas seeps emanating from Yanartaş (Chimera), Turkey, have been documented for thousands of years. Active serpentinization produces hydrogen and a range of carbon gases that may provide fuel for life. Here we report a newly discovered, ephemeral fluid seep emanating from a small gas vent at Yanartaş.

Chemolithotrophic primary production in a subglacial ecosystem.

Glacial comminution of bedrock generates fresh mineral surfaces capable of sustaining chemotrophic microbial communities under the dark conditions that pervade subglacial habitats. Geochemical and isotopic evidence suggests that pyrite oxidation is a dominant weathering process generating protons that drive mineral dissolution in many subglacial systems. Here, we provide evidence correlating pyrite oxidation with chemosynthetic primary productivity and carbonate dissolution in subglacial sediments sampled from Robertson Glacier (RG), Alberta, Canada.

Sphalerite is a geochemical catalyst for carbon-hydrogen bond activation.

Reactions among minerals and organic compounds in hydrothermal systems are critical components of the Earth's deep carbon cycle, provide energy for the deep biosphere, and may have implications for the origins of life. However, there is limited information as to how specific minerals influence the reactivity of organic compounds. Here we demonstrate mineral catalysis of the most fundamental component of an organic reaction: the breaking and making of a covalent bond.