Barite in hydrothermal environments as a recorder of subseafloor processes: a multiple‐isotope study from the Loki's Castle vent field

Barite chimneys are known to form in hydrothermal systems where barium‐enriched fluids generated by leaching of the oceanic basement are discharged and react with seawater sulfate. They also form at cold seeps along continental margins, where marine (or pelagic) barite in the sediments is remobilized because of subseafloor microbial sulfate reduction. We test the possibility of using multiple sulfur isotopes (δ³⁴S, Δ³³S, ?³⁶S) of barite to identify microbial sulfate reduction in a hydrothermal system. In addition to multiple sulfur isotopes, we present oxygen (δ¹⁸O) and strontium (⁸⁷Sr/⁸⁶Sr) isotopes for one of numerous barite chimneys in a low‐temperature (~20 °C) venting area of the Loki's Castle black smoker field at the ultraslow‐spreading Arctic Mid‐Ocean Ridge (AMOR). The chemistry of the venting fluids in the barite field identifies a contribution of at least 10% of high‐temperature black smoker fluid, which is corroborated by ⁸⁷Sr/⁸⁶Sr ratios in the barite chimney that are less radiogenic than in seawater. In contrast, oxygen and multiple sulfur isotopes indicate that the fluid from which the barite precipitated contained residual sulfate that was affected by microbial sulfate reduction. A sulfate reduction zone at this site is further supported by the multiple sulfur isotopic composition of framboidal pyrite in the flow channel of the barite chimney and in the hydrothermal sediments in the barite field, as well as by low SO₄ and elevated H₂S concentrations in the venting fluids compared with conservative mixing values. We suggest that the mixing of ascending H₂‐ and CH₄‐rich high‐temperature fluids with percolating seawater fuels microbial sulfate reduction, which is subsequently recorded by barite formed at the seafloor in areas where the flow rate is sufficient. Thus, low‐temperature precipitates in hydrothermal systems are promising sites to explore the interactions between the geosphere and biosphere in order to evaluate the microbial impact on these systems.     

Organic sulfur metabolisms in hydrothermal environments

Sulfur is central to the metabolisms of many organisms that inhabit extreme environments. While biotic and abiotic cycling of organic sulfur compounds has been well documented in low‐temperature anaerobic environments, cycling of organic sulfur in hydrothermal environments has received less attention. Recently published thermodynamic data have been used to estimate aqueous alkyl thiol and sulfide activities in deep‐sea hydrothermal systems. Here we use geochemical mixing models to predict fluid compositions that result from mixing end‐member hydrothermal fluid from the East Pacific Rise with bottom seawater. These fluid compositions are combined with estimates of methanethiol and dimethylsulfide activities to evaluate energy yields for potential organic sulfur‐based metabolisms under hydrothermal conditions. Aerobic respiration has the highest energy yields (over ?240 kJ/mol e^?) at lower temperature; however, oxygen is unlikely to persist at high temperatures, restricting aerobic respiration to mesophilic communities. Nitrite reduction to N₂ has the highest energy yields at higher temperatures (greater than ～40 °C). Nitrate and nitrite reduction to ammonium also yield significant energy (up to ?70 kJ/mol e^?). Much lower, but still feasible energy yields are calculated for sulfate reduction, disproportionation, and reduction with H₂. Organic compound family and the activity of methanethiol and dimethylsulfide were less important than metabolic strategy in determining overall energy yields. All metabolic strategies considered were exergonic within some portion of the mixing regime suggesting that organic sulfur‐based metabolisms may be prevalent within deep‐sea hydrothermal vent microbial communities.     

Bioenergetic Constraints on Microbial Hydrogen Utilization in Precambrian Deep Crustal Fracture Fluids

Precambrian Shield rocks host the oldest fracture fluids on Earth, with residence times up to a billion years or more. Water–rock reactions in these fracture systems over geological time have produced highly saline fluids, which can contain millimolar concentrations of H₂. Mixing of these ancient Precambrian fluids with meteoric or palaeo-meteoric water can occur through tectonic fracturing, providing microbial inocula and redox couples to fuel blooms of subsurface growth. Here, we present geochemical and microbiological data from a series of borehole fluids of varying ionic strength (0.6–6.4 M) from the Thompson Mine (Manitoba) within the Canadian Precambrian Shield. Thermodynamic calculations demonstrate sufficient energy for H₂-based catabolic reactions across the entire range of ionic strengths during mixing of high ionic strength fracture fluids with meteoric water, although microbial H₂ consumption and cultivable H₂-utilizing microbes were only detected in fluids of ≤1.9 M ionic strength. This pattern of microbial H₂ utilization can be explained by the higher potential bioenergetic cost of organic osmolyte synthesis at increasing ionic strengths. We propose that further research into the bioenergetics of osmolyte regulation in halophiles is warranted to better constrain the habitability zones of hydrogenotrophic ecosystems in both terrestrial subsurface, including potential future radioactive waste disposal sites, and other planetary body crustal environments, including Mars.     

Energy landscapes shape microbial communities in hydrothermal systems on the Arctic Mid-Ocean Ridge

Methods developed in geochemical modelling combined with recent advances in molecular microbial ecology provide new opportunities to explore how microbial communities are shaped by their chemical surroundings. Here, we present a framework for analyses of how chemical energy availability shape chemotrophic microbial communities in hydrothermal systems through an investigation of two geochemically different basalt-hosted hydrothermal systems on the Arctic Mid-Ocean Ridge: the Soria Moria Vent field (SMVF) and the Loki''s Castle Vent Field (LCVF). Chemical energy landscapes were evaluated through modelling of the Gibbs energy from selected redox reactions under different mixing ratios between seawater and hydrothermal fluids. Our models indicate that the sediment-influenced LCVF has a much higher potential for both anaerobic and aerobic methane oxidation, as well as aerobic ammonium and hydrogen oxidation, than the SMVF. The modelled energy landscapes were used to develop microbial community composition models, which were compared with community compositions in environmental samples inside or on the exterior of hydrothermal chimneys, as assessed by pyrosequencing of partial 16S rRNA genes. We show that modelled microbial communities based solely on thermodynamic considerations can have a high predictive power and provide a framework for analyses of the link between energy availability and microbial community composition.     

Temporal Changes in Archaeal Diversity and Chemistry in a Mid-Ocean Ridge Subseafloor Habitat

The temporal variation in archaeal diversity in vent fluids from a midocean ridge subseafloor habitat was examined using PCR-amplified 16S rRNA gene sequence analysis and most-probable-number (MPN) cultivation techniques targeting hyperthermophiles. To determine how variations in temperature and chemical characteristics of subseafloor fluids affect the microbial communities, we performed molecular phylogenetic and chemical analyses on diffuse-flow vent fluids from one site shortly after a volcanic eruption in 1998 and again in 1999 and 2000. The archaeal population was divided into particle-attached (>3-μm-diameter cells) and free-living fractions to test the hypothesis that subseafloor microorganisms associated with active hydrothermal systems are adapted for a lifestyle that involves attachment to solid surfaces and formation of biofilms. To delineate between entrained seawater archaea and the indigenous subseafloor microbial community, a background seawater sample was also examined and found to consist only of Group I Crenarchaeota and Group II Euryarchaeota, both of which were also present in vent fluids. The indigenous subseafloor archaeal community consisted of clones related to both mesophilic and hyperthermophilic Methanococcales, as well as many uncultured Euryarchaeota, some of which have been identified in other vent environments. The particle-attached fraction consistently showed greater diversity than the free-living fraction. The fluid and MPN counts indicate that while culturable hyperthermophiles represent less than 1% of the total microbial community, the subseafloor at new eruption sites does support a hyperthermophilic microbial community. The temperature and chemical indicators of the degree of subseafloor mixing appear to be the most important environmental parameters affecting community diversity, and it is apparent that decreasing fluid temperatures correlated with increased entrainment of seawater, decreased concentrations of hydrothermal chemical species, and increased incidence of seawater archaeal sequences.     

Relative Importance of H2 and H2S as Energy Sources for Primary Production in Geothermal Springs

Geothermal waters contain numerous potential electron donors capable of supporting chemolithotrophy-based primary production. Thermodynamic predictions of energy yields for specific electron donor and acceptor pairs in such systems are available, although direct assessments of these predictions are rare. This study assessed the relative importance of dissolved H₂ and H₂S as energy sources for the support of chemolithotrophic metabolism in an acidic geothermal spring in Yellowstone National Park. H₂S and H₂ concentration gradients were observed in the outflow channel, and vertical H₂S and O₂ gradients were evident within the microbial mat. H₂S levels and microbial consumption rates were approximately three orders of magnitude greater than those of H₂. Hydrogenobaculum-like organisms dominated the bacterial component of the microbial community, and isolates representing three distinct 16S rRNA gene phylotypes (phylotype = 100% identity) were isolated and characterized. Within a phylotype, O₂ requirements varied, as did energy source utilization: some isolates could grow only with H₂S, some only with H₂, while others could utilize either as an energy source. These metabolic phenotypes were consistent with in situ geochemical conditions measured using aqueous chemical analysis and in-field measurements made by using gas chromatography and microelectrodes. Pure-culture experiments with an isolate that could utilize H₂S and H₂ and that represented the dominant phylotype (70% of the PCR clones) showed that H₂S and H₂ were used simultaneously, without evidence of induction or catabolite repression, and at relative rate differences comparable to those measured in ex situ field assays. Under in situ-relevant concentrations, growth of this isolate with H₂S was better than that with H₂. The major conclusions drawn from this study are that phylogeny may not necessarily be reliable for predicting physiology and that H₂S can dominate over H₂ as an energy source in terms of availability, apparent in situ consumption rates, and growth-supporting energy.     

Enhanced Synthesis of Alkyl Amino Acids in Miller’s 1958 H<Subscript>2</Subscript>S Experiment

Stanley Miller’s 1958 H₂S-containing experiment, which included a simulated prebiotic atmosphere of methane (CH₄), ammonia (NH₃), carbon dioxide (CO₂), and hydrogen sulfide (H₂S) produced several alkyl amino acids, including the α-, β-, and γ-isomers of aminobutyric acid (ABA) in greater relative yields than had previously been reported from his spark discharge experiments. In the presence of H₂S, aspartic and glutamic acids could yield alkyl amino acids via the formation of thioimide intermediates. Radical chemistry initiated by passing H₂S through a spark discharge could have also enhanced alkyl amino acid synthesis by generating alkyl radicals that can help form the aldehyde and ketone precursors to these amino acids. We propose mechanisms that may have influenced the synthesis of certain amino acids in localized environments rich in H₂S and lightning discharges, similar to conditions near volcanic systems on the early Earth, thus contributing to the prebiotic chemical inventory of the primordial Earth.     

Temporal changes in archaeal diversity and chemistry in a mid-ocean ridge subseafloor habitat

The temporal variation in archaeal diversity in vent fluids from a midocean ridge subseafloor habitat was examined using PCR-amplified 16S rRNA gene sequence analysis and most-probable-number (MPN) cultivation techniques targeting hyperthermophiles. To determine how variations in temperature and chemical characteristics of subseafloor fluids affect the microbial communities, we performed molecular phylogenetic and chemical analyses on diffuse-flow vent fluids from one site shortly after a volcanic eruption in 1998 and again in 1999 and 2000. The archaeal population was divided into particle-attached (>3-microm-diameter cells) and free-living fractions to test the hypothesis that subseafloor microorganisms associated with active hydrothermal systems are adapted for a lifestyle that involves attachment to solid surfaces and formation of biofilms. To delineate between entrained seawater archaea and the indigenous subseafloor microbial community, a background seawater sample was also examined and found to consist only of Group I Crenarchaeota and Group II Euryarchaeota, both of which were also present in vent fluids. The indigenous subseafloor archaeal community consisted of clones related to both mesophilic and hyperthermophilic Methanococcales, as well as many uncultured Euryarchaeota, some of which have been identified in other vent environments. The particle-attached fraction consistently showed greater diversity than the free-living fraction. The fluid and MPN counts indicate that while culturable hyperthermophiles represent less than 1% of the total microbial community, the subseafloor at new eruption sites does support a hyperthermophilic microbial community. The temperature and chemical indicators of the degree of subseafloor mixing appear to be the most important environmental parameters affecting community diversity, and it is apparent that decreasing fluid temperatures correlated with increased entrainment of seawater, decreased concentrations of hydrothermal chemical species, and increased incidence of seawater archaeal sequences.     

Links Between Hydrothermal Environments,Pyrophosphate, Na<Superscript>+</Superscript>, and Early Evolution

The discovery that photosynthetic bacterial membrane-bound inorganic pyrophosphatase (PPase) catalyzed light-induced phosphorylation of orthophosphate (Pi) to pyrophosphate (PPi) and the capability of PPi to drive energy requiring dark reactions supported PPi as a possible early alternative to ATP. Like the proton-pumping ATPase, the corresponding membrane-bound PPase also is a H⁺-pump, and like the Na⁺-pumping ATPase, it can be a Na⁺-pump, both in archaeal and bacterial membranes. We suggest that PPi and Na⁺ transport preceded ATP and H⁺ transport in association with geochemistry of the Earth at the time of the origin and early evolution of life. Life may have started in connection with early plate tectonic processes coupled to alkaline hydrothermal activity. A hydrothermal environment in which Na⁺ is abundant exists in sediment-starved subduction zones, like the Mariana forearc in the W Pacific Ocean. It is considered to mimic the Archean Earth. The forearc pore fluids have a pH up to 12.6, a Na⁺-concentration of 0.7 mol/kg seawater. PPi could have been formed during early subduction of oceanic lithosphere by dehydration of protonated orthophosphates. A key to PPi formation in these geological environments is a low local activity of water.     

Linking geology,fluid chemistry,and microbial activity of basalt‐ and ultramafic‐hosted deep‐sea hydrothermal vent environments

Hydrothermal fluids passing through basaltic rocks along mid‐ocean ridges are known to be enriched in sulfide, while those circulating through ultramafic mantle rocks are typically elevated in hydrogen. Therefore, it has been estimated that the maximum energy in basalt‐hosted systems is available through sulfide oxidation and in ultramafic‐hosted systems through hydrogen oxidation. Furthermore, thermodynamic models suggest that the greatest biomass potential arises from sulfide oxidation in basalt‐hosted and from hydrogen oxidation in ultramafic‐hosted systems. We tested these predictions by measuring biological sulfide and hydrogen removal and subsequent autotrophic CO₂ fixation in chemically distinct hydrothermal fluids from basalt‐hosted and ultramafic‐hosted vents. We found a large potential of microbial hydrogen oxidation in naturally hydrogen‐rich (ultramafic‐hosted) but also in naturally hydrogen‐poor (basalt‐hosted) hydrothermal fluids. Moreover, hydrogen oxidation–based primary production proved to be highly attractive under our incubation conditions regardless whether hydrothermal fluids from ultramafic‐hosted or basalt‐hosted sites were used. Site‐specific hydrogen and sulfide availability alone did not appear to determine whether hydrogen or sulfide oxidation provides the energy for primary production by the free‐living microbes in the tested hydrothermal fluids. This suggests that more complex features (e.g., a combination of oxygen, temperature, biological interactions) may play a role for determining which energy source is preferably used in chemically distinct hydrothermal vent biotopes.     

Sources of organic nitrogen at the serpentinite‐hosted Lost City hydrothermal field

The reaction of ultramafic rocks with water during serpentinization at moderate temperatures results in alkaline fluids with high concentrations of reduced chemical compounds such as hydrogen and methane. Such environments provide unique habitats for microbial communities capable of utilizing these reduced compounds in present‐day and, possibly, early Earth environments. However, these systems present challenges to microbial communities as well, particularly due to high fluid pH and possibly the availability of essential nutrients such as nitrogen. Here we investigate the source and cycling of organic nitrogen at an oceanic serpentinizing environment, the Lost City hydrothermal field (30°N, Mid‐Atlantic Ridge). Total hydrolizable amino acid (THAA) concentrations in the fluids range from 736 to 2300 nm and constitute a large fraction of the dissolved organic carbon (2.5–15.1%). The amino acid distributions, and the relative concentrations of these compounds across the hydrothermal field, indicate they most likely derived from chemolithoautotrophic production. Previous studies have identified the presence of numerous nitrogen fixation genes in the fluids and the chimneys. Organic nitrogen in actively venting chimneys has δ¹⁵N values as low as 0.1‰ which is compatible with biological nitrogen fixation. Total hydrolizable amino acids in the chimneys are enriched in ¹³C by 2–7‰ compared to bulk organic matter. The distribution and absolute δ¹³C_THAA values are compatible with a chemolithoautotrophic source, an attribution also supported by molar organic C/N ratios in most active chimneys (4.1–5.5) which are similar to those expected for microbial communities. In total, these data indicate nitrogen is readily available to microbial communities at Lost City.     

Prebiotic Synthesis of Glycine from Ethanolamine in Simulated Archean Alkaline Hydrothermal Vents

Submarine hydrothermal vents are generally considered as the likely habitats for the origin and evolution of early life on Earth. In recent years, a novel hydrothermal system in Archean subseafloor has been proposed. In this model, highly alkaline and high temperature hydrothermal fluids were generated in basalt-hosted hydrothermal vents, where H₂ and CO₂ could be abundantly provided. These extreme conditions could have played an irreplaceable role in the early evolution of life. Nevertheless, sufficient information has not yet been obtained for the abiotic synthesis of amino acids, which are indispensable components of life, at high temperature and alkaline condition. This study aims to propose a new method for the synthesis of glycine in simulated Archean submarine alkaline vent systems. We investigated the formation of glycine from ethanolamine under conditions of high temperature (80–160 °C) and highly alkaline solutions (pH = 9.70). Experiments were performed in an anaerobic environment under mild pressure (0.1–8.0 MPa) at the same time. The results suggested that the formation of glycine from ethanolamine occurred rapidly and efficiently in the presence of metal powders, and was favored by high temperatures and high pressures. The experiment provides a new pathway for prebiotic glycine formation and points out the phenomenal influence of high-temperature alkaline hydrothermal vents in origin of life in the early ocean.     

The Abiotic Formation of Hydrocarbons from Dissolved CO2 Under Hydrothermal Conditions with Cobalt-Bearing Magnetite

Conversion of CO₂ to organic compounds in hydrothermal systems is important in understanding prebiotic chemical evolution leading to the origin of life. However, organic compounds with carbon number of more than 3 have never been produced from dissolved CO₂ in simulated hydrothermal experiments. In this paper, we report that not only CH₄, C₂H₆ and C₃H₈, but also n-C₄H₁₀ and n-C₅H₁₂ could be produced from dissolved CO₂ and H₂ in the presence of cobalt-bearing magnetite at 300°C and 30 MPa. It is shown that unbranched alkanes in Anderson–Schulz–Flory distribution were the dominant hydrocarbon products produced from dissolved CO₂ catalyzed by cobalt-bearing magnetite under certain hydrothermal conditions. It is proposed that magnetite with other transition metals may act potentially as effective mineral catalysts for abiotic formation of organic compounds from dissolved CO₂ in hydrothermal systems.     

The energetics of organic synthesis inside and outside the cell

Thermodynamic modelling of organic synthesis has largely been focused on deep-sea hydrothermal systems. When seawater mixes with hydrothermal fluids, redox gradients are established that serve as potential energy sources for the formation of organic compounds and biomolecules from inorganic starting materials. This energetic drive, which varies substantially depending on the type of host rock, is present and available both for abiotic (outside the cell) and biotic (inside the cell) processes. Here, we review and interpret a library of theoretical studies that target organic synthesis energetics. The biogeochemical scenarios evaluated include those in present-day hydrothermal systems and in putative early Earth environments. It is consistently and repeatedly shown in these studies that the formation of relatively simple organic compounds and biomolecules can be energy-yielding (exergonic) at conditions that occur in hydrothermal systems. Expanding on our ability to calculate biomass synthesis energetics, we also present here a new approach for estimating the energetics of polymerization reactions, specifically those associated with polypeptide formation from the requisite amino acids.     

Plausible Emergence and Self Assembly of a Primitive Phospholipid from Reduced Phosphorus on the Primordial Earth

How life arose on the primitive Earth is one of the biggest questions in science. Biomolecular emergence scenarios have proliferated in the literature but accounting for the ubiquity of oxidized (+?5) phosphate (PO₄^3?) in extant biochemistries has been challenging due to the dearth of phosphate and molecular oxygen on the primordial Earth. A compelling body of work suggests that exogenous schreibersite ((Fe,Ni)₃P) was delivered to Earth via meteorite impacts during the Heavy Bombardment (ca. 4.1–3.8 Gya) and there converted to reduced P oxyanions (e.g., phosphite (HPO₃^2?) and hypophosphite (H₂PO₂^?)) and phosphonates. Inspired by this idea, we review the relevant literature to deduce a plausible reduced phospholipid analog of modern phosphatidylcholines that could have emerged in a primordial hydrothermal setting. A shallow alkaline lacustrine basin underlain by active hydrothermal fissures and meteoritic schreibersite-, clay-, and metal-enriched sediments is envisioned. The water column is laden with known and putative primordial hydrothermal reagents. Small system dimensions and thermal- and UV-driven evaporation further concentrate chemical precursors. We hypothesize that a reduced phospholipid arises from Fischer–Tropsch-type (FTT) production of a C8 alkanoic acid, which condenses with an organophosphinate (derived from schreibersite corrosion to hypophosphite with subsequent methylation/oxidation), to yield a reduced protophospholipid. This then condenses with an α-amino nitrile (derived from Strecker-type reactions) to form the polar head. Preliminary modeling results indicate that reduced phospholipids do not aggregate rapidly; however, single layer micelles are stable up to aggregates with approximately 100 molecules.

     

深海热液生物共附生可培养硫氧化细菌的多样性及硫氧化特性

[背景]深海热液环境中存在大量H2S及含硫化合物,许多微生物与大型生物形成了紧密的共生体系,例如硫氧化细菌,它们利用其独特的代谢体系协助宿主更好地适应极端环境,但目前尚未对热液底栖生物共附生的硫氧化细菌进行培养鉴定和功能分析.[目的]了解深海热液生物共附生硫氧化细菌的种群特征和功能特征,筛选出深海热液生物共附生微生物中...     

Geochemical constraints on the origin of organic compounds in hydrothermal systems

It is proposed that abiotic synthesis of organic compounds occurs in metastable states. These states are permitted by kinetic barriers which inhibit the approach to stable equilibrium in the C-H-O-N system. Evidence for metastable equilibrium among organic compounds in sedimentary basins is reviewed, and further evidence is elucidated from hydrous pyrolysis experiments reported in the literature. This analysis shows that at hydrothermal conditions, organic compounds are formed or destroyed primarily through oxidation/reduction reactions, and that the role of temperature is to lower the kinetic barriers to these reactions. These lines of evidence allow the development of a scenario in which abiotic synthesis can occur at hydrothermal conditions through the reduction of CO₂ and N₂. This scenario can be tested quantitatively with distribution of species calculations as functions of temperature, pressure, hydrogen fugacity (fH₂) and initial composition. One example of such a test is given for an early, sudden outgassing of the Earth, in which CO₂, H₂O, and N₂ are transported from the mantle to the atmosphere by hydrothermal solutions. Activities of metastable aqueous organic species which form as a consequence of this process are evaluated at conditions appropriate for seafloor hydrothermal systems, and are found to maximize at about 200 °C and between the oxidation states set by two mineral assemblages common in the oceanic crust.     

Engineering Multifunctional Collaborative Catalytic Interface Enabling Efficient Hydrogen Evolution in All pH Range and Seawater

Developing electrocatalysts with high compatibility to the reaction systems with complicated chemical properties represents an important frontier of catalyst design. Herein, a strategy by engineering a multifunctional collaborative catalytic interface to propel the hydrogen evolution reaction (HER) in the full pH range and seawater is reported. Collaborative catalytic interfaces among MXene, bimetallic carbide, and hybridized carbon are demonstrated to afford overall enhancement in electrical conductivity, exposure of reactive sites, water dissociation kinetics, H⁺/water adsorption, and intermediate H binding capability, which satisfy highly variable chemical environment for HER under different pH conditions. Therefore, the HER performance of resultant electrocatalysts can compete with commercial Pt/C in 0.5 m H₂SO₄ or 1.0 m KOH but outperform it under pH 2.2–11.2. They also show exceptional performance for HER in natural seawater with stringent requirements in catalytic activity and stability, exhibiting the best combination of Pt‐like activity, long durability (225 h, 64 times that of Pt/C), and 98% Faradaic efficiency, comparable with commercial Pt/C and the best documented electrocatalysts by far. This work may shed fresh light into the design of effective electrocatalytic interface for regulating the energy chemistry over wide operation conditions, and also inspires the exploration of hydrogen energy utilization technologies and beyond.     

Colonization of subsurface microbial observatories deployed in young ocean crust

Oceanic crust comprises the largest hydrogeologic reservoir on Earth, containing fluids in thermodynamic disequilibrium with the basaltic crust. Little is known about microbial ecosystems that inhabit this vast realm and exploit chemically favorable conditions for metabolic activities. Crustal samples recovered from ocean drilling operations are often compromised for microbiological assays, hampering efforts to resolve the extent and functioning of a subsurface biosphere. We report results from the first in situ experimental observatory systems that have been used to study subseafloor life. Experiments deployed for 4 years in young (3.5 Ma) basaltic crust on the eastern flank of the Juan de Fuca Ridge record a dynamic, post-drilling response of crustal microbial ecosystems to changing physical and chemical conditions. Twisted stalks exhibiting a biogenic iron oxyhydroxide signature coated the surface of mineral substrates in the observatories; these are biosignatures indicating colonization by iron oxidizing bacteria during an initial phase of cool, oxic, iron-rich conditions following observatory installation. Following thermal and chemical recovery to warmer, reducing conditions, the in situ microbial structure in the observatory shifted, becoming representative of natural conditions in regional crustal fluids. Firmicutes, metabolic potential of which is unknown but may involve N or S cycling, dominated the post-rebound bacterial community. The archaeal community exhibited an extremely low diversity. Our experiment documented in situ conditions within a natural hydrological system that can pervade over millennia, exemplifying the power of observatory experiments for exploring the subsurface basaltic biosphere, the largest but most poorly understood biotope on Earth.     

The inevitable journey to being

Life is evolutionarily the most complex of the emergent symmetry-breaking, macroscopically organized dynamic structures in the Universe. Members of this cascading series of disequilibria-converting systems, or engines in Cottrell''s terminology, become ever more complicated—more chemical and less physical—as each engine extracts, exploits and generates ever lower grades of energy and resources in the service of entropy generation. Each one of these engines emerges spontaneously from order created by a particular mother engine or engines, as the disequilibrated potential daughter is driven beyond a critical point. Exothermic serpentinization of ocean crust is life''s mother engine. It drives alkaline hydrothermal convection and thereby the spontaneous production of precipitated submarine hydrothermal mounds. Here, the two chemical disequilibria directly causative in the emergence of life spontaneously arose across the mineral precipitate membranes separating the acidulous, nitrate-bearing CO₂-rich, Hadean sea from the alkaline and CH₄/H₂-rich serpentinization-generated effluents. Essential redox gradients—involving hydrothermal CH₄ and H₂ as electron donors, CO₂ and nitrate, nitrite, and ferric iron from the ambient ocean as acceptors—were imposed which functioned as the original ‘carbon-fixing engine’. At the same time, a post-critical-point (milli)voltage pH potential (proton concentration gradient) drove the condensation of orthophosphate to produce a high energy currency: ‘the pyrophosphatase engine’.