Abiotic synthesis of organic compounds in deep-sea hydrothermal environments

100% Conversion of CO2-CH4 with Non-Precious Co@ZnO Catalyst in Hot Water

The combination of solar energy and natural hydrothermal systems will innovate the chemistry of CO2 hydrogenation; however, the approach remains challenging due to the lack of robust and cost-effective catalytic system. Here, Zn which can be recycled with solar energy-induced approach was chosen as the reductant and Co as catalyst to achieve robust hydrothermal CO2 methanation. Nanosheets of honeycomb ZnO were grown in situ on the Co surface, resulting in a new motif (Co@ZnO catalyst) that inhibits Co deactivation through ZnO-assisted CoOx reduction. The stabilized Co and interaction between Co and ZnO functioned collaboratively toward the full conversion of CO2-CH4. In situ hydrothermal infrared spectroscopy confirmed the formation of formic acid as an intermediate, thereby avoiding CO formation and unwanted side reaction pathways. This study presents a straightforward one-step process for both highly efficient CO2 conversion and catalyst synthesis, paving the way for solar-driven CO2 methanation.

Origin of the RNA World in Cold Hadean Geothermal Fields Enriched in Zinc and Potassium: Abiogenesis as a Positive Fallout from the Moon-Forming Impact?

The ubiquitous, evolutionarily oldest RNAs and proteins exclusively use rather rare zinc as transition metal cofactor and potassium as alkali metal cofactor, which implies their abundance in the habitats of the first organisms. Intriguingly, lunar rocks contain a hundred times less zinc and ten times less potassium than the Earth's crust; the Moon is also depleted in other moderately volatile elements (MVEs). Current theories of impact formation of the Moon attribute this depletion to the MVEs still being in a gaseous state when the hot post-impact disk contracted and separated from the nascent Moon. The MVEs then fell out onto juvenile Earth's protocrust; zinc, as the most volatile metal, precipitated last, just after potassium. According to our calculations, the top layer of the protocrust must have contained up to 1019 kg of metallic zinc, a powerful reductant. The venting of hot geothermal fluids through this MVE-fallout layer, rich in metallic zinc and radioactive potassium, both capable of reducing carbon dioxide and dinitrogen, must have yielded a plethora of organic molecules released with the geothermal vapor. In the pools of vapor condensate, the RNA-like molecules may have emerged through a pre-Darwinian selection for low-volatile, associative, mineral-affine, radiation-resistant, nitrogen-rich, and polymerizable molecules.

Higher Microbial Biomass Accumulation on El Médano 464 Meteorite Compared with Adjacent Soils in the Atacama Desert

Chondritic meteorites can be appropriate substrates for the colonization of terrestrial microorganisms. However, determining whether organic compounds are intrinsic to the meteorite or come from external (terrestrial) contamination is still controversial. This research explores the molecular distribution and carbon isotopic composition of three lipid families (hydrocarbons, alkanoic acids, and alcohols) as well as DNA extracted from the interior of a CO carbonaceous chondrite named El Médano 464 (EM 464), discovered in the Atacama Desert in 2019. Three soil samples from the discovery area of EM 464 were collected and used as a background control for the composition and distribution of organic compounds. Our results revealed a higher abundance of the three lipid families in EM 464 compared with the surrounding soil samples. The organic compounds in EM 464 showed a mean δ13C value of -27.8 ± 0.5 for hydrocarbons (N = 20), -27.6 ± 1.1 for alkanoic acids (N = 17), and -27.5 ± 2.2‰ for alcohols (N = 18). These δ13C-depleted values are compatible with terrestrial biosignatures and are within isotopic values produced as a result of carbon fixation due to the Calvin cycle (δ13C of ca. from -19 to -34‰) widely used by photosynthetic terrestrial microorganisms. The DNA analysis (based on the bacterial 16S rRNA gene) showed a dominance of Proteobacteria (now Pseudomonadota) and Actinobacteriota in both meteorite and soils but exhibited different bacterial composition at the family level. This suggests that the microbial material inside the meteorite may have partially come from the adjacent soils, but we cannot rule out other sources, such as windborne microbes from distant locations. In addition, the meteorite showed higher bacterial diversity (H' = 2.4-2.8) compared with the three soil samples (H' = 0.3-1.8). Based on the distribution and δ13C value of organic compounds as well as DNA analysis, we suggest that most, if not all, of the organic compounds detected in the studied CO chondrite are of terrestrial origin (i.e., contamination). The terrestrial contamination of EM 464 by a diverse microbial community indicates that Atacama chondrites can offer distinctive ecological conditions for microorganisms to thrive in the harsh desert environment, which can result in an accumulation of microbial biomass and preservation of molecular fossils over time.

Energetic and genomic potential for hydrogenotrophic, formatotrophic, and acetoclastic methanogenesis in surface-expressed serpentinized fluids of the Samail Ophiolite

Serpentinization, the reaction of water with ultramafic rock, produces reduced, hyperalkaline, and H2-rich fluids that support a variety of hydrogenotrophic microbial metabolisms. Previous work indicates the occurrence of methanogenesis in fluids from the actively serpentinizing Samail Ophiolite in the Sultanate of Oman. While those fluids contain abundant H2 to fuel hydrogenotrophic methanogenesis (CO2 + 4H2 ➔ CH4 + 2H2O), the concentration of CO2 is very low due to the hyperalkalinity (> pH 11) and geochemistry of the fluids. As a result, species such as formate and acetate may be important as alternative methanogenic substrates. In this study we quantified the impact of inorganic carbon, formate and acetate availability for methanogenic metabolisms, across a range of fluid chemistries, in terms of (1) the potential diffusive flux of substrates to the cell, (2) the Affinity (Gibbs energy change) associated with methanogenic metabolism, and (3) the energy "inventory" per kg fluid. In parallel, we assessed the genomic potential for the conduct of those three methanogenic modes across the same set of fluids and consider the results within the quantitative framework of energy availability. We find that formatotrophic methanogenesis affords a higher Affinity (greater energetic yield) than acetoclastic and hydrogenotrophic methanogenesis in pristine serpentinized fluids and, in agreement with previous studies, find genomic evidence for a methanogen of the genus Methanobacterium to carry out formatotrophic and hydrogenotrophic methanogenesis, with the possibility of even using bicarbonate as a supply of CO2. Acetoclastic methanogenesis is also shown to be energetically favorable in these fluids, and we report the first detection of a potential acetoclastic methanogen of the family Methanosarcinaceae, which forms a distinct clade with a genome from the serpentinizing seafloor hydrothermal vent field, Lost City. These results demonstrate the applicability of an energy availability framework for interpreting methanogen ecology in serpentinizing systems.

Unraveling abiotic organic synthesis pathways in the mafic crust of mid-ocean ridges

The aqueous alteration of the oceanic lithosphere provides significant energy that impacts the synthesis and diversity of organic compounds, which are crucial for the deep carbon cycle and may have provided the first building blocks for life. Although abiotic organic synthesis has been documented in mantle-derived rocks, the formation mechanisms and complexity of organic compounds in crustal rocks remain largely unknown. Here, we show the specific association of aliphatic carbonaceous matter with Fe oxyhydroxides in mafic crustal rocks of the Southwest Indian Ridge (SWIR). We determine potential Fe-based pathways for abiotic organic synthesis from CO2 and H2 using multimodal and molecular nano-geochemical tools. Quantum mechanical modeling is further employed to constrain the catalytical activity of Fe oxyhydroxides, revealing that the catalytic cycle of hydrogen may play a key role in carbon-carbon bond formation. This approach offers the possibility of interpreting physicochemical organic formation and condensation mechanisms at an atomic scale. The findings expand our knowledge of the existence of abiotic organic carbon in the oceanic crustal rocks and emphasize the mafic oceanic crust of the SWIR as a potential site for low-temperature abiotic organic synthesis.

Prebiotic Synthesis of Microdroplets from Formate over a Bimetallic Cobalt-Nickel Nanomotif

The hypothesis underlying the abiogenic origin of life suggests that the nonenzymatic synthesis of long-chain fatty acids led to the construction of vesicles for compartmentalization in an early stage during the transition from geochemistry to biochemistry. However, evidence for this theory remains elusive as C5+ carboxylic acids cannot be synthesized using current laboratory simulations. Here, we report the synthesis of long-chain carboxylic acids (C3-C7) with a 42 mmol/gCo+Ni yield and 87.7% selectivity from formate (an intermediate of the acetyl-CoA pathway) over a cobalt-nickel alloy under alkaline hydrothermal conditions and the subsequent formation of microdroplets from organics. Density functional theory (DFT) calculations confirmed that the synergistic effect of the bimetal catalyst is key for catalyzing C-C coupling. Investigations by infrared spectroscopy, electron paramagnetic resonance, and isotope-labeled experiments revealed that HCO* serves as a reaction intermediate and is involved in the subsequent elementary steps for synthesizing long-chain carboxylic acids from formate. Taken together, these findings may help explain how the first protocells emerged geochemically and provide support for the hypothesis of the abiogenic origin of life. The hydrothermal system developed may also be applicable for the sustainable synthesis of long-chain carboxylates from one-carbon substrates using nonnoble metal catalysts.

A long section of serpentinized depleted mantle peridotite

The upper mantle is critical for our understanding of terrestrial magmatism, crust formation, and element cycling between Earth's solid interior, hydrosphere, atmosphere, and biosphere. Mantle composition and evolution have been primarily inferred by surface sampling and indirect methods. We recovered a long (1268-meter) section of serpentinized abyssal mantle peridotite interleaved with thin gabbroic intrusions. We find depleted compositions with notable variations in mantle mineralogy controlled by melt flow. Dunite zones have predominantly intermediate dips, in contrast to the originally steep mantle fabrics, indicative of oblique melt transport. Extensive hydrothermal fluid-rock interaction is recorded across the full depth of the core and is overprinted by oxidation in the upper 200 meters. Alteration patterns are consistent with vent fluid composition in the nearby Lost City hydrothermal field.

Recent advances in CO2 reduction with renewable reductants under hydrothermal conditions: towards efficient and net carbon benefit CO2 conversion

The ever-growing atmospheric CO2 concentration threatening the environmental sustainability of humankind makes the reduction of CO2 to chemicals or fuels an ideal solution. Two priorities are anticipated for the conversion technology, high efficiency and net carbon benefit, to ensure the mitigation of the CO2 problem both promptly and sustainably. Until now, catalytic hydrogenation or solar/electro-chemical CO2 conversion have achieved CO2 reduction promisingly while, to some extent, compromising to fulfill the two rules, and thus alternative approaches for CO2 reduction are necessary. Natural geochemical processes as abiotic CO2 reductions give hints for efficient CO2 reduction by building hydrothermal reaction systems, and this type of reaction atmosphere provides room for introducing renewable substances as reductants, which offers the possibility to achieve CO2 reduction with net carbon benefit. While the progress in CO2 reduction has been abundantly summarized, reviews on hydrothermal CO2 reduction are relatively scarce and, more importantly, few have focused on CO2 reduction with renewable reductants with the consideration of both scale of efficiency and sustainability. This review provides a fundamental and critical review of metal, biomass and polymer waste as reducing agents for hydrothermal CO2 reduction. Various products including formic acid, methanol, methane and multi-carbon chemicals can be formed, and effects of operational parameters such as temperature, batch holding time, pH value and water filing as well as detailed reaction mechanisms are illustrated. Particularly, the critical roles of high temperature and pressure water as reaction promotor and catalyst in hydrothermal CO2 conversion are discussed at the mechanistic level. More importantly, this review compares hydrothermal CO2 reduction with other methods such as catalytic hydrogenation and photo/electrocatalysis, evaluating their efficiency and potential for net carbon benefit. The aim of this review is to promote the understanding of CO2 activation under a hydrothermal environment and provide insights into the efficient and sustainable strategy of hydrothermal CO2 conversion for future fundamental research and industrial applications.

Interpreting an Archaean paleoenvironment through 3D imagery of microbialites

While stromatolites, and to a lesser extent thrombolites, have been extensively studied in order to unravel Precambrian (>539 Ma) biological evolution, studies of clastic-dominated microbially induced sedimentary structures (MISS) are relatively scarce. The lack of a consolidated record of clastic microbialites creates questions about how much (and what) information on depositional and taphonomic settings can be gleaned from these fossils. We used μCT scanning, a non-destructive X-ray-based 3D imaging method, to reconstruct morphologies of ancient MISS and mat textures in two previously described coastal Archaean samples from the ~3.48 Ga Dresser Formation, Pilbara, Western Australia. The aim of this study was to test the ability of μCT scanning to visualize and make 3D measurements that can be used to interpret the biotic-environmental interactions. Fossil MISS including mat laminae with carpet-like textures in one sample and mat rip-up chips in the second sample were investigated. Compiled δ13C and δ34S analyses of specimens from the Dresser Fm. are consistent with a taxonomically diverse community that could be capable of forming such MISS. 3D measurements of fossil microbial mat chips indicate significant biostabilization and suggest formation in flow velocities >25 cm s-1. Given the stratigraphic location of these chips in a low-flow lagoonal layer, we conclude that these chips formed due to tidal influence, as these assumed velocities are consistent with recent modeling of Archaean tides. The success of μCT scanning in documenting these microbialite features validates this technique both as a first step analysis for rare samples prior to the use of more destructive techniques and as a valuable tool for gaining insight into microbialite taphonomy.

Soil processes modify the composition of volatile organic compounds (VOCs) from CO2- and CH4-dominated geogenic and landfill gases: A comprehensive study

Degradation mechanisms affecting non-methane volatile organic compounds (VOCs) during gas uprising from different hypogenic sources to the surface were investigated through extensive sampling surveys in areas encompassing a high enthalpy hydrothermal system associated with active volcanism, a CH4-rich sedimentary basin and a municipal waste landfill. For a comprehensive framework, published data from medium-to-high enthalpy hydrothermal systems were also included. The investigated systems were characterised by peculiar VOC suites that reflected the conditions of the genetic environments in which temperature, contents of organic matter, and gas fugacity had a major role. Differences in VOC patterns between source (gas vents and landfill gas) and soil gases indicated VOC transformations in soil. Processes acting in soil preferentially degraded high-molecular weight alkanes with respect to the low-molecular weight ones. Alkenes and cyclics roughly behaved like alkanes. Thiophenes were degraded to a larger extent with respect to alkylated benzenes, which were more reactive than benzene. Furan appeared less degraded than its alkylated homologues. Dimethylsulfoxide was generally favoured with respect to dimethylsulfide. Limonene and camphene were relatively unstable under aerobic conditions, while α-pinene was recalcitrant. O-bearing organic compounds (i.e., aldehydes, esters, ketones, alcohols, organic acids and phenol) acted as intermediate products of the ongoing VOC degradations in soil. No evidence for the degradation of halogenated compounds and benzothiazole was observed. This study pointed out how soil degradation processes reduce hypogenic VOC emissions and the important role played by physicochemical and biological parameters on the effective VOC attenuation capacity of the soil.

Serpentinization-associated travertines as spatio-temporal archives for lipid biomarkers key for the search for life on Mars

Serpentinization is a well-known aqueous alteration process that may have played important roles in the origins and early evolution of life on Earth, and perhaps Mars, but there are still aspects related to biomarker distribution, partitioning, and preservation that merit further study. To assess the role that precipitation of carbonate phases in serpentinization settings may have on biomarker preservation, we search for life signs in one of the world's largest outcrops of subcontinental peridotites (Ronda, South Spain). We investigate the organic record of groundwater and associated carbonate deposits (travertines) in seven hyperalkaline springs, and reconstruct the biological activity and metabolic interactions of the serpentinization-hosted ecosystem. We identified lipid biomarkers and isotopic evidences of life, whose concentration and variety were much lower in groundwater than travertine deposits (ppb/ppt versus ppm level). Groundwater carried organics of abiotic (n-alkanes with values of CPI ∼ 1) and/or biotic origin, of fresher (e.g. acids or alcohols) or more diagenetized (mature hopanes and n-alkanes) nature. In contrast, associated travertines held a more prolific record of biomarkers incorporating (molecular and isotopic) fingerprints of surface (mostly phototrophs) and subsurface (chemolithotrophs, methanogens and/or methanotrophs) life. Serpentinization-associated travertines seem to act as biomolecule archives over time fed by autochthonous and allochthonous sources, hence amplifying the dim biological signal of groundwater. These results illustrate the relevance of serpentinization-associated surface mineral deposits in searching for traces of life on analogous environments on Mars. We highlight the diversity of lipids produced in serpentinizing land environments and emphasize the potential of these geostable biomolecules to preserve fingerprints of life.

Resource partitioning and amino acid assimilation in a terrestrial geothermal spring

High-temperature geothermal springs host simplified microbial communities; however, the activities of individual microorganisms and their roles in the carbon cycle in nature are not well understood. Here, quantitative stable isotope probing (qSIP) was used to track the assimilation of 13C-acetate and 13C-aspartate into DNA in 74 °C sediments in Gongxiaoshe Hot Spring, Tengchong, China. This revealed a community-wide preference for aspartate and a tight coupling between aspartate incorporation into DNA and the proliferation of aspartate utilizers during labeling. Both 13C incorporation into DNA and changes in the abundance of taxa during incubations indicated strong resource partitioning and a significant phylogenetic signal for aspartate incorporation. Of the active amplicon sequence variants (ASVs) identified by qSIP, most could be matched with genomes from Gongxiaoshe Hot Spring or nearby springs with an average nucleotide similarity of 99.4%. Genomes corresponding to aspartate primary utilizers were smaller, near-universally encoded polar amino acid ABC transporters, and had codon preferences indicative of faster growth rates. The most active ASVs assimilating both substrates were not abundant, suggesting an important role for the rare biosphere in the community response to organic carbon addition. The broad incorporation of aspartate into DNA over acetate by the hot spring community may reflect dynamic cycling of cell lysis products in situ or substrates delivered during monsoon rains and may reflect N limitation.

Microbial biosignatures in ancient deep-sea hydrothermal sulfides

Deep-sea hydrothermal systems provide ideal conditions for prebiotic reactions and ancient metabolic pathways and, therefore, might have played a pivotal role in the emergence of life. To understand this role better, it is paramount to examine fundamental interactions between hydrothermal processes, non-living matter, and microbial life in deep time. However, the distribution and diversity of microbial communities in ancient deep-sea hydrothermal systems are still poorly constrained, so evolutionary, and ecological relationships remain unclear. One important reason is an insufficient understanding of the formation of diagnostic microbial biosignatures in such settings and their preservation through geological time. This contribution centers around microbial biosignatures in Precambrian deep-sea hydrothermal sulfide deposits. Intending to provide a valuable resource for scientists from across the natural sciences whose research is concerned with the origins of life, we first introduce different types of biosignatures that can be preserved over geological timescales (rock fabrics and textures, microfossils, mineral precipitates, carbonaceous matter, trace metal, and isotope geochemical signatures). We then review selected reports of biosignatures from Precambrian deep-sea hydrothermal sulfide deposits and discuss their geobiological significance. Our survey highlights that Precambrian hydrothermal sulfide deposits potentially encode valuable information on environmental conditions, the presence and nature of microbial life, and the complex interactions between fluids, micro-organisms, and minerals. It further emphasizes that the geobiological interpretation of these records is challenging and requires the concerted application of analytical and experimental methods from various fields, including geology, mineralogy, geochemistry, and microbiology. Well-orchestrated multidisciplinary studies allow us to understand the formation and preservation of microbial biosignatures in deep-sea hydrothermal sulfide systems and thus help unravel the fundamental geobiology of ancient settings. This, in turn, is critical for reconstructing life's emergence and early evolution on Earth and the search for life elsewhere in the universe.

An Overview of Lipid Biomarkers in Terrestrial Extreme Environments with Relevance for Mars Exploration

Lipid molecules are organic compounds, insoluble in water, and based on carbon-carbon chains that form an integral part of biological cell membranes. As such, lipids are ubiquitous in life on Earth, which is why they are considered useful biomarkers for life detection in terrestrial environments. These molecules display effective membrane-forming properties even under geochemically hostile conditions that challenge most of microbial life, which grants lipids a universal biomarker character suitable for life detection beyond Earth, where a putative biological membrane would also be required. What discriminates lipids from nucleic acids or proteins is their capacity to retain diagnostic information about their biological source in their recalcitrant hydrocarbon skeletons for thousands of millions of years, which is indispensable in the field of astrobiology given the time span that the geological ages of planetary bodies encompass. This work gathers studies that have employed lipid biomarker approaches for paleoenvironmental surveys and life detection purposes in terrestrial environments with extreme conditions: hydrothermal, hyperarid, hypersaline, and highly acidic, among others; all of which are analogous to current or past conditions on Mars. Although some of the compounds discussed in this review may be abiotically synthesized, we focus on those with a biological origin, namely lipid biomarkers. Therefore, along with appropriate complementary techniques such as bulk and compound-specific stable carbon isotope analysis, this work recapitulates and reevaluates the potential of lipid biomarkers as an additional, powerful tool to interrogate whether there is life on Mars, or if there ever was.

Hydrothermal Reduction of CO2 to Value-Added Products by In Situ Generated Metal Hydrides

An integrated process by coupling hydrothermal reactions, including CO₂ reduction and H₂O dissociation with metals, is proposed. The hydrogen could be rapidly produced under hydrothermal conditions, owing to the special characteristics of high temperature water, generating metal hydrides as intermediates. Hydrogen production from the H₂O dissociation under hydrothermal conditions is one of the most ideal processes due to its environmentally friendly impact. Recent experimental and theoretical studies on the hydrothermal reduction of CO₂ to value-added products by in situ generated metal hydrides are introduced, including the production of formic acid, methanol, methane, and long-chain hydrocarbons. These results indicate that this process holds promise in respect to the conversion of CO₂ to useful chemicals and fuels, and for hydrogen storage, which could help alleviate the problems of climate change and energy shortage.

Evaluating the abiotic synthesis potential and the stability of building blocks of life beneath an impact-induced steam atmosphere

A prerequisite for prebiotic chemistry is the accumulation of critical building blocks of life. Some studies argue that more frequent impact events on the primitive Earth could have induced a more reducing steam atmosphere and thus favor widespread and more efficient synthesis of life building blocks. However, elevated temperature is also proposed to threaten the stability of organics and whether life building blocks could accumulate to appreciable levels in the reducing yet hot surface seawater beneath the steam atmosphere is still poorly examined. Here, we used a thermodynamic tool to examine the synthesis affinity of various life building blocks using inorganic gasses as reactants at elevated temperatures and corresponding steam pressures relevant with the steam-seawater interface. Our calculations show that although the synthesis affinity of all life building blocks decreases when temperature increases, many organics, including methane, methanol, and carboxylic acids, have positive synthesis affinity over a wide range of temperatures, implying that these species were favorable to form (>10-6 molal) in the surface seawater. However, cyanide and formaldehyde have overall negative affinities, suggesting that these critical compounds would tend to undergo hydrolysis in the surface seawaters. Most of the 18 investigated amino acids have positive affinities at temperature <220°C and their synthesis affinity increases under more alkaline conditions. Sugars, ribose, and nucleobases have overall negative synthesis affinities at the investigated range of temperatures. Synthesis affinities are shown to be sensitive to the hydrogen fugacity. Higher hydrogen fugacity (in equilibrium with FQI or IW) favors the synthesis and accumulation of nearly all the investigated compounds, except for HCN and its derivate products. In summary, our results suggest that reducing conditions induced by primitive impacts could indeed favor the synthesis/accumulation of some life building blocks, but some critical species, particularly HCN and nucleosides, were still unfavorable to accumulate to appreciable levels. Our results can provide helpful guidance for future efforts to search for or understand the stability of biomolecules on other planets like Mars and icy moons. We advocate examining craters formed by more reducing impactors to look for the preservation of prebiotic materials.

Self-Similar Patterns from Abiotic Decarboxylation Metabolism through Chemically Oscillating Reactions: A Prebiotic Model for the Origin of Life

The origin of life must have included an abiotic stage of carbon redox reactions that involved electron transport chains and the production of lifelike patterns. Chemically oscillating reactions (COR) are abiotic, spontaneous, out-of-equilibrium, and redox reactions that involve the decarboxylation of carboxylic acids with strong oxidants and strong acids to produce CO₂ and characteristic self-similar patterns. Those patterns have circular concentricity, radial geometries, characteristic circular twins, colour gradients, cavity structures, and branching to parallel alignment. We propose that COR played a role during the prebiotic cycling of carboxylic acids, furthering the new model for geology where COR can also explain the patterns of diagenetic spheroids in sediments. The patterns of COR in Petri dishes are first considered and compared to those observed in some eukaryotic lifeforms. The molecular structures and functions of reactants in COR are then compared to key biological metabolic processes. We conclude that the newly recognised similarities in compositions and patterns warrant future research to better investigate the role of halogens in biochemistry; COR in life-forms, including in humans; and the COR-stage of prebiotic carbon cycling on other planets, such as Mars.

Plausible Sources of Membrane-Forming Fatty Acids on the Early Earth: A Review of the Literature and an Estimation of Amounts

The first cells were plausibly bounded by membranes assembled from fatty acids with at least 8 carbons. Although the presence of fatty acids on the early Earth is widely assumed within the astrobiology community, there is no consensus regarding their origin and abundance. In this Review, we highlight three possible sources of fatty acids: (1) delivery by carbonaceous meteorites, (2) synthesis on metals delivered by impactors, and (3) electrochemical synthesis by spark discharges. We also discuss fatty acid synthesis by UV or particle irradiation, gas-phase ion-molecule reactions, and aqueous redox reactions. We compare estimates for the total mass of fatty acids supplied to Earth by each source during the Hadean eon after an extremely massive asteroid impact that would have reset Earth's fatty acid inventory. We find that synthesis on iron-rich surfaces derived from the massive impactor in contact with an impact-generated reducing atmosphere could have contributed ∼10² times more total mass of fatty acids than subsequent delivery by either carbonaceous meteorites or electrochemical synthesis. Additionally, we estimate that a single carbonaceous meteorite would not deliver a high enough concentration of fatty acids (∼15 mM for decanoic acid) into an existing body of water on the Earth's surface to spontaneously form membranes unless the fatty acids were further concentrated by another mechanism, such as subsequent evaporation of the water. Our estimates rely heavily on various assumptions, leading to significant uncertainties; nevertheless, these estimates provide rough order-of-magnitude comparisons of various sources of fatty acids on the early Earth. We also suggest specific experiments to improve future estimates. Our calculations support the view that fatty acids would have been available on the early Earth. Further investigation is needed to assess the mechanisms by which fatty acids could have been concentrated sufficiently to assemble into membranes during the origin of life.

Plausible Sources of Membrane-Forming Fatty Acids on the Early Earth: A Review of the Literature and an Estimation of Amounts

The first cells were plausibly bounded by membranes assembled from fatty acids with at least 8 carbons. Although the presence of fatty acids on the early Earth is widely assumed within the astrobiology community, there is no consensus regarding their origin and abundance. In this Review, we highlight three possible sources of fatty acids: (1) delivery by carbonaceous meteorites, (2) synthesis on metals delivered by impactors, and (3) electrochemical synthesis by spark discharges. We also discuss fatty acid synthesis by UV or particle irradiation, gas-phase ion-molecule reactions, and aqueous redox reactions. We compare estimates for the total mass of fatty acids supplied to Earth by each source during the Hadean eon after an extremely massive asteroid impact that would have reset Earth's fatty acid inventory. We find that synthesis on iron-rich surfaces derived from the massive impactor in contact with an impact-generated reducing atmosphere could have contributed ∼10² times more total mass of fatty acids than subsequent delivery by either carbonaceous meteorites or electrochemical synthesis. Additionally, we estimate that a single carbonaceous meteorite would not deliver a high enough concentration of fatty acids (∼15 mM for decanoic acid) into an existing body of water on the Earth's surface to spontaneously form membranes unless the fatty acids were further concentrated by another mechanism, such as subsequent evaporation of the water. Our estimates rely heavily on various assumptions, leading to significant uncertainties; nevertheless, these estimates provide rough order-of-magnitude comparisons of various sources of fatty acids on the early Earth. We also suggest specific experiments to improve future estimates. Our calculations support the view that fatty acids would have been available on the early Earth. Further investigation is needed to assess the mechanisms by which fatty acids could have been concentrated sufficiently to assemble into membranes during the origin of life.

Parapatric speciation of Meiothermus in serpentinite-hosted aquifers in Oman

The factors that control the distribution and evolution of microbial life in subsurface environments remain enigmatic due to challenges associated with sampling fluids from discrete depth intervals via boreholes while avoiding mixing of fluids. Here, using an inflatable packer system, fracture waters were isolated and collected from three discrete depth intervals spanning >130 m in a borehole intersecting an ultramafic rock formation undergoing serpentinization in the Samail Ophiolite, Sultanate of Oman. Near surface aquifer waters were moderately reducing and had alkaline pH while deeper aquifer waters were reduced and had hyperalkaline pH, indicating extensive influence by serpentinization. Metagenomic sequencing and analysis of DNA from filtered biomass collected from discrete depth intervals revealed an abundance of aerobes in near surface waters and a greater proportion of anaerobes at depth. Yet the abundance of the putatively obligate aerobe, Meiothermus, increased with depth, providing an opportunity to evaluate the influence of chemical and spatial variation on its distribution and speciation. Two clades of Meiothermus metagenome assembled genomes (MAGs) were identified that correspond to surface and deep populations termed Types I (S) and II (D), respectively; both clades comprised an apparently Oman-specific lineage indicating a common ancestor. Type II (D) clade MAGs encoded fewer genes and were undergoing slower genome replication as inferred from read mapping. Further, single nucleotide variants (SNVs) and mobile genetic elements identified among MAGs revealed detectable, albeit limited, evidence for gene flow/recombination between spatially segregated Type I (S) and Type II (D) populations. Together, these observations indicate that chemical variation generated by serpentinization, combined with physical barriers that reduce/limit dispersal and gene flow, allowed for the parapatric speciation of Meiothermus in the Samail Ophiolite or a geologic precursor. Further, Meiothermus genomic data suggest that deep and shallow aquifer fluids in the Samail Ophiolite may mix over shorter time scales than has been previously estimated from geochemical data.

A naturalist perspective of microbiology: Examples from methanogenic archaea

Storytelling has been the primary means of knowledge transfer over human history. The effectiveness and reach of stories are improved when the message is appropriate for the target audience. Oftentimes, the stories that are most well received and recounted are those that have a clear purpose and that are told from a variety of perspectives that touch on the varied interests of the target audience. Whether scientists realize or not, they are accustomed to telling stories of their own scientific discoveries through the preparation of manuscripts, presentations, and lectures. Perhaps less frequently, scientists prepare review articles or book chapters that summarize a body of knowledge on a given subject matter, meant to be more holistic recounts of a body of literature. Yet, by necessity, such summaries are often still narrow in their scope and are told from the perspective of a particular discipline. In other words, interdisciplinary reviews or book chapters tend to be the rarity rather than the norm. Here, we advocate for and highlight the benefits of interdisciplinary perspectives on microbiological subjects.

Microbial ecology of a shallow alkaline hydrothermal vent: Strýtan Hydrothermal Field, Eyjafördur, northern Iceland

Strýtan Hydrothermal Field (SHF) is a submarine system located in Eyjafördur in northern Iceland composed of two main vents: Big Strýtan and Arnarnesstrýtan. The vents are shallow, ranging from 16 to 70 m water depth, and vent high pH (up to 10.2), moderate temperature (T max ∼70°C), anoxic, fresh fluids elevated in dissolved silica, with slightly elevated concentrations of hydrogen and methane. In contrast to other alkaline hydrothermal vents, SHF is unique because it is hosted in basalt and therefore the high pH is not created by serpentinization. While previous studies have assessed the geology and geochemistry of this site, the microbial diversity of SHF has not been explored in detail. Here we present a microbial diversity survey of the actively venting fluids and chimneys from Big Strýtan and Arnarnesstrýtan, using 16S rRNA gene amplicon sequencing. Community members from the vent fluids are mostly aerobic heterotrophic bacteria; however, within the chimneys oxic, low oxygen, and anoxic habitats could be distinguished, where taxa putatively capable of acetogenesis, sulfur-cycling, and hydrogen metabolism were observed. Very few archaea were observed in the samples. The inhabitants of SHF are more similar to terrestrial hot spring samples than other marine sites. It has been hypothesized that life on Earth (and elsewhere in the solar system) could have originated in an alkaline hydrothermal system, however all other studied alkaline submarine hydrothermal systems to date are fueled by serpentinization. SHF adds to our understandings of hydrothermal vents in relationship to microbial diversity, evolution, and possibly the origin of life.

Deep-branching acetogens in serpentinized subsurface fluids of Oman

Little is known of acetogens in contemporary serpentinizing systems, despite widely supported theories that serpentinite-hosted environments supported the first life on Earth via acetogenesis. To address this knowledge gap, genome-resolved metagenomics was applied to subsurface fracture water communities from an area of active serpentinization in the Samail Ophiolite, Sultanate of Oman. Two deeply branching putative bacterial acetogen types were identified in the communities belonging to the Acetothermia (hereafter, types I and II) that exhibited distinct distributions among waters with lower and higher water-rock reaction (i.e., serpentinization influence), respectively. Metabolic reconstructions revealed contrasting core metabolic pathways of type I and II Acetothermia, including in acetogenic pathway components (e.g., bacterial- vs. archaeal-like carbon monoxide dehydrogenases [CODH], respectively), hydrogen use to drive acetogenesis, and chemiosmotic potential generation via respiratory (type I) or canonical acetogen ferredoxin-based complexes (type II). Notably, type II Acetothermia metabolic pathways allow for use of serpentinization-derived substrates and implicate them as key primary producers in contemporary hyperalkaline serpentinite environments. Phylogenomic analyses indicate that 1) archaeal-like CODH of the type II genomes and those of other serpentinite-associated Bacteria derive from a deeply rooted horizontal transfer or origin among archaeal methanogens and 2) Acetothermia are among the earliest evolving bacterial lineages. The discovery of dominant and early-branching acetogens in subsurface waters of the largest near-surface serpentinite formation provides insight into the physiological traits that likely facilitated rock-supported life to flourish on a primitive Earth and possibly on other rocky planets undergoing serpentinization.

High-pressure synthesis and storage of solid organic compounds in active subduction zones

Recent thermodynamic and experimental studies have suggested that volatile organic compounds (e.g., methane, formate, and acetate) can be produced and stabilized in subduction zones, potentially playing an important role in the deep carbon cycle. However, field evidence for the high-pressure production and storage of solid organic compounds is missing. Here, we examine forearc serpentinite clasts recovered by drilling mud volcanoes above the Mariana subduction zone. Notable correlations between carbon and iron stable-isotope signatures and fluid-mobile element (B, As and Sb) concentrations provide evidence for the percolation of slab-derived CO₂-rich aqueous fluids through the forearc mantle. The presence of carbonaceous matter rich in aliphatic moieties within high-temperature clasts (>350°C) demonstrates that molecular hydrogen production associated with forearc serpentinization is an efficient mechanism for the reduction and conversion of slab-derived CO₂-rich fluids into solid organic compounds. These findings emphasize the need to consider the forearc mantle as an important reservoir of organic carbon on Earth.

Metabolic Strategies Shared by Basement Residents of the Lost City Hydrothermal Field

Alkaline fluids venting from chimneys of the Lost City hydrothermal field flow from a potentially vast microbial habitat within the seafloor where energy and organic molecules are released by chemical reactions within rocks uplifted from Earth's mantle. In this study, we investigated hydrothermal fluids venting from Lost City chimneys as windows into subseafloor environments where the products of geochemical reactions, such as molecular hydrogen (H2), formate, and methane, may be the only available sources of energy for biological activity. Our deep sequencing of metagenomes and metatranscriptomes from these hydrothermal fluids revealed a few key species of archaea and bacteria that are likely to play critical roles in the subseafloor microbial ecosystem. We identified a population of Thermodesulfovibrionales (belonging to phylum Nitrospirota) as a prevalent sulfate-reducing bacterium that may be responsible for much of the consumption of H2 and sulfate in Lost City fluids. Metagenome-assembled genomes (MAGs) classified as Methanosarcinaceae and Candidatus Bipolaricaulota were also recovered from venting fluids and represent potential methanogenic and acetogenic members of the subseafloor ecosystem. These genomes share novel hydrogenases and formate dehydrogenase-like sequences that may be unique to hydrothermal environments where H2 and formate are much more abundant than carbon dioxide. The results of this study include multiple examples of metabolic strategies that appear to be advantageous in hydrothermal and subsurface alkaline environments where energy and carbon are provided by geochemical reactions. IMPORTANCE The Lost City hydrothermal field is an iconic example of a microbial ecosystem fueled by energy and carbon from Earth's mantle. Uplift of mantle rocks into the seafloor can trigger a process known as serpentinization that releases molecular hydrogen (H2) and creates unusual environmental conditions where simple organic carbon molecules are more stable than dissolved inorganic carbon. This study provides an initial glimpse into the kinds of microbes that live deep within the seafloor where serpentinization takes place, by sampling hydrothermal fluids exiting from the Lost City chimneys. The metabolic strategies that these microbes appear to be using are also shared by microbes that inhabit other sites of serpentinization, including continental subsurface environments and natural springs. Therefore, the results of this study contribute to a broader, interdisciplinary effort to understand the general principles and mechanisms by which serpentinization-associated processes can support life on Earth and perhaps other worlds.

Comparative Metagenomics Highlight a Widespread Pathway Involved in Catabolism of Phosphonates in Marine and Terrestrial Serpentinizing Ecosystems

Serpentinizing hydrothermal systems result from water circulating into the subsurface and interacting with mantle-derived rocks notably near mid-ocean ridges or continental ophiolites. Serpentinization and associated reactions produce alkaline fluids enriched in molecular hydrogen, methane, and small organic molecules that are assumed to feed microbial inhabitants. In this study, we explored the relationships linking serpentinization to associated microbial communities by comparative metagenomics of serpentinite-hosted systems, basalt-hosted vents, and hot springs. The shallow Prony bay hydrothermal field (PBHF) microbiome appeared to be more related to those of ophiolitic sites than to the Lost City hydrothermal field (LCHF) microbiome, probably because of the meteoric origin of its fluid, like terrestrial alkaline springs. This study emphasized the ubiquitous importance of a set of genes involved in the catabolism of phosphonates and highly enriched in all serpentinizing sites compared to other ecosystems. Because most of the serpentinizing systems are depleted in inorganic phosphate, the abundance of genes involved in the carbon-phosphorus lyase pathway suggests that the phosphonates constitute a source of phosphorus in these ecosystems. Additionally, hydrocarbons such as methane, released upon phosphonate catabolism, may contribute to the overall budget of organic molecules in serpentinizing systems.

IMPORTANCE This first comparative metagenomic study of serpentinite-hosted environments provides an objective framework to understand the functioning of these peculiar ecosystems. We showed a taxonomic similarity between the PBHF and other terrestrial serpentinite-hosted ecosystems. At the same time, the LCHF microbial community was closer to deep basalt-hosted hydrothermal fields than continental ophiolites, despite the influence of serpentinization. This study revealed shared functional capabilities among serpentinite-hosted ecosystems in response to environmental stress, the metabolism of abundant dihydrogen, and the metabolism of phosphorus. Our results are consistent with the generalized view of serpentinite environments but provide deeper insight into the array of factors that may control microbial activities in these ecosystems. Moreover, we show that metabolism of phosphonate is widespread among alkaline serpentinizing systems and could play a crucial role in phosphorus and methane biogeochemical cycles. This study opens a new line of investigation of the metabolism of reduced phosphorus compounds in serpentinizing environments.

Transformation of low molecular weight organic acids by microbial endoliths in subsurface mafic and ultramafic igneous rock

A growing body of work indicates that continental subsurface rocks host a substantial portion of the Earth's biosphere. However, the activities of microbial cells inhabiting pore spaces and microfractures in subsurface rocks remain underexplored. Here, we develop and optimize microcosm assays to detect organic acid transformation activities of cells residing in mafic to ultramafic igneous rocks. Application of this assay to gabbro core from the Stillwater Mine, Montana, USA, revealed maximal methane production from acetate at temperatures approximating that of the mine. Controls show that these activities are not due to contamination introduced during drilling, exhumation, or laboratory processing of the core. The assay was then applied to rocks cored from the Samail Ophiolite, Oman, which is undergoing low temperature serpentinization. Production of i) carbon dioxide from acetate and formate and ii) methane from formate were detected in a dunite/harzburgite rock core interfacing pH 9.6 waters, and estimates of microbial activities were up to three orders of magnitude higher in the rock core pore space than in corresponding waters. The detection of endolithic microbial activities in igneous rocks has implications for life detection on other planetary bodies where similar rock types prevail, such as Mars, Europa, and Enceladus. This article is protected by copyright. All rights reserved.

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.

Organic synthesis associated with serpentinization and carbonation on early Mars

Water-rock interactions are relevant to planetary habitability, influencing mineralogical diversity and the production of organic molecules. We examine carbonates and silicates in the martian meteorite Allan Hills 84001 (ALH 84001), using colocated nanoscale analyses, to characterize the nature of water-rock reactions on early Mars. We find complex refractory organic material associated with mineral assemblages that formed by mineral carbonation and serpentinization reactions. The organic molecules are colocated with nanophase magnetite; both formed in situ during water-rock interactions on Mars. Two potentially distinct mechanisms of abiotic organic synthesis operated on early Mars during the late Noachian period (3.9 to 4.1 billion years ago).

Hydrothermal synthesis of long-chain hydrocarbons up to C24 with NaHCO3-assisted stabilizing cobalt

Abiotic CO₂ reduction on transition metal minerals has been proposed to account for the synthesis of organic compounds in alkaline hydrothermal systems, but this reaction lacks experimental support, as only short-chain hydrocarbons (<C₅) have been synthesized in artificial simulation. This presents a question: What particular hydrothermal conditions favor long-chain hydrocarbon synthesis? Here, we demonstrate the hydrothermal bicarbonate reduction at ∼300 °C and 30 MPa into long-chain hydrocarbons using iron (Fe) and cobalt (Co) metals as catalysts. We found the Co⁰ promoter responsible for synthesizing long-chain hydrocarbons to be extraordinarily stable when coupled with Fe-OH formation. Under these hydrothermal conditions, the traditional water-induced deactivation of Co is inhibited by bicarbonate-assisted CoO_x reduction, leading to honeycomb-native Co nanosheets generated in situ as a new motif. The Fe-OH formation, confirmed by operando infrared spectroscopy, enhances CO adsorption on Co, thereby favoring further reduction to long-chain hydrocarbons (up to C₂₄). These results not only advance theories for an abiogenic origin for some petroleum accumulations and the hydrothermal hypothesis of the emergence of life but also introduce an approach for synthesizing long-chain hydrocarbons by nonnoble metal catalysts for artificial CO₂ utilization.

Simple Ion-Gas Mixtures as a Source of Key Molecules Relevant to Prebiotic Chemistry

Very simple chemistry can result in the rapid and high-yield production of key prebiotic inorganic molecules. The two reactions investigated here involve such simple systems, (a) carbon disulfide (CS₂) and acetate (CH₃COO¯) and (b) sulfur dioxide (SO₂) and formate (HCOO¯). They have been carried out under non-aqueous conditions, either in an organic solvent or with a powdered salt exposed to the requisite gas. Under such dry conditions the first reaction generated the thioacetate anion [CH₃COS]¯ while the second produced the radical [SO₂^·]¯anion. Anhydrous conditions are not rare and may have arisen on the early earth at sites where an interface between different phases (liquid/gas or solid/gas) could be generated. This is one way to rationalize the formation of molecules and ions (such as we have produced) necessary in the prebiotic world. Interpretation of our results provides insight into scenarios consistent with the more prominent theories of abiogenesis.

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.

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 (PO43-) 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 (HPO32-) 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.

Pathways of Iron and Sulfur Acquisition, Cofactor Assembly, Destination, and Storage in Diverse Archaeal Methanogens and Alkanotrophs

Archaeal methanogens, methanotrophs, and alkanotrophs have a high demand for iron (Fe) and sulfur (S); however, little is known of how they acquire, traffic, deploy, and store these elements. Here, we examined the distribution of homologs of proteins mediating key steps in Fe/S metabolism in model microorganisms, including iron(II) sensing/uptake (FeoAB), sulfide extraction from cysteine (SufS), and the biosynthesis of iron-sulfur [Fe-S] clusters (SufBCDE), siroheme (Pch2 dehydrogenase), protoheme (AhbABCD), cytochrome c (Cyt c) (CcmCF), and iron storage/detoxification (Bfr, FtrA, and IssA), among 326 publicly available, complete or metagenome-assembled genomes of archaeal methanogens/methanotrophs/alkanotrophs. The results indicate several prevalent but nonuniversal features, including FeoB, SufBC, and the biosynthetic apparatus for the basic tetrapyrrole scaffold, as well as its siroheme (and F₄₃₀) derivatives. However, several early-diverging genomes lacked SufS and pathways to synthesize and deploy heme. Genomes encoding complete versus incomplete heme biosynthetic pathways exhibited equivalent prevalences of [Fe-S] cluster binding proteins, suggesting an expansion of catalytic capabilities rather than substitution of heme for [Fe-S] in the former group. Several strains with heme binding proteins lacked heme biosynthesis capabilities, while other strains with siroheme biosynthesis capability lacked homologs of known siroheme binding proteins, indicating heme auxotrophy and unknown siroheme biochemistry, respectively. While ferritin proteins involved in ferric oxide storage were widespread, those involved in storing Fe as thioferrate were unevenly distributed. Collectively, the results suggest that differences in the mechanisms of Fe and S acquisition, deployment, and storage have accompanied the diversification of methanogens/methanotrophs/alkanotrophs, possibly in response to differential availability of these elements as these organisms evolved.

IMPORTANCE Archaeal methanogens, methanotrophs, and alkanotrophs, argued to be among the most ancient forms of life, have a high demand for iron (Fe) and sulfur (S) for cofactor biosynthesis, among other uses. Here, using comparative bioinformatic approaches applied to 326 genomes, we show that major differences in Fe/S acquisition, trafficking, deployment, and storage exist in this group. Variation in these characters was generally congruent with the phylogenetic placement of these genomes, indicating that variation in Fe/S usage and deployment has contributed to the diversification and ecology of these organisms. However, incongruency was observed among the distribution of cofactor biosynthesis pathways and known protein destinations for those cofactors, suggesting auxotrophy or yet-to-be-discovered pathways for cofactor biosynthesis.

Rational Design of Bioinspired Nanocomposites with Tunable Catalytic Activity

Biological assembly processes offer inspiration for ordering building blocks across multiple length scales into advanced functional materials. Such bioinspired strategies are attractive for assembling supported catalysts, where shaping and structuring across length scales are essential for their performance but still remain tremendously difficult to achieve. Here, we present a simple bioinspired route toward supported catalysts with tunable activity and selectivity. We coprecipitate shape-controlled nanocomposites with large specific surface areas of barium carbonate nanocrystals that are uniformly embedded in a silica support. Subsequently, we exchange the barium carbonate to cobalt while preserving the nanoscopic layout and microscopic shape, and demonstrate their catalytic performances in the Fischer-Tropsch synthesis as a case study. Control over the crystal size between 10 and 17 nm offers tunable activity and selectivity for shorter (C₅-C₁₁) and longer (C₂₀₊) hydrocarbons, respectively. Hence, these results open simple, versatile, and scalable routes to tunable and highly reactive bioinspired catalysts.

Characteristics of hydrocarbons in hydrothermal products of the Clam hydrothermal field from the Okinawa trough

The hydrothermal products of the Clam hydrothermal field from the Okinawa Trough were analyzed by gas chromatography-mass spectrometry to determine abundances of hydrocarbons. The n-alkanes in the hydrothermal products conformed to a bimodal distribution and exhibited an odd-to-even predominance of high molecular weight and an even-to-odd predominance of low molecular weight n-alkanes with maxima at C₁₆ and C₁₈. The total concentration of n-alkanes in hydrothermal sediment was much higher than that in hydrothermal sulfide and altered rock. The carbon isotopic value of individual n-alkanes in hydrothermal sediment was slightly higher than that in pelagic sediment. The concentrations and individual carbon isotopic compositions of n-alkanes suggest that the n-alkanes in hydrothermal products may be mainly the result of the metabolic activity of submarine microorganisms. Additionally, the present results suggest that the abiogenic contribution to source of hydrocarbons in hydrothermal products of the Clam hydrothermal field from the Okinawa Trough should not be ignored.

Pyrolysis of Carboxylic Acids in the Presence of Iron Oxides: Implications for Life Detection on Missions to Mars

The search for, and characterization of, organic matter on Mars is central to efforts in identifying habitable environments and detecting evidence of life in the martian surface and near surface. Iron oxides are ubiquitous in the martian regolith and are known to be associated with the deposition and preservation of organic matter in certain terrestrial environments, thus iron oxide-rich sediments are potential targets for life-detection missions. The most frequently used protocol for martian organic matter characterization (also planned for use on ExoMars) has been thermal extraction for the transfer of organic matter to gas chromatography-mass spectrometry (GC-MS) detectors. For the effective use of thermal extraction for martian samples, it is necessary to explore how potential biomarker organic molecules evolve during this process in the presence of iron oxides. We have thermally decomposed iron oxides simultaneously with (z)-octadec-9-enoic and n-octadecanoic acids and analyzed the products through pyrolysis-GC-MS. We found that the thermally driven dehydration, reduction, and recrystallization of iron oxides transformed fatty acids. Overall detectability of products greatly reduced, molecular diversity decreased, unsaturated products decreased, and aromatization increased. The severity of this effect increased as reduction potential of the iron oxide and inferred free radical formation increased. Of the iron oxides tested hematite showed the least transformative effects, followed by magnetite, goethite, then ferrihydrite. It was possible to identify the saturation state of the parent carboxylic acid at high (0.5 wt %) concentrations by the distribution of n-alkylbenzenes in the pyrolysis products. When selecting life-detection targets on Mars, localities where hematite is the dominant iron oxide could be targeted preferentially, otherwise thermal analysis of carboxylic acids, or similar biomarker molecules, will lead to enhanced polymerization, aromatization, and breakdown, which will in turn reduce the fidelity of the original biomarker, similar to changes normally observed during thermal maturation.

Diversification of methanogens into hyperalkaline serpentinizing environments through adaptations to minimize oxidant limitation

Metagenome assembled genomes (MAGs) and single amplified genomes (SAGs) affiliated with two distinct Methanobacterium lineages were recovered from subsurface fracture waters of the Samail Ophiolite, Sultanate of Oman. Lineage Type I was abundant in waters with circumneutral pH, whereas lineage Type II was abundant in hydrogen rich, hyperalkaline waters. Type I encoded proteins to couple hydrogen oxidation to CO₂ reduction, typical of hydrogenotrophic methanogens. Surprisingly, Type II, which branched from the Type I lineage, lacked homologs of two key oxidative [NiFe]-hydrogenases. These functions were presumably replaced by formate dehydrogenases that oxidize formate to yield reductant and cytoplasmic CO₂ via a pathway that was unique among characterized Methanobacteria, allowing cells to overcome CO₂/oxidant limitation in high pH waters. This prediction was supported by microcosm-based radiotracer experiments that showed significant biological methane generation from formate, but not bicarbonate, in waters where the Type II lineage was detected in highest relative abundance. Phylogenetic analyses and variability in gene content suggested that recent and ongoing diversification of the Type II lineage was enabled by gene transfer, loss, and transposition. These data indicate that selection imposed by CO₂/oxidant availability drove recent methanogen diversification into hyperalkaline waters that are heavily impacted by serpentinization.

Coenzymes and Their Role in the Evolution of Life

The evolution of coenzymes, or their impact on the origin of life, is fundamental for understanding our own existence. Having established reasonable hypotheses about the emergence of prebiotic chemical building blocks, which were probably created under palaeogeochemical conditions, and surmising that these smaller compounds must have become integrated to afford complex macromolecules such as RNA, the question of coenzyme origin and its relation to the evolution of functional biochemistry should gain new impetus. Many coenzymes have a simple chemical structure and are often nucleotide-derived, which suggests that they may have coexisted with the emergence of RNA and may have played a pivotal role in early metabolism. Based on current theories of prebiotic evolution, which attempt to explain the emergence of privileged organic building blocks, this Review discusses plausible hypotheses on the prebiotic formation of key elements within selected extant coenzymes. In combination with prebiotic RNA, coenzymes may have dramatically broadened early protometabolic networks and the catalytic scope of RNA during the evolution of life.

Ingredients for microbial life preserved in 3.5 billion-year-old fluid inclusions

It is widely hypothesised that primeval life utilised small organic molecules as sources of carbon and energy. However, the presence of such primordial ingredients in early Earth habitats has not yet been demonstrated. Here we report the existence of indigenous organic molecules and gases in primary fluid inclusions in c. 3.5-billion-year-old barites (Dresser Formation, Pilbara Craton, Western Australia). The compounds identified (e.g., H₂S, COS, CS₂, CH₄, acetic acid, organic (poly-)sulfanes, thiols) may have formed important substrates for purported ancestral sulfur and methanogenic metabolisms. They also include stable building blocks of methyl thioacetate (methanethiol, acetic acid) – a putative key agent in primordial energy metabolism and thus the emergence of life. Delivered by hydrothermal fluids, some of these compounds may have fuelled microbial communities associated with the barite deposits. Our findings demonstrate that early Archaean hydrothermal fluids contained essential primordial ingredients that provided fertile substrates for earliest life on our planet.

It is widely hypothesised that primeval life utilized small organic molecules as sources of carbon and energy, however, the presence of such primordial ingredients in early Earth habitats has not yet been demonstrated. Here the authors report the existence of indigenous organic molecules and gases in primary fluid inclusions in c. 3.5- billion-year-old rocks from Western Australia.

Nanoscale Anatomy of Iron-Silica Self-Organized Membranes: Implications for Prebiotic Chemistry

Iron-silica self-organized membranes, so-called chemical gardens, behave as fuel cells and catalyze the formation of amino/carboxylic acids and RNA nucleobases from organics that were available on early Earth. Despite their relevance for prebiotic chemistry, little is known about their structure and mineralogy at the nanoscale. Studied here are focused ion beam milled sections of iron-silica membranes, grown from synthetic and natural, alkaline, serpentinization-derived fluids thought to be widespread on early Earth. Electron microscopy shows they comprise amorphous silica and iron nanoparticles of large surface areas and inter/intraparticle porosities. Their construction resembles that of a heterogeneous catalyst, but they can also exhibit a bilayer structure. Surface-area measurements suggest that membranes grown from natural waters have even higher catalytic potential. Considering their geochemically plausible precipitation in the early hydrothermal systems where abiotic organics were produced, iron-silica membranes might have assisted the generation and organization of the first biologically relevant organics.

The Role of Glycerol and Its Derivatives in the Biochemistry of Living Organisms, and Their Prebiotic Origin and Significance in the Evolution of Life

The emergence and evolution of prebiotic biomolecules on the early Earth remain a question that is considered crucial to understanding the chemistry of the origin of life. Amongst prebiotic molecules, glycerol is significant due to its ubiquity in biochemistry. In this review, we discuss the significance of glycerol and its various derivatives in biochemistry, their plausible roles in the origin and evolution of early cell membranes, and significance in the biochemistry of extremophiles, followed by their prebiotic origin on the early Earth and associated catalytic processes that led to the origin of these compounds. We also discuss various scenarios for the prebiotic syntheses of glycerol and its derivates and evaluate these to determine their relevance to early Earth biochemistry and geochemistry, and recapitulate the utilization of various minerals (including clays), condensation agents, and solvents that could have led to the successful prebiotic genesis of these biomolecules. Furthermore, important prebiotic events such as meteoritic delivery and prebiotic synthesis reactions under astrophysical conditions are also discussed. Finally, we have also highlighted some novel features of glycerol, including glycerol nucleic acid (GNA), in the origin and evolution of the life.

Carbon-Metal Bonds: Rare and Primordial in Metabolism

Submarine hydrothermal vents are rich in hydrogen (H₂), an ancient source of electrons and chemical energy for life. Geochemical H₂ stems from serpentinization, a process in which rock-bound iron reduces water to H₂. Reactions involving H₂ and carbon dioxide (CO₂) in hydrothermal systems generate abiotic methane and formate; these reactions resemble the core energy metabolism of methanogens and acetogens. These organisms are strict anaerobic autotrophs that inhabit hydrothermal vents and harness energy via H₂-dependent CO₂ reduction. Serpentinization also generates native metals, which can reduce CO₂ to formate and acetate in the laboratory. The enzymes that channel H₂, CO₂, and dinitrogen (N₂) into methanogen and acetogen metabolism are the backbone of the most ancient metabolic pathways. Their active sites share carbon-metal bonds which, although rare in biology, are conserved relics of primordial biochemistry present at the origin of life.

Single-Cell Genomics of Novel Actinobacteria With the Wood-Ljungdahl Pathway Discovered in a Serpentinizing System

Serpentinite-hosted systems represent modern-day analogs of early Earth environments. In these systems, water-rock interactions generate highly alkaline and reducing fluids that can contain hydrogen, methane, and low-molecular-weight hydrocarbons-potent reductants capable of fueling microbial metabolism. In this study, we investigated the microbiota of Hakuba Happo hot springs (∼50°C; pH∼10.5-11), located in Nagano (Japan), which are impacted by the serpentinization process. Analysis of the 16S rRNA gene amplicon sequences revealed that the bacterial community comprises Nitrospirae (47%), "Parcubacteria" (19%), Deinococcus-Thermus (16%), and Actinobacteria (9%), among others. Notably, only 57 amplicon sequence variants (ASV) were detected, and fifteen of these accounted for 90% of the amplicons. Among the abundant ASVs, an early-branching, uncultivated actinobacterial clade identified as RBG-16-55-12 in the SILVA database was detected. Ten single-cell genomes (average pairwise nucleotide identity: 0.98-1.00; estimated completeness: 33-93%; estimated genome size: ∼2.3 Mb) that affiliated with this clade were obtained. Taxonomic classification using single copy genes indicates that the genomes belong to the actinobacterial class-level clade UBA1414 in the Genome Taxonomy Database. Based on metabolic pathway predictions, these actinobacteria are anaerobes, capable of glycolysis, dissimilatory nitrate reduction and CO₂ fixation via the Wood-Ljungdahl (WL) pathway. Several other genomes within UBA1414 and two related class-level clades also encode the WL pathway, which has not yet been reported for the Actinobacteria phylum. For the Hakuba actinobacterium, the energy metabolism related to the WL pathway is likely supported by a combination of the Rnf complex, group 3b and 3d [NiFe]-hydrogenases, [FeFe]-hydrogenases, and V-type (H⁺/Na⁺ pump) ATPase. The genomes also harbor a form IV ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) complex, also known as a RubisCO-like protein, and contain signatures of interactions with viruses, including clustered regularly interspaced short palindromic repeat (CRISPR) regions and several phage integrases. This is the first report and detailed genome analysis of a bacterium within the Actinobacteria phylum capable of utilizing the WL pathway. The Hakuba actinobacterium is a member of the clade UBA1414/RBG-16-55-12, formerly within the group "OPB41." We propose to name this bacterium 'Candidatus Hakubanella thermoalkaliphilus.'

Microbial Residents of the Atlantis Massif's Shallow Serpentinite Subsurface

The International Ocean Discovery Program Expedition 357—“Serpentinization and Life”—utilized seabed drills to collect rocks from the oceanic crust. The recovered rock cores represent the shallow serpentinite subsurface of the Atlantis Massif, where reactions between uplifted mantle rocks and water, collectively known as serpentinization, produce environmental conditions that can stimulate biological activity and are thought to be analogous to environments that were prevalent on the early Earth and perhaps other planets. The methodology and results of this project have implications for life detection experiments, including sample return missions, and provide a window into the diversity of microbial communities inhabiting subseafloor serpentinites.

The Atlantis Massif rises 4,000 m above the seafloor near the Mid-Atlantic Ridge and consists of rocks uplifted from Earth’s lower crust and upper mantle. Exposure of the mantle rocks to seawater leads to their alteration into serpentinites. These aqueous geochemical reactions, collectively known as the process of serpentinization, are exothermic and are associated with the release of hydrogen gas (H₂), methane (CH₄), and small organic molecules. The biological consequences of this flux of energy and organic compounds from the Atlantis Massif were explored by International Ocean Discovery Program (IODP) Expedition 357, which used seabed drills to collect continuous sequences of shallow (<16 m below seafloor) marine serpentinites and mafic assemblages. Here, we report the census of microbial diversity in samples of the drill cores, as measured by environmental 16S rRNA gene amplicon sequencing. The problem of contamination of subsurface samples was a primary concern during all stages of this project, starting from the initial study design, continuing to the collection of samples from the seafloor, handling the samples shipboard and in the lab, preparing the samples for DNA extraction, and analyzing the DNA sequence data. To distinguish endemic microbial taxa of serpentinite subsurface rocks from seawater residents and other potential contaminants, the distributions of individual 16S rRNA gene sequences among all samples were evaluated, taking into consideration both presence/absence and relative abundances. Our results highlight a few candidate residents of the shallow serpentinite subsurface, including uncultured representatives of the Thermoplasmata, Acidobacteria, Acidimicrobia, and Chloroflexi.

IMPORTANCE The International Ocean Discovery Program Expedition 357—“Serpentinization and Life”—utilized seabed drills to collect rocks from the oceanic crust. The recovered rock cores represent the shallow serpentinite subsurface of the Atlantis Massif, where reactions between uplifted mantle rocks and water, collectively known as serpentinization, produce environmental conditions that can stimulate biological activity and are thought to be analogous to environments that were prevalent on the early Earth and perhaps other planets. The methodology and results of this project have implications for life detection experiments, including sample return missions, and provide a window into the diversity of microbial communities inhabiting subseafloor serpentinites.

Autocatalytic chemical networks at the origin of metabolism

Modern cells embody metabolic networks containing thousands of elements and form autocatalytic sets of molecules that produce copies of themselves. How the first self-sustaining metabolic networks arose at life's origin is a major open question. Autocatalytic sets smaller than metabolic networks were proposed as transitory intermediates at the origin of life, but evidence for their role in prebiotic evolution is lacking. Here, we identify reflexively autocatalytic food-generated networks (RAFs)-self-sustaining networks that collectively catalyse all their reactions-embedded within microbial metabolism. RAFs in the metabolism of ancient anaerobic autotrophs that live from H2 and CO2 provided with small-molecule catalysts generate acetyl-CoA as well as amino acids and bases, the monomeric components of protein and RNA, but amino acids and bases without organic catalysts do not generate metabolic RAFs. This suggests that RAFs identify attributes of biochemical origins conserved in metabolic networks. RAFs are consistent with an autotrophic origin of metabolism and furthermore indicate that autocatalytic chemical networks preceded proteins and RNA in evolution. RAFs uncover intermediate stages in the emergence of metabolic networks, narrowing the gaps between early Earth chemistry and life.

Habitability of the marine serpentinite subsurface: a case study of the Lost City hydrothermal field

The Lost City hydrothermal field is a dramatic example of the biological potential of serpentinization. Microbial life is prevalent throughout the Lost City chimneys, powered by the hydrogen gas and organic molecules produced by serpentinization and its associated geochemical reactions. Microbial life in the serpentinite subsurface below the Lost City chimneys, however, is unlikely to be as dense or active. The marine serpentinite subsurface poses serious challenges for microbial activity, including low porosities, the combination of stressors of elevated temperature, high pH and a lack of bioavailable ∑CO₂. A better understanding of the biological opportunities and challenges in serpentinizing systems would provide important insights into the total habitable volume of Earth's crust and for the potential of the origin and persistence of life in Earth's subsurface environments. Furthermore, the limitations to life in serpentinizing subsurface environments on Earth have significant implications for the habitability of subsurface environments on ocean worlds such as Europa and Enceladus. Here, we review the requirements and limitations of life in serpentinizing systems, informed by our research at the Lost City and the underwater mountain on which it resides, the Atlantis Massif. This article is part of a discussion meeting issue 'Serpentinite in the Earth System'.

Chemical and isotopic analyses of hydrocarbon-bearing fluid inclusions in olivine-rich rocks

We examined the mineralogical, chemical and isotopic compositions of secondary fluid inclusions in olivine-rich rocks from two active serpentinization systems: the Von Damm hydrothermal field (Mid-Cayman Rise) and the Zambales ophiolite (Philippines). Peridotite, troctolite and gabbroic rocks in these systems contain abundant CH4-rich secondary inclusions in olivine, with less abundant inclusions in plagioclase and clinopyroxene. Olivine-hosted secondary inclusions are chiefly composed of CH4 and minor H2, in addition to secondary minerals including serpentine, brucite, magnetite and carbonates. Secondary inclusions in plagioclase are dominated by CH4 with variable amounts of H2 and H2O, while those in clinopyroxene contain only CH4. We determined hydrocarbon abundances and stable carbon isotope compositions by crushing whole rocks and analysing the released volatiles using isotope ratio monitoring-gas chromatography mass spectrometry. Bulk rock gas analyses yielded appreciable quantities of CH4 and C2H6 in samples from Cayman (4-313 nmol g-1 CH4 and 0.02-0.99 nmol g-1 C2H6), with lesser amounts in samples from Zambales (2-37 nmol g-1 CH4 and 0.004-0.082 nmol g-1 C2H6). Mafic and ultramafic rocks at Cayman exhibit δ13CCH4 values of -16.7‰ to -4.4‰ and δ13CC2H6 values of -20.3‰ to +0.7‰. Ultramafic rocks from Zambales exhibit δ13CCH4 values of -12.4‰ to -2.8‰ and δ13CC2H6 values of -1.2‰ to -0.9‰. Similarities in the carbon isotopic compositions of CH4 and C2H6 in plutonic rocks, Von Damm hydrothermal fluids, and Zambales gas seeps suggest that leaching of fluid inclusions may provide a significant contribution of abiotic hydrocarbons to deep-sea vent fluids and ophiolite-hosted gas seeps. Isotopic compositions of CH4 and C2H6 from a variety of hydrothermal fields hosted in olivine-rich rocks that are similar to those in Von Damm vent fluids further support the idea that a significant portion of abiotic hydrocarbons in ultramafic-influenced vent fluids is derived from fluid inclusions. This article is part of a discussion meeting issue 'Serpentinite in the Earth system'.