Arguments for an abiotic origin of low-molecular weight organic compounds in deep-sea hot springs are compelling owing to implications for the sustenance of deep biosphere microbial communities and their potential role in the origin of life. Theory predicts that warm H 2 -rich fluids, like those emanating from serpentinizing hydrothermal systems, create a favorable thermodynamic drive for the abiotic generation of organic compounds from inorganic precursors. Here, we constrain two distinct reaction pathways for abiotic organic synthesis in the natural environment at the Von Damm hydrothermal field and delineate spatially where inorganic carbon is converted into bioavailable reduced carbon. We reveal that carbon transformation reactions in a single system can progress over hours, days, and up to thousands of years. Previous studies have suggested that CH 4 and higher hydrocarbons in ultramafic hydrothermal systems were dependent on H 2 generation during active serpentinization. Rather, our results indicate that CH 4 found in vent fluids is formed in H 2 -rich fluid inclusions, and higher n-alkanes may likely be derived from the same source. This finding implies that, in contrast with current paradigms, these compounds may form independently of actively circulating serpentinizing fluids in ultramafic-influenced systems. Conversely, widespread production of formate by ΣCO 2 reduction at Von Damm occurs rapidly during shallow subsurface mixing of the same fluids, which may support anaerobic methanogenesis. Our finding of abiogenic formate in deep-sea hot springs has significant implications for microbial life strategies in the present-day deep biosphere as well as early life on Earth and beyond.abiotic organic synthesis | hydrothermal systems | methane | formate | fluid-vapor inclusions S eawater-derived hydrothermal fluids venting at oceanic spreading centers are a net source for dissolved carbon to the deep sea, with vent fluid carbon contents directly tied to the sustenance of the subseafloor biosphere (1). Highly reducing fluids rich in dissolved H 2 , such as those discharging from serpentinizing hydrothermal systems, are of particular interest because of the potential for abiotic reduction of dissolved inorganic carbon ðΣCO 2 = CO 2 + HCO − 3 + CO 2− 3 Þ to organic compounds (2-6) and their potential role as precursor compounds for prebiotic chemistry associated with the origin of life (7). Although there is increasing evidence that supports an abiotic origin for CH 4 and other low-molecular weight organic compounds in ultramafic-hosted hydrothermal systems (8-10), the physical conditions, reaction pathways, and timescales that control abiotic organic synthesis at oceanic spreading centers remain elusive. Working models for the formation of abiotic CH 4 and other hydrocarbons observed in vent fluids involve reduction of ΣCO 2 and/or CO through Fischer-Tropsch-type processes during active circulation of seawater-derived hydrothermal fluids that are highly enriched in dissolved H 2 because of serpentinization of...

Thirty years after the first discovery of high-temperature submarine venting, the vast majority of the global mid-ocean ridge remains unexplored for hydrothermal activity. Of particular interest are the world's ultraslow spreading ridges that were the last to be demonstrated to host high-temperature venting but may host systems particularly relevant to prebiotic chemistry and the origins of life. Here we report evidence for previously unknown, diverse, and very deep hydrothermal vents along the ∼110 km long, ultraslow spreading Mid-Cayman Rise (MCR). Our data indicate that the MCR hosts at least three discrete hydrothermal sites, each representing a different type of water-rock interaction, including both mafic and ultramafic systems and, at ∼5,000 m, the deepest known hydrothermal vent. Although submarine hydrothermal circulation, in which seawater percolates through and reacts with host lithologies, occurs on all mid-ocean ridges, the diversity of vent types identified here and their relative geographic isolation make the MCR unique in the oceans. These new sites offer prospects for an expanded range of vent-fluid compositions, varieties of abiotic organic chemical synthesis and extremophile microorganisms, and unparalleled faunal biodiversity-all in close proximity.hydrothermal activity | mid-ocean ridge | microbiology | ocean chemistry | astrobiology

SummaryWarm fluids emanating from hydrothermal vents can be used as windows into the rocky subseafloor habitat and its resident microbial community. Two new vent systems on the Mid‐Cayman Rise each exhibits novel geologic settings and distinctively hydrogen‐rich vent fluid compositions. We have determined and compared the chemistry, potential energy yielding reactions, abundance, community composition, diversity, and function of microbes in venting fluids from both sites: Piccard, the world's deepest vent site, hosted in mafic rocks; and Von Damm, an adjacent, ultramafic‐influenced system. Von Damm hosted a wider diversity of lineages and metabolisms in comparison to Piccard, consistent with thermodynamic models that predict more numerous energy sources at ultramafic systems. There was little overlap in the phylotypes found at each site, although similar and dominant hydrogen‐utilizing genera were present at both. Despite the differences in community structure, depth, geology, and fluid chemistry, energetic modelling and metagenomic analysis indicate near functional equivalence between Von Damm and Piccard, likely driven by the high hydrogen concentrations and elevated temperatures at both sites. Results are compared with hydrothermal sites worldwide to provide a global perspective on the distinctiveness of these newly discovered sites and the interplay among rocks, fluid composition and life in the subseafloor.

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