Pathways for abiotic organic synthesis at submarine hydrothermal fields

Significance Arguments for an abiotic origin of organic compounds in deep-sea hot springs are compelling because of their potential role in the origin of life and sustaining microbial communities. Theory predicts that warm H2-rich fluids circulating through serpentinizing systems create a favorable thermodynamic drive for inorganic carbon reduction to organic compounds. We show that abiotic synthesis proceeds by two spatially and temporally distinct mechanisms. Abundant dissolved CH4 and higher hydrocarbons are likely formed in H2-rich fluid inclusions over geologic timescales. Conversely, formate production by ΣCO2 reduction occurs rapidly during subsurface mixing, which may support anaerobic methanogenesis. We confirm models for abiotic metastable organic compound formation and argue that alkanes in all ultramafic-influenced vents may form independently of actively circulating serpentinizing fluids. 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 H2-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 CH4 and higher hydrocarbons in ultramafic hydrothermal systems were dependent on H2 generation during active serpentinization. Rather, our results indicate that CH4 found in vent fluids is formed in H2-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 ΣCO2 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.