Pathways for abiotic organic synthesis at submarine hydrothermal fields

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-06, Vol.112 (25), p.7668-7672
• Main Authors: McDermott, Jill M, Jeffrey S. Seewald, Christopher R. German, Sean P. Sylva
• Format: Article
• Language: English
• Subjects: abiotic organic synthesis; Biosphere; carbon; fluid–vapor inclusions; formate; formates; hot springs; Hydrocarbons; hydrothermal systems; methane; methane production; microbial communities; mixing; Molecular weight; Organic chemicals; Physical Sciences; Thermodynamics
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1506295112

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. 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 H ₂-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 CH ₄ and higher hydrocarbons are likely formed in H ₂-rich fluid inclusions over geologic timescales. Conversely, formate production by ΣCO ₂ reduction occurs rapidly during subsurface mixing, which may support anaerobic methanogenesis. We confirm models for abiotic metas