All life on Earth is built of organic molecules, so the primordial sources of reduced carbon remain a major open question in studies of the origin of life. A variant of the alkaline-hydrothermal-vent theory for life’s emergence suggests that organics could have been produced by the reduction of CO2 via H2 oxidation, facilitated by geologically sustained pH gradients. The process would be an abiotic analog—and proposed evolutionary predecessor—of the Wood–Ljungdahl acetyl-CoA pathway of modern archaea and bacteria. The first energetic bottleneck of the pathway involves the endergonic reduction of CO2 with H2 to formate (HCOO–), which has proven elusive in mild abiotic settings. Here we show the reduction of CO2 with H2 at room temperature under moderate pressures (1.5 bar), driven by microfluidic pH gradients across inorganic Fe(Ni)S precipitates. Isotopic labeling with 13C confirmed formate production. Separately, deuterium (2H) labeling indicated that electron transfer to CO2 does not occur via direct hydrogenation with H2 but instead, freshly deposited Fe(Ni)S precipitates appear to facilitate electron transfer in an electrochemical-cell mechanism with two distinct half-reactions. Decreasing the pH gradient significantly, removing H2, or eliminating the precipitate yielded no detectable product. Our work demonstrates the feasibility of spatially separated yet electrically coupled geochemical reactions as drivers of otherwise endergonic processes. Beyond corroborating the ability of early-Earth alkaline hydrothermal systems to couple carbon reduction to hydrogen oxidation through biologically relevant mechanisms, these results may also be of significance for industrial and environmental applications, where other redox reactions could be facilitated using similarly mild approaches.

24All life on Earth is built of organic molecules, so the primordial sources of reduced carbon are a major 25 open question in studies of the origin of life. A variant of the alkaline-vent theory suggests that 26 organics could have been produced by the reduction of CO2 via H2 oxidation, facilitated by 27 geologically sustained pH gradients. The process would be an abiotic analog-and proposed 28 evolutionary predecessor-of the modern Wood-Ljungdahl acetyl-Co-A pathway of extant archaea 29 and bacteria. The first energetic bottleneck of the pathway involves the endergonic reduction of CO2 30 with H2 to formate, which has proven elusive in low-temperature abiotic settings. Here we show the 31 reduction of CO2 with H2 at moderate pressures (1.5 bar), driven by microfluidic pH gradients across 32 inorganic Fe(Ni)S precipitates. Isotopic labelling with 13 C confirmed production of formate. 33 Separately, deuterium ( 2 H) labelling indicated that electron transfer to CO2 did not occur via direct 34 hydrogenation with H2. Instead, freshly deposited Fe(Ni)S precipitates appear to facilitate electron 35 transfer in an electrochemical-cell mechanism with two distinct half-reactions. Decreasing the pH 36 gradient significantly, or removing either H2 or the precipitate, yielded no detectable product. Our 37 work demonstrates the feasibility of spatially separated, yet electrically coupled geochemical reactions 38 as drivers of otherwise endergonic processes. Beyond corroborating the ability of early-Earth alkaline 39 hydrothermal systems to couple carbon reduction to hydrogen oxidation through geologically 40 plausible and biologically relevant mechanisms, these results may also be of significance for industrial 41 and environmental applications, where other redox reactions could be facilitated using similarly mild 42 approaches. 43 Keywords 44 carbon fixation, origin of life, Wood-Ljungdahl, acetyl co-A, 45 catalysis, membranes, iron-nickel-sulfur, electrosynthesis 46 A dependable supply of reduced organic molecules is essential for any scenario of the origin of 48 life. On the early Earth, one way in which this supply could have been attained was by reduction of 49 CO2 with H2 (1-5). Geological models indicate that CO2 was at comparatively high concentrations in 50 the ocean during the Hadean eon, whereas H2 was the product of serpentinization processes in the 51 Earth's crust and would have emanated as part of the efflux of alkaline hydrothermal vents (3,(6)(7)(8). 52Upon meeting at the vent-ocean interface, the model suggests, the reaction between the two dissolved 53 gases would have produced hydrocarbons, which would in turn take roles in the transition from 54 geochemistry to biochemistry (2, 9). Under standard conditions (1 atm, 25 ºC, pH 7), the reaction 55 between CO2 and H2 to produce formate (HCOO -) is thermodynamically disfavored, 56 with ΔG 0 ' = +3.5 kJ mol -1 (10, 11). In ancient alkaline vents, however ( Figure 1A), H2 was present in 57 the OH --rich effluent of the vent, which would have favored its oxidat...