A prevailing scenario of the origin of life postulates thioesters as key intermediates in protometabolism, but there is no experimental support for the prebiotic CO2 fixation routes to thioesters. Here we demonstrate that, under a simulated geoelectrochemical condition in primordial ocean hydrothermal systems (–0.6 to –1.0 V versus the standard hydrogen electrode), nickel sulfide (NiS) gradually reduces to Ni0, while accumulating surface-bound carbon monoxide (CO) due to CO2 electroreduction. The resultant partially reduced NiS realizes thioester (S-methyl thioacetate) formation from CO and methanethiol even at room temperature and neutral pH with the yield up to 35% based on CO. This thioester formation is not inhibited, or even improved, by 50:50 coprecipitation of NiS with FeS or CoS (the maximum yields; 27 or 56%, respectively). Such a simple thioester synthesis likely occurred in Hadean deep-sea vent environments, setting a stage for the autotrophic origin of life.

Chemical evolution is an abiotic reaction process in which complex organic molecules arise from a combination of simple inorganic and organic chemical compounds. To assess the possible ongoing chemical evolution in the subsurface ocean of Saturn's icy satellite Enceladus, we explored the water−rock aqueous reactions and the peptide formation capability under a hydrothermal environment similar to that on Enceladus. It has been suggested that the core of Enceladus has not experienced high temperatures from the time of satellite formation to the present. The major components of the core are assumed to be carbonaceous chondrites; thus, simple organic substances, including amino acids, are likely present in the alkaline seawater of Enceladus. In this study, we conducted a laboratory-based simulation experiment to describe the chemical alteration of six prebiotically abundant amino acids over 147 days under high pressure with thermal cycling (30 to 100 °C) to simulate the water−rock interface of the ocean on Enceladus. As a result, we detected 28 out of 36 possible dipeptide species during the entire reaction period. We propose that peptide-bond formation is coupled to rock surface chemisorption of amino acids under alkaline condition, which was further supported by the elemental analysis showing carbon and nitrogen signature on the rock surface only when amino acids are added. The above result suggests that ongoing chemical evolution on Enceladus is likely producing short abiotic peptides on the porous core surface.

Thioester synthesis by CO dehydrogenase/acetyl-CoA synthase is among the most ancient autotrophic metabolisms. Although the preceding prebiotic CO2 fixation routes to thioesters are often suggested, none has any experimentally supported evidence. Here we demonstrate that, under an electrochemical condition realizable in early ocean hydrothermal systems, nickel sulfide (NiS) gradually reduces to Ni0, while accumulating surface-bound CO due to CO2 electroreduction. The resultant partially reduced NiS facilitates thioester (S-methyl thioacetate) formation from CO and methanethiol even at room temperature and neutral pH. This thioester formation can further be enhanced up to a selectivity of 56% by NiS coprecipitating with FeS or CoS. Considering the central role of Ni in the enzymatic process mentioned above, our demonstrated thioester synthesis with the partially reduced NiS could have a direct implication to the autotrophic origin of life.