Enceladus-like geochemistry fuels methanogenesis under extreme CO₂ limitation

Saturn's icy moon Enceladus features chemical signatures consistent with an alkaline soda ocean with H₂ production via hydrothermal water-rock reactions. Chemolithoautotrophic methanogenesis is thermodynamically favorable under these conditions, but it is unknown whether CO₂ scarcity in the alkaline Enceladus soda ocean limits autotrophy. Here, we show that simulated Enceladus' ocean chemistry containing high concentrations of dissolved inorganic carbon and H₂ from mineral-water reactions enables growth of the chemolithoautotroph H₂-oxidizing methanogen Methanothermococcus okinawensis until pH 11, far exceeding its previously known pH limit. Transcriptomics revealed that growing cells overcome CO₂ scarcity in the simulated Enceladus soda ocean by overexpressing the reductive acetyl-CoA pathway for CO₂ fixation, which enabled efficient scavenging of CO₂ at extremely low concentrations. The carbon and energy metabolism of M. okinawensis was entirely fueled by the abiotic H₂ derived from the simulated Enceladus mineral-water reactions.