Enceladus-like geochemistry fuels methanogenesis under extreme CO 2 limitation

Abstract

Saturn’s icy moon Enceladus features chemical signatures consistent with an alkaline soda ocean with H 2 production via hydrothermal water-rock reactions. Chemolithoautotrophic methanogenesis is thermodynamically favorable under these conditions, but it is unknown whether CO 2 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 H 2 from mineral-water reactions enables growth of the chemolithoautotroph H 2 -oxidizing methanogen Methanothermococcus okinawensis until pH 11, far exceeding its previously known pH limit. Transcriptomics revealed that growing cells overcome CO 2 scarcity in the simulated Enceladus soda ocean by overexpressing the reductive acetyl-CoA pathway for CO 2 fixation, which enabled efficient scavenging of CO 2 at extremely low concentrations. The carbon and energy metabolism of M. okinawensis was entirely fueled by the abiotic H 2 derived from the simulated Enceladus mineral-water reactions.