Methanogenesis at 7-12 mbar under a simulated Martian atmosphere: New revelations from transcriptomics on the habitability of the shallow Martian subsurface

A growing body of literature assessing methanogen survival under extreme conditions characterizing Mars Special Regions – the most "habitable" environments on the planet today – demonstrably indicates the shallow subsurface of modern Mars may not be as hostile to microbial methanogenesis as previously thought. Numerous studies have presented data supporting biological methanogenesis following methanogens' prolonged exposure to desiccation, high salinity, strong oxidants, and extremes in temperature, pH, and pressure. By coupling these survival experiments with RNA sequencing and comparative transcriptomics, we recently demonstrated how the methanogen Methanosarcina barkeri can regulate its gene expression to not only survive and make methane in the presence of perchlorate salts, but also to reduce these chaotropic oxidants to less toxic chloride species (Harris et al., 2021).

Here we follow up on our 2021 study, investigating methanogenesis and accompanying gene expression in M. barkeri in response to sustained low hydrostatic pressure (7 – 12 mbar), freezing temperatures (0˚C), and one of two headspace conditions: (1) 80:20 H2:CO2 or (2) a simulated Martian atmosphere (95% CO2 + balance N2, Ar, O2, H2O, and H2). Notably, we observed statistically quantifiable CH4 production under all assayed conditions, and complementary transcriptomic analyses demonstrated ongoing expression of the methanogenesis pathway, with significant down-regulation only occurring under low-pressure, Mars-like atmospheric conditions. We conclude that it is not low-pressure conditions alone, but the partial pressure of H2, that limits methanogenesis in M. barkeri under simulated Mars conditions. With these results we intend to offer new perspectives on Martian habitability, its enigmatic methane cycle, and science objectives for Mars Sample Return and the proposed Mars Life Explorer mission.