Methane Production on Mars-Relevant Clay Minerals and Simulant Regolith

The presence of atmospheric carbon dioxide and potential subsurface molecular hydrogen (H₂), in addition to potential subsurface liquid water sources, suggest that the martian subsurface may currently be habitable, particularly to autotrophic chemosynthetic microorganisms. In addition, the widespread nature of clays and other minerals on Mars could provide sufficient nutrients to support microbial life. Here we tested four methanogenic species (Methanosarcina barkeri, Methanobacterium formicicum, Methanothermobacter wolfeii, and Methanococcus maripaludis) in the presence of illite, nontronite, and one martian regolith simulant, Mojave Mars Simulant (MMS), in their optimal growth medium. We aimed to determine whether the presence of certain clay minerals and regolith simulants inhibited, promoted, or had no effect on methane (CH₄) production by these microorganisms. We also tested the same methanogens in the presence of montmorillonite, H₂, sodium sulfide (Na₂S), and bicarbonate buffer to determine if this clay could support growth (as measured by CH₄ production). Results indicated that three of the four methanogens tested, M. barkeri, M. formicicum, and M. wolfeii, were capable of growth in the presence of both clay minerals and MMS, although most cultures demonstrated lower CH₄ production compared to growth in optimal media without clay minerals. Additionally, all three methanogens were capable of CH₄ production in cultures containing only 10% (w/v) montmorillonite, H₂, Na₂S, and bicarbonate buffer. Conversely, M. maripaludis, a halophile, showed the greatest sensitivity of the four methanogens tested, being unable to produce any CH₄ in cultures containing standard methanogenic growth medium and 2% (w/v) illite or in bicarbonate buffer containing H₂, Na₂S, and 10% (w/v) montmorillonite. However, in one experiment assessing the minimum medium requirements for this organism, the presence of 5% (w/v) montmorillonite enabled greater CH₄ production compared to cultures containing the organism’s standard growth medium alone. Overall, these results suggest that the presence of clay minerals on Mars does not preclude the survivability and growth of methanogens in a potential subsurface habitat on the planet. In fact, these geological components may provide sufficient nutrients to support growth and survivability.