Potential use of highly insoluble carbonates as carbon sources by methanogens in the subsurface of Mars

Methanogens, microorganisms in the domain Archaea, have been studied as life forms that might inhabit the subsurface of Mars. These organisms can use carbon dioxide as a carbon source, a compound that is abundant in the martian atmosphere. But if they exist in the deep subsurface where the carbon dioxide may not penetrate, they would have to rely on another source of carbon. Magnesium carbonate and calcium carbonate have been detected at the martian surface, and there is no reason to believe that they would not be in the subsurface as well. In the research reported here, we asked if these carbonates could possibly serve as carbon sources for four species of methanogens. Methanothermobacter wolfeii, Methanobacterium formicicum and Methanococcus maripaludis were able to produce a small amount of methane (approximately 0.4–0.8% headspace gas) when either carbonate was the carbon source available while Methanosarcina barkeri only produced significant methane (also 0.4–0.8%) when calcium carbonate was the carbon source. The amounts produced were dependent on methanogenic species, carbonate used and pH. At equilibrium, a small amount of carbon dioxide (approximately 0.05–0.15% headspace gas as well as in liquid media) was generated by these carbonates, and this carbon dioxide was most likely the carbon compound that was being metabolized. Background carbon dioxide from the atmosphere was not sufficient for measureable methane production.