13C-ENRICHED METHANE PRODUCED BIOLOGICALLY AT HYPERSALINE MARS ANALOG SITES

Methane is one of the biosignature gases targeted for detection in the atmosphere of Mars by the 2012 Mars Science Laboratory and the 2016 ExoMars Trace Gas Orbiter. Both missions will have instruments to measure both concentration and stable carbon isotopes of methane. To give context to the Mars methane data when it returns, we have been working at hypersaline Mars analog sites here on Earth, environments which are similar to the southern highlands of Mars where chloride minerals have been found. We have examined methanogenesis in hypersaline ponds (salinities of ~55 to ~300 ppt) in Baja California Sur and in the Don Edwards National Wildlife Refuge in northern California. At the lower salinity ponds, thick microbial mats dominate; at the higher salinity ponds, gypsum/halite crusts of containing endolithic microbial communities form. Incubations of the mats and crusts resulted in the production of methane, with the highest production (~20 nmol/g/d) occurring within the gypsum crust. The carbon isotopic composition of this produced methane averaged -36 ‰, a value that would typically be considered non-biogenic. Values at all sites ranged from about -60 ‰ (at lower salinity sites) to -40 ‰ (at higher salinity sites). To understand the biologically-produced, isotopically-enriched values at the high salinity sites, we also measured the concentration and isotopic composition of the particulate organic carbon (POC) and determined the main substrates used by the methanogens. POC content was low, generally less than 1% of the total mass. The isotopic composition of the POC ranged from -13 to -21 ‰. The main substrates used by the methanogens were the non-competitive substrates, the methylamines and methanol. When increasing amounts (up to 1000 µM) of trimethylamine were added to incubation vials containing gypsum/halite crusts, the resultant methane became increasingly depleted in ¹³C, resembling methane emitted from marine and freshwater environments. This suggests that the methanogens living in these evaporitic crusts are substrate-limited in situ, and so do not fractionate the methane carbon isotopes to the fullest extent possible. Such substrate limitation for methanogens resulting in abnormally high biogenic methane δ¹³C values may be characteristic of these endolithic microbial communities.