The Effect of Bacterial Sulfate Reduction Inhibition on the Production and Stable Isotopic Composition of Methane in Hypersaline Environments

Abstract

The aim of this research was to investigate the competition between methanogens and sulfate-reducing bacteria in hypersaline environments. Samples of photosynthetic microbial mats, both soft mats (salinities of 55–126 ppt) and gypsum-hosted endoevaporite mats (salinities of 77–320 ppt), were obtained from hypersaline environments in California, USA, Mexico and Chile. Methane production was determined from the increase in headspace methane concentration within incubation vials containing mat samples. At the end of the incubation period, the δ13C values of produced methane were measured. Soft microbial mat vials containing molybdate, a specific inhibitor of bacterial sulfate reduction, exhibited dramatically higher methane production rates and higher (enriched in 13C) methane δ13C values than the controls. This suggests that the inhibition of sulfate reduction allowed the methanogens at these sites to use the competitive substrates (H2 and/or acetate) made available. Further, the higher δ13C values of the produced methane suggest that substrates (both competitive and non-competitive) were used to near completion. At the endoevaporite sites, which have much higher salinities than the soft mat sites, methane production was not significantly different and the methane δ13C values either remained the same or decreased (depleted in 13C) with added molybdate. We suggest that substrate availability increased enough to allow for somewhat greater isotopic fractionation resulting in the lower methane δ13C values that were observed, but not enough to significantly increase measured production rates. Where no changes in either methane production rates or δ13C values occurred, we hypothesize that salinity itself was inhibiting sulfate reduction and thus controlling microbe populations and rates of metabolism.