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

• Published in: Aquatic geochemistry 2019-12, Vol.25 (5-6), p.237-251
• Main Authors: Kelley, Cheryl A., Bebout, Brad M., Chanton, Jeffrey P., Detweiler, Angela M., Frisbee, Adrienne, Nicholson, Brooke E., Poole, Jennifer, Tazaz, Amanda, Winkler, Claire
• Format: Article
• Language: English
• Subjects: Acetates; Acetic acid; Analytical Chemistry; Bacteria; Biogeosciences; Competition; Earth and Environmental Science; Earth Sciences; Earth System Sciences; Fractionation; Geochemistry; Gypsum; Headspace; Incubation period; Isotope fractionation; Metabolism; Methane; Methanogenic bacteria; Microbial mats; Microorganisms; Molybdate; Oceanography; Photosynthesis; Ponds; Reduction; Salinity; Substrates; Sulfate reduction; Sulfate-reducing bacteria; Sulfates; Sulphate reduction
• ISSN: 1380-6165; 1573-1421
• DOI: 10.1007/s10498-019-09362-x

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 δ 13 C 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 13 C) methane δ 13 C values than the controls. This suggests that the inhibition of sulfate reduction allowed the methanogens at these sites to use the competitive substrates (H 2 and/or acetate) made available. Further, the higher δ 13 C 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 δ 13 C values either remained the same or decreased (depleted in 13 C) with added molybdate. We suggest that substrate availability increased enough to allow for somewhat greater isotopic fractionation resulting in the lower methane δ 13 C values that were observed, but not enough to significantly increase measured production rates. Where no changes in either methane production rates or δ 13 C values occurred, we hypothesize that salinity itself was inhibiting sulfate reduction and thus controlling microbe populations and rates of metabolism.