Dominant substrates used by methanogens in hypersaline environments
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] The central goal of this study was to determine the dominant substrates used by methanogens in the salt ponds of Elkhorn Slough, the Don Edwards National Wildlife Refuge, and the and the Exportadora de Sal saltern. Microbial mat and/or sediments were amended with site water only or with site water and various substrates and incubated in N2(g) flushed serum vials. It was hypothesized that trimethylamine (TMA) would be a dominant substrate in all ponds. Additionally, samples were incubated with substrates also reported to be of importance in hypersaline environments: methanol (MeOH); dimethylsulfide (DMS); monomethylamine (MMA); and acetate. As expected, high concentrations of TMA and MMA stimulated methane production to a greater extent than the controls across the entire salinity gradient sampled and there was also demonstrated use of 13C-labeled methanol at all of the Guerrero Negro sites. Apparent fractionation factors were calculated to determine if they could be used to predict dominant substrates of hypersaline methanogenesis. Because TMA and MeOH were used, apparent fractionation factors were expected to be high and in the range of 1.04 to 1.09 (the published range for TMA and MeOH). However, calculated apparent fractionation factors were lower and ranged from 1.02 to 1.07, indicating use of other substrates. The difference in expected versus measured apparent fractionation factors could be a result of methanogens being substrate limited because of high salinities or low POC concentrations, thus reducing the expressed fractionation of the stable isotopes.
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