The Effects of Trimethylamine and Organic Matter Additions on the Stable Carbon Isotopic Composition of Methane Produced in Hypersaline Microbial Mat Environments

Abstract:

Methane production has been observed in a number of hypersaline environments, and it is generally thought that this methane is produced through the use of non-competitive substrates, such as the methylamines, methanol and dimethylsulfide. The stable carbon isotopic composition of the produced methane has suggested that the methanogens are operating under conditions of substrate limitation. We investigated substrate limitation in gypsum-hosted endoevaporite and soft mat hypersaline environments by the additions of trimethylamine, a non-competitive substrate for methanogenesis, and dried microbial mat, a source of natural organic matter. The δ¹³C values of the methane produced after amendments were compared to those in unamended control vials. At all hypersaline sites investigated, the δ¹³C values of the methane produced in the amended vials were statistically lower (by 10 to 71 ‰) than the unamended controls, supporting the hypothesis of substrate limitation at these sites. When substrates were added to the incubation vials, the methanogens within the vials fractionated carbon isotopes to a greater degree, resulting in the production of more ¹³C-depleted methane. Trimethylamine-amended samples produced lower methane δ¹³C values than the mat-amended samples. This difference in the δ¹³C values between the two types of amendments could be due to differences in isotope fractionation associated with the dominant methane production pathway (or substrate used) within the vials, with trimethylamine being the main substrate used in the trimethylamine-amended vials. We hypothesize that increased natural organic matter in the mat-amended vials would increase fermentation rates, leading to higher H₂ concentrations and increased CO₂/H₂ methanogenesis.