Carbon isotopic fractionation in Methanosarcina barkeri and the study of anaerobic microbial communities of saline springs in West Central Manitoba

Abstract

Stable carbon isotope fractionation during methanogenesis is affected by the availability of substrates. The effects of different substrates on methanogen biomass, total lipid extract, biomarkers and methane under both abundant and limiting substrate conditions were studied. Methanosarcina barkeri was grown with methanol, acetate, trimethylamine (TMA) and H2/CO2, and carbon isotope fractionation in methane production was greatest with methanol, followed by H2/CO2, TMA and acetate. In contrast, biomass was isotopically lightest in M.barkeri grown on methanol, followed by TMA, H2/CO2 and acetate. Generally, fractionation was greater in cultures grown with abundant substrate availability as compared to those supplied with limiting substrate. During autotrophic growth, fractionation was greatest during slower growth for both methane and biomass production. The results of these fractionation studies under controlled laboratory conditions can be applied to the interpretation of isotopic signatures for methane and methanogen biomarkers, and ecological processes, in marine environments. Several hypersaline springs off the western shore of Lake Winnipegosis, MB support unique microbial mat communities. These low temperature springs contain water with a mean salinity as high as 6.1%. Studies were undertaken to contrast the anaerobic microbial communities of these springs, specifically the methanogens and sulphate-reducing bacteria (SRB), and their contributions to biogeochemical cycling in these mats. Comparisons of lipid profiles revealed changes in the proportions of the dominant fatty acids related to the amount of mat growth. Cultures of SRB and methanogens were established with six different substrates. Methanogenic cultures grew best on TMA and methanol, but could use formate, H2/CO2 and glycine betaine as well. In contrast, H2/CO2 was the preferred substrate of the SRB enrichment cultures, which were also able to use formate, but not TMA, the breakdown product of the compatible solute glycine betaine. Maximum methane production occurred at 5% salinity. The lipid composition of the mats, including methanogen biomarkers, and the results of the enrichments on different substrates and at different salinities, suggest that methanogenesis in these springs is supported by compatible solutes whereas sulphate reduction is linked to availability of hydrogen and formate.