Friederike Marie Luise Grundger

We investigated the stable carbon and hydrogen isotopic signatures of methane, CO₂ and water during microbial formation of methane from mineral oil-related compounds in order to determine the variability in the methane signatures. The isotopic discrimination for carbon and hydrogen between substrate and methane was calculated and resulted in ε C_{DIC - CH4} 26-60‰, ε C_{substrate - CH4} 16-33‰, ε H_{H2 O - CH4} 257-336‰ and ε H_{substrate - CH4} 174-318‰, respectively. These carbon and hydrogen isotope signatures fell in to a relatively narrow range, suggesting a coupling of fermentation with acetoclastic and CO₂ reducing methanogenesis. In order to characterize the microbial consortia involved in the methanogenic degradation of hexadecane, a methanogenic enrichment culture was incubated with 1-¹³C-hexadecane and its biomass was analyzed for the pattern and isotopic signature of carboxylic acids. The highest extent of labelling was detected in the n-C17 fatty acid with methyl groups at C-4, presumably indicative of Syntrophus sp. To determine if the isotope composition of methane can be used as an indicator for methanogenesis during growth with oil-related compounds in field studies, we analyzed the isotope composition of methane in a confined mineral oil contaminated aquifer. The variability in carbon and hydrogen isotope composition was almost identical to the values obtained from enrichment cultures, thereby providing a tool for screening for microbial methane formation during hydrocarbon exploration.