Isotopic fingerprinting of methane and CO2 formation from aliphatic and aromatic hydrocarbons

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 εCDIC-CH4 26-60‰, εCsubstrate-CH4 16-33‰, εHH2O-CH4 257-336‰ and εHsubstrate-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.