Anaerobic degradation and carbon isotopic fractionation of alkylbenzenes in crude oil by sulphate-reducing bacteria

A mesophilic enrichment culture of sulphate-reducing bacteria isolated from the water phase of a North Sea oil tank using oil from the same tank as sole source of carbon and energy specifically depletes certain CL-CS alkylbenzenes in crude oil during growth. The enrichment culture grows on oils of different origin and composition resulting in similar patterns of alkylbenzene depletion. Two pure cultures of sulphate-reducing bacteria, strain oXyS1 and mXyS1 which were isolated on o-xylene and m-xylene, respectively, also grow on crude oil. Strain oXyS1 preferentially depletes o-xylene and o-ethyltoluene while strain mXyS1 preferentially depletes In-xylene and m-ethyltoluene. Roth strains also utilize toluene. The degradative patterns of the pure cultures are complementary and their combination results in the degradative pattern of the enrichment culture. During growth of the enrichment culture and the pure strains on crude oil alkylated benzoic acids were the main metabolic products, which were isolated from the water phases of the incubation experiments. The patterns of alkylated benzoic acids produced by the pure cultures are again complementary with respect to the pattern observed for the enrichment culture. The spectrum of alkylated benzoic acids suggests that partial oxidation of alkylbenzenes, which do not support growth, takes place resulting in the formation of dead-end metabolites. Alkylphenylsuccinic and fumaric acids were produced in trace amounts only. The portion of alkylbenzenes remaining in crude oil becomes enriched in C-13 during growth of bacteria. From the data obtained in this study it can be estimated that the carbon isotopic fractionation of the initial reaction of alkylbenzene degradation by the present bacteria is between -26 and -33 parts per thousand. We suggest that the variability in alkylbenzene concentrations and their carbon isotopic signature together with the occurrence of alkylated benzoic acids may be used as a specific indicator of initial biodegradation of crude oils and fossil fuel products by sulphate-reducing bacteria in various environments. (C) 2000 Elsevier Science Ltd. All rights reserved.