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 so le source of carbon and energy specifically depletes certain CL-CS alkylben zenes in crude oil during growth. The enrichment culture grows on oils of d ifferent origin and composition resulting in similar patterns of alkylbenze ne 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-eth yltoluene while strain mXyS1 preferentially depletes In-xylene and m-ethylt oluene. Roth strains also utilize toluene. The degradative patterns of the pure cultures are complementary and their combination results in the degrad ative pattern of the enrichment culture. During growth of the enrichment cu lture and the pure strains on crude oil alkylated benzoic acids were the ma in metabolic products, which were isolated from the water phases of the inc ubation experiments. The patterns of alkylated benzoic acids produced by th e pure cultures are again complementary with respect to the pattern observe d for the enrichment culture. The spectrum of alkylated benzoic acids sugge sts that partial oxidation of alkylbenzenes, which do not support growth, t akes place resulting in the formation of dead-end metabolites. Alkylphenyls uccinic and fumaric acids were produced in trace amounts only. The portion of alkylbenzenes remaining in crude oil becomes enriched in C-13 during gro wth of bacteria. From the data obtained in this study it can be estimated t hat the carbon isotopic fractionation of the initial reaction of alkylbenze ne degradation by the present bacteria is between -26 and -33 parts per tho usand. We suggest that the variability in alkylbenzene concentrations and t heir carbon isotopic signature together with the occurrence of alkylated be nzoic acids may be used as a specific indicator of initial biodegradation o f crude oils and fossil fuel products by sulphate-reducing bacteria in vari ous environments. (C) 2000 Elsevier Science Ltd. All rights reserved.