Anaerobic phenanthrene mineralization by a carboxylating sulfate-reducing bacterial enrichment

Information on the susceptibility of higher molecular weight polynuclear aromatic hydrocarbons to anaerobic biodegradation is relatively rare. We obtained a sulfate-reducing bacterial enrichment capable of phenanthrene metabolism from a hydrocarbon-contaminated marine sediment. Phenanthrene degradation was in stoichiometric agreement with the theoretically expected amount of sulfate reduction and inhibited by molybdate. Mineralization of 14C-phenanthrene by the enrichment was confirmed by the recovery of the expected amount of 14CO2. Stable isotope studies with protonated or deuterated phenanthrene resulted in the detection of the correspondingly labeled phenanthrene carboxylic acid by gas chromatography–mass spectrometry. Comparison of the metabolite profile with a synthesized standard confirmed that the parent molecule was carboxylated at the C-2 position. Incorporation of 13C-bicarbonate into the carboxyl group implicated a direct carboxylation of phenanthrene as a likely key initial reaction. Denaturing gradient gel electrophoresis analysis of the enrichment showed only two major bands and 16S rRNA sequences obtained by cloning clustered with known hydrocarbon-degrading sulfate-reducing δ-proteobacteria, indicating their possible involvement in the anaerobic oxidation of phenanthrene via carboxylation as the initial activation reaction.