Investigation of anaerobic biodegradation of phenanthrene by a sulfate-dependent Geobacter sulfurreducens strain PheS2.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous and harmful contaminants, which can be degraded aerobically. However, the persistence of PAHs in anoxic environments indicates that anaerobic biodegradation of PAHs should also be investigated. Pure-culture and biotransformation processes for anaerobic phenanthrene biodegradation with sulfate as a terminal electron acceptor remains in its infancy. In this study, we investigated anaerobic biodegradation of PAHs by PheS2, an isolated phenanthrene-utilizing sulfate-reducer, using phenanthrene as a model compound. PheS2 was phylogenetically closely related to Geobacter sulfurreducens and reduced sulfate to sulfide during anaerobic phenanthrene biodegradation. Phenanthrene biodegradation processes were detected using gas chromatography-mass spectrometry, genome, and reverse transcription quantitative PCR analyses. Carboxylation was the initial step of anaerobic phenanthrene biodegradation based upon detection of 2- and 4-phenanthroic acid, its isotopically labeled analogs when using C-labeled bicarbonate and fully deuterated-phenanthrene (C14D10), and genes encoding enzymes putatively involved in the biodegradation. Further, ring-system reducing and cleavage occurred, and substituted benzene series and cyclohexane derivatives were detected in downstream biotransformation metabolites. Additionally, PheS2 can degrade benzene, naphthalene, anthracene, and benz[a]anthracene, but not pyrene and benz[a]pyrene. This study describes the isolation of an anaerobic phenanthrene-degrading sulfate-reducer, the first pure-culture evidence of phenanthrene biotransformation processes with sulfate as an electron acceptor.