Microbial communities associated with anaerobic benzene degradation in a petroleum-contaminated aquifer

Microbial community composition associated with benzene oxidation under in situ Fe(III)-reducing conditions in a petroleum-contaminated aquifer locate d in Bemidji, Minn., was investigated. Community structure associated with benzene degradation was compared to sediment communities that did not anaer obically oxidize benzene which were obtained from two adjacent Fe(III)-redu cing sites and from methanogenic and uncontaminated zones. Denaturing gradi ent gel electrophoresis of 16S rDNA sequences amplified with bacterial or G eobacteraceae-specific primers indicated significant differences in the com position of the microbial communities at the different sites. Most notable was a selective enrichment of microorganisms in the Geobacter cluster seen in the benzene-degrading sediments. This finding was in accordance with pho spholipid fatty acid analysis and most-probable-number-PCR enumeration, whi ch indicated that members of the family Geobacteraceae were more numerous i n these sediments. A benzene-oxidizing Pe(III)-reducing enrichment culture was established from benzene-degrading sediments and contained an organism closely related to the uncultivated Geobacter spp. This genus contains the only known organisms that can oxidize aromatic compounds with the reduction of Fe(III). Sequences closely related to the Fe(III) reducer Geothrix ferm entans and the aerobe Variovorax paradoxus were also amplified from the ben zene-degrading enrichment and were present in the benzene-degrading sedimen ts. However, neither G. fermentans nor V. paradoxus is known to oxidize aro matic compounds with the reduction of Fe(III), and there was no apparent en richment of these organisms in the benzene-degrading sediments. These resul ts suggest that Geobacter spp, play an important role in the anaerobic oxid ation of benzene in the Bemidji aquifer and that molecular community analys is may be a powerful tool for predicting a site's capacity for anaerobic be nzene degradation.