Anaerobic monoaromatic hydrocarbon degradation by a novel chlorate reducing organism, Dechloromonas aromatica strain RCB

Benzene is a highly toxic industrial compound and due to its carcinogenic effects, much attention has focused on microbial benzene biodegradation in the environment, especially in the absence of oxygen. In this study, a novel organism, Dechloromonas strain RCB was isolated from sediments collected from the Potomac River, MD, which completely mineralizes benzene to CO2 in the absence of O2, and is the first example of an organism of any description capable of anaerobic benzene degradation. Phylogenetic analysis identified strain RCB as a member of the Dechloromonas genus in the beta-subclass of the Proteobacteria. Detailed characterization studies on this novel organism were performed. Cells of strain RCB are facultatively anaerobic, motile, non-spore forming rods growing optimally at 30°C and at a pH of 7. Strain RCB utilizes perchlorate, chlorate, nitrate, manganese or oxygen as terminal electron acceptors, while degrading monoaromatics like benzene and toluene completely to CO2. Strain RCB is capable of growth with benzene as the sole source of carbon and energy. The metabolic pathway by which this organism oxidizes benzene anaerobically was also investigated. Nitrate-dependent benzene degradation by strain RCB involves an initial hydroxylation step forming phenol followed by subsequent carboxylation and loss of the hydroxyl group to form benzoate. Detailed studies focusing on the initial hydroxylation step in the pathway revealed that while oxygen and water played no role, hydroxyl free-radicals catalyze this reaction. Strain RCB is also capable of degrading other monoaromatic compounds including ethylbenzene and the meta-, para- and ortho-isomers of xylene with nitrate or chlorate as the terminal electron acceptor. The addition of Dechloromonas strain RCB to benzene contaminated soil stimulated 14C-benzene oxidation under nitrate reducing conditions. Finally, the applicability of this organism to bioremediative strategy for the treatment of benzene in hydrocarbon contaminated soil by bioaugmentation was investigated. Laboratory incubations with benzene contaminated soil and subsequent field trials with strain RCB and NO3 - resulted in the removal of 90% and 80% of benzene respectively relative to control treatments, thus demonstrating great potential in large scale applications of this process in the future.