Anaerobic transformation of brominated aromatic compounds by Dehalococcoides mccartyi strain CBDB1

Abstract

Brominated flame retardants are widely used compounds for fire safety in our daily life. Many of them have been reported to be toxic to humans and identified as environmental contaminants. Dehalococcoides mccartyi strains are well known for their dependence on organohalide respiration as an energy conserving process and have therefore been intensively studied. So far, much less investigations were done on brominated than on chlorinated compounds with D. mccartyi strains. D. mccartyi strain CBDB1 uses a wide range of halogenated compounds as electron acceptors for organohalide respiration. Most of its electron acceptors e.g. chlorinated benzenes, dioxins and polychlorinated biphenyls have the basic chemical structure of aromatic compounds and are bigger molecules than simple halogenated acyclic hydrocarbons such as chlorinated ethenes. From this point, brominated flame retardants share structural similarity with halogenated compounds which were demonstrated to be dehalogenated by D. mccartyi strain CBDB1 before. Therefore, in this study, D. mccartyi strain CBDB1 was chosen as the model organism and incubated with several brominated organic compounds including brominated flame retardants to investigate reductive debromination. D. mccartyi strain CBDB1 completely dehalogenated brominated benzenes, tetrabromobisphenol A and bromophenol blue to the non-brominated forms as the final products. Such debromination processes revealed a further dehalogenation extent compared to reductive dechlorination catalyzed by the strain. Neither debromination activities nor cell growth were detected in the cultures of strain CBDB1 incubated with either decabromodiphenyl ether or hexabromocyclododecane. Growth yields of 2.4 × 1013 to 4.6 × 1013 cells mol-1 bromide released were obtained in the cultures of strain CBDB1 incubated with hexabromobenzene or 1,3,5-tribromobenzene as the electron acceptor. Reductive debromination of the two brominated phenols was achieved only when they were supplied at low concentration. Growth yields of 2.7 × 1014 to 3.6 × 1014 cells mol-1 bromide released were obtained in the cultures incubated with bromophenol blue but no cell growth was detected in cultures incubated with tetrabromobisphenol A. This suggests different extents of toxicity are caused by the two brominated phenols. Toxicity tests with bromophenol blue revealed that the debromination reaction and cell growth of strain CBDB1 were continuously delayed with the increase of initial concentrations of bromophenol blue. Resting cell activity assays analyzed by gas chromatography demonstrated that strain CBDB1 debrominated tetrabromobenzenes to tribromobenzenes. With a photometric activity assays, both cultures of strain CBDB1 grown with hexabromobenzene or 1,3,5-tribromobenzene showed higher specific activities on 1,2,4-tribromobenzene and 1,2-dibromobenzene but lower specific activities on 1,3,5-tribromobenzene and the other tested halogenated benzenes. Results of shotgun proteomics showed that the same dominant reductive dehalogenases were involved in the dehalogenation of brominated benzenes and chlorinated benzenes indicating that these reductive dehalogenases are not strictly substrate-specific. Additionally, several reductive dehalogenase homologous proteins were specifically induced by hexabromobenzene or oligocyclic brominated phenols in cultures of strain CBDB1. This suggests the molecular size and chemical properties of an electron acceptor can influence the expression of reductive dehalogenases. Compound specific isotope analysis revealed identical carbon isotope enrichment factors for 1,2-dibromobenzene and 1,3-dibromobenzene, but significant lower enrichment factors were determined for 1,2,4-tribromobenzene and 1,3,5-tribromobenzene. Identical carbon isotope enrichment factors were determined for live cultures and in vitro activity assay with the same electron acceptor indicating the isotope fractionation was not affected by the physiological status of the cells.