Diastereomer-specific transformation of hexabromocyclododecane by soil bacterial communities

Abstract

For decades, the brominated fire retardant hexabromocyclododecane (HBCD) has been extensively used to reduce fire risk. Due to the inherent properties associated with environmental and biological persistence of HBCD, the legacy environmental problems caused by HBCD require investigation. In particular, the stereoisomer-specific biochemical processes that occur during microbial transformation of HBCD remain unclear. In this study, biotransformation of HBCD isomers by soil bacterial communities was systematically investigated. After 30 days, the removal rate of technical HBCD was 57.0%, and the composition of diastereomers changed due to the preference for β-HBCD transformation. The removal rates of each isomer were in the order β-HBCD > γ-HBCD > α-HBCD. High-throughput Illumina sequencing of 16S rRNA genes revealed that soil bacterial communities responded to the stress of α-HBCD and β-HBCD by increasing Proteobacteria dominance, while they responded to the stress of γ-HBCD by enhancing species diversity. Moreover, Acinetobacter hemolyticum strain HW-2 was isolated and demonstrated the same isomer specificity, attributed to the differential expression of the four key genes (had1, had1, had1, dehH). The strain successfully colonized the soil bacterial communities during bioaugmentation and enhanced the HBCD removal rates. These findings reveal that the diastereomer-specific biotransformation process is expedited by bioaugmentation with strain HW-2, minimizing ecotoxicities caused by HBCD. These results improve our understanding of the HBCD biotransformation process and provide a theoretical basis for the control of HBCD in ecosystems.