Abstract
Extensive utilization of polybrominated diphenyl ethers (PBDEs) as flame retardants since the 1960s in a variety of commercial products has resulted in ubiquitous environmental distribution of commercial PBDE mixtures. Dangers posed to biological populations became apparent after the discovery of elevated levels of PBDEs in biota, most notably in human breast milk and tissues. Environmental persistence of PBDEs results in significant transboundary displacement, threatening fragile ecosystems globally. Despite efforts to curtail usage of PBDEs, public concern remains about the effects of legacy PBDEs contamination and continued discharge of PBDEs in regions lacking restrictions on usage and manufacture. Among available technologies for remediation of PBDEs such as ex-situ soil washing, electrokinetic degradation, and biodegradation, this review focuses on bioremediation by microbes under anaerobic conditions. Bioremediation is generally preferred as it is less disruptive to contaminated ecosystems, is cost-effective, and can be implemented at sites that may be inaccessible to more traditional ex-situ methods. The aims of this review are to (1) summarize current knowledge of anaerobic microbes that debrominate PBDEs and their associated synergistic partnerships with non-dehalogenating microbes; (2) explore current understandings of the metabolic reductive debromination of PBDE congeners; (3) discuss recent discoveries on dehalogenase genes involved in debromination of PBDEs.
Highlights
Polybrominated diphenyl ethers (PBDEs) have been widely used as flame retardant additives in a variety of manufactured products, from paints and plastics to textiles and televisions since 1960s
Though restrictions and bans on manufacture and usage of polybrominated diphenyl ethers (PBDEs) have been in place for several years, these legislations have no effect on the release of PBDEs from existing products or from recycled materials containing PBDEs
This review aims to summarize current knowledge of microbial reductive debromination of PBDEs under anaerobic conditions, the debromination pathways involved, and dehalogenase genes identified so far
Summary
Polybrominated diphenyl ethers (PBDEs) have been widely used as flame retardant additives in a variety of manufactured products, from paints and plastics to textiles and televisions since 1960s. PBDE formulations have been primarily manufactured as three different technical mixtures (Figure 1): penta-BDEs, octaBDEs, and deca-BDEs. BDE-209 has historically been the most widely used PBDE congener, BDE −47, −99, −100, and −153 are the most commonly observed PBDEs in the environment (Darnerud et al, 2001), indicating that environmental transformation of highly brominated BDEs is an important pathway contributing to the dispersal and environmental impact of PBDEs. The partial debromination of deca- and octa-BDEs is worrisome because the less brominated metabolites like tetra- and penta-BDEs are of higher toxicity (Palm et al, 2002). Anaerobic and anoxic sediments and soils are major sinks and environmental reservoirs for PBDEs, making anoxic debromination by microorganisms an important route for eliminating PBDEs in the environment. Investigation of PBDEs debrominating microbes and exploration of the underlying mechanisms of debromination will enable more effective tracking the fate of PBDEs in the environment
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