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Anaerobic dehalogenation of halogenated organic compounds: Novel strategies for bioremediation of contaminated sediments

Anaerobic dehalogenation of halogenated organic compounds: Novel strategies for bioremediation of contaminated sediments. Halogenated organic compounds constitute one of the largest groups of environmental chemicals and the development and production of new halogenated organic compounds, including aliphatic, aromatic and heterocyclic derivatives, has increased over the last century. Aquatic sediments are ultimate receptors of many halogenated contaminants, including polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), and most recently brominated flame retardants. Microbial degradation is one of the key factors that determine the ultimate fate of organohalides in the environment, with cleavage of the carbon-halogen bond being one of the critical steps. Microbial reductive dechlorination is an important environmental process, because it has the potential of decreasing the toxicity of PCDD/Fs if lateral chlorines are removed. Dechlorination may also be advantageous because lesser chlorinated congeners of PCDD/Fs are more susceptible to subsequent aerobic degradation. Our lab has a long-time interest in characterizing microbial dehalogenation of organochlorine and organobromine compounds in anoxic environments. Our overall objective are to determine the capacity for dehalogenation and degradation of organohalides by anaerobic microbial communities in anoxic sediments and how these processes can be harnessed for bioremediation of contaminated sites.

Results from micro- and mesocosm experiments using contaminated sediments (e.g., Anacostia River MD, Hackensack River NJ and Kymijoki River Finland) have revealed diverse communities of dehalogenating microorganisms. Although Dehalococcoides species are the most likely candidates for PCDD/F and PCB dechlorination, there are other Chloroflexi microorganisms that have been shown to be active in dechlorination. The addition of halogenated co-amendments might be one tool to enhance dechlorination of PCBs and PCDD/Fs in historically contaminated sediments. The enhanced dechlorination correlates with increased numbers of dehalorespirer populations and reductive dehalogenase genes, supporting our hypothesis that the halogenated co-substrates enhance dechlorination of historic pollutants by supporting growth and activity of indigenous dehalogenating bacteria. A combined bioaugmentation/biostimulation approach may thus be feasible for the bioremediation of sediments contaminated with PCBs and PCDD/Fs.

The increased industrial use of brominated flame retardants, which has doubled in less than a decade, has created growing concern over the health effects resulting from the environmental accumulation of these compounds. For instance, tetrabromobisphenol-A [TBBPA, 4,4'-isopropylidenebis(2,6-dibromophenol)] is one of the highest-volume brominated flame retardants and its increased use over the past two decades has created growing concern over the health effects resulting from the environmental accumulation. TBBPA has been detected in environmental samples worldwide and more recently even in human breast milk and tissues. TBBPA is persistent and is only slowly degraded in the environment. We have demonstrated that TBBPA is reductively dehalogenated under anaerobic conditions to yield bisphenol A and while TBBPA is not degraded aerobically it can be biotransformed to its corresponding mono- and dimethyl derivatives. These microbial transformations of TBBPA yield metabolites with different chemical properties than the parent compound including an increase in lipophilicity and potential for increased environmental accumulation of the TBBPA dimethyl ether. The biotransformation of TBBPA and its derivatives may prove to be important for health risk evaluation, as TBBPA induces a number of toxicological effects including cellular oxidative stress and neurotransmitter inhibition, while BPA is a suggested endocrine disruptor. The environmental and health effects of the methyl derivatives of tetrabromobisphenol A are completely unknown. When evaluating the human and environmental health effects of TBBPA it is thus crucial to also consider its different biotransformation products, e.g. bisphenol A and the mono- and dimethyl ether derivatives. The aims of our work are to determine the extent to which brominated flame retardants are biotransformed by microbes indigenous to sediments and soils and to assess the fate and ecotoxicity of brominated flame retardants in the ecosystem.

Seoyean Sohn: Dehalogenation of halogenated aromatic compounds by indigenous microorganisms in sediments of the Hackensack River, New Jersey

Hang Dam: Assessment of microbially mediated reductive dehalogenation of polychlorodibenzo–p–dioxins (PCDDs) in contaminated sites

Sanna Kuokka (University of Helsinki, Finland): Assessing the Potential for Anaerobic Microbial Dechlorination of PCDD/Fs in the Kymijoki River sediments

Collaborators: Donna Fennell, Anna–Lea Rantalainen

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