Current Research Projects

Identification of Reductive Dehalogenase Genes That Encode Dechlorination of Specific PCB Congeners

Background

Polychlorinated biphenyls (PCBs) are widespread contaminants of freshwater and estuarine sediments worldwide. They are also potential human carcinogens and have been associated with adverse health effects in wildlife and humans. PCBs bioaccumulate in fatty tissues and biomagnify in the food chain. As a consequence, fishing and recreational activities in PCB-contaminated rivers, lakes, and harbors are restricted. Microbial dechlorination of PCBs occurs naturally in many sediments and could significantly reduce the persistence and toxicity of PCBs. However, PCB dechlorination in situ usually does not progress far enough to achieve these ends. This is the case in the Housatonic River. Previous studies by the PI and her collaborators have clearly demonstrated that PCB-dechlorinating microorganisms are present in Housatonic River sediment and can be activated, even in situ.

In 1992 we began the first successful field test to stimulate PCB dechlorination in situ. The test was conducted at ambient temperatures in the Woods Pond stretch of the Housatonic River in western Massachusetts. By activating the indigenous bacteria to attack PCBs in the sediment, we achieved a 62% decrease in PCBs with six or more chlorines in just 3 months and a 74% decrease in a year. This dechlorination greatly reduces the toxicity of PCBs and makes them less persistent because PCBs with fewer chlorines are more easily metabolized by a variety of organisms.

Three major patterns of PCB dechlorination, N, P, and LP, have been identified in these sediments. These patterns can be distinguished on the basis of congener selectivity, position of the chlorine removed, and temperature and pH range for the dechlorination.

Current research

Until very recently, none of the organisms responsible had been identified, mainly because of the difficulty in establishing stable sediment-free cultures that retained the full spectrum of PCB dechlorination activity observed in situ. Our laboratory has succeeded in developing and characterizing the first sediment-free cultures that carry out a major dechlorination process observed in nature. (See publications) Furthermore, we have now established that strict anaerobic bacteria of the group Dehalococcoides are responsible for Process N dechlorination. Dehalococcoides are a recently discovered group of bacteria with a very restrictive metabolism. They require a halogenated organic compound for growth and derive energy for growth by removing chlorine atoms and replacing them with hydrogen atoms, a process known as halorespiration.

Dehalococcoides are best known for their ability to dechlorinate chlorinated aliphatics, especially TCE, DCE, and vinyl chloride. We have now established that some Dehalococcoides can also dechlorinate a broad suite of PCB congeners, and it is these that we are studying in this laboratory. We are using the latest molecular techniques to identify the genes that encode the reductive dehalogenase enzymes that dechlorinate specific PCB congeners and families of congeners.

Long term goals: (1) Isolation of the PCB dechlorinating bacteria. (2) Acquisition of sufficient understanding of the ecology of PCB-dechlorinators to enable development of safe and economical ways to stimulate their activity (and hence PCB remediation) in situ. (3) Isolation and characterization of novel PCB dechlorinating enzymes.

Significance: This research will expand our understanding of important environmental processes and of anaerobic sediment communities. It is expected to lead to effective and environmentally compatible ways to detoxify and degrade PCBs in situ. It will also lead to the discovery of novel enzymes that may be important in the chemical industry.

Funding: This research is funded by NSF

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