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Bacterium Could Treat PCBs Without the Need for Dredging

Researchers at Rensselaer Polytechnic Institute have discovered a tiny bacterium that could one day transform the way we remove polychlorinated biphenyls (PCBs) from our environment//. The organism could be the key to developing methods that help detoxify commercial PCB compounds on site — without the need for dredging.

The results will appear in the April 15 issue of Applied and Environmental Microbiology.

Commercial PCBs, which were banned from production in the United States in 1977, were once commonly used by industry. The compounds are mixtures of 70-90 different molecular forms that vary in the number and positions of chlorine atoms, making them difficult to degrade. To date, the most commonly used method to remove PCBs from the environment is to dredge and then deposit the sediments in a landfill.

In order to detoxify PCBs the strong bonds between the chlorine atoms and the biphenyl compounds that make up the PCB atomic structure need to be broken, a process known as dechlorination. More than two decades ago, scientists discovered that PCBs were slowly being dechlorinated by naturally occurring microbes, but despite years of research, the exact microbes responsible have remained elusive — until now.

"For the first time we have been able to cultivate in defined media naturally occurring bacteria that can extensively dechlorinate PCBs right at the site of the contamination," said Donna Bedard, professor of biology at Rensselaer and lead author of the paper. "This is a major step toward the development of cost-effective methods for on-site PCB remediation."

Bedard used sediments from the Housatonic River in Massachusetts — an area known to be contaminated with PCBs — to develop sediment-free cultures and to identify the bacteria that were breaking down the PCBs. Using molecular techniques, the research team determined that the microbes that are dechlorinating the PCBs belong to a group of bacteria known as Dehalococcoides (Dhc).

Dhc are "strict anaerobic" bacteria, which means they cannot survive in the presence of oxygen. They are frequently involved in natural remediation of chlorinated solvents such as trichloroethylene (TCE), but this is the first time it has been demonstrated that Dhc can dechlorinate complex commercial PCB mixtures.

After identifying the Dhc bacteria, Bedard and her team proved that the anaerobic bacteria thrive on the PCBs, much as humans thrive on oxygen. The microbes replace the chlorines on the PCBs with hydrogen, which fuels their growth and begins the PCB degradation process.

The discovery of the Dhc bacteria’s unique abilities could one day alter the way we treat PCB contaminated water bodies, according to Bedard.

"Now that we have identified the PCB-dechlorinating bacteria and learned how to cultivate them in the laboratory, we can begin to understand the processes that they use to dechlorinate PCBs and tap their unique abilities to create new technologies that efficiently and safely remove commercial PCBs from our environment," she said.


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