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Three New Bacteria For PCB Decontamination

AsianScientist (Sep. 15, 2014) – Researchers have developed a novel approach that could greatly enhance the effectiveness of destroying toxic polychlorinated biphenyls (PCBs) in the environment. Their approach, using bacteria from the genus Dehalococcoides, has been published in the Proceedings of the National Academy of Sciences.

PCBs are synthetic organic chemical compounds of chlorine and biphenyl that have widely been used as coolant fluids in many electrical products. However, they are known to be toxic, causing cancer and neurological effects. Though PCBs have been banned since the 1970s, they are virtually indestructible and can possibly remain in the environment for a long time, continuing to contaminate rivers, lakes and harbors worldwide.

Currently, the only treatment is capping or dredging and landfilling the PCBs. An example is the ongoing SuperFund project to dredge the upper Hudson River to remove PCBs which has already cost nearly US$1 billion.

Applying microbes directly to break down (dechlorinate) PCBs on-site would be very effective for PCB bioremediation. However, these microbes are extremely hard to culture, thereby limiting efforts to characterize them for such applications.

There have only been seven known enzymes associated with Dehalococcoides found to have confirmed function on chlorinated compounds. Now, researchers from the National University of Singapore (NUS) in collaboration with scientists from the Genome Institute of Singapore (GIS) have added the three new bacteria to the list—each with distinct specificities.

Associate professor He Jianzhong, who is from the NUS Department of Civil & Environmental Engineering, explained, “While the scientific community has found out that certain bacteria can dechlorinate PCBs and make them more susceptible to oxidation, it was not until three decades ago that some were identified.”

“However, challenges still remain in growing these organisms in quantities that will make an impact. Their low biomass has also prevented us from studying closely the process, especially in identifying the enzymes responsible. Furthermore, as PCBs are extremely insoluble, they are unsuitable as substrates for culturing the helpful bacteria.”

To overcome this problem, the NUS team came out with an alternative substrate called Terrachloroethene (PCE) which can be used to boost the cell numbers of PCB dechlorinators.

Dr. Niranjan Nagarajan, who led the research at GIS, said, “Through synergy generated from traditional culture techniques combined with state of the art genomic technologies, we could successfully cultivate and characterize three PCB dechlorinating microbial strains. From these efforts, we were able to be the first to identify the functional genes responsible for breaking down PCBs. These genes could be very useful as biomarkers for monitoring PCB bioremediation.”

The research findings make bioaugmentation feasible, paving the way for in situbioremediation. Future research will be focused on application of genomic technologies for in situ degradation of PCBs and other halogenated compounds.

The article can be found at: Wang et al. (2014) Genomic Characterization of Three UniqueDehalococcoides that Respire on Persistent Polychlorinated Biphenyls.

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