By Erica Goldman

Photo by Erica Goldman

Baltimore Harbor is an anaerobic microbiologist's dream. "The sediments are black and oily, almost like black Jell-o," says Kevin Sowers, a scientist at the University of Maryland Biotechnology Institute's Center of Marine Biotechnology.

Sowers studies anaerobes — bacteria in the sediment that thrive without oxygen. "Lots of people don't think about the anaerobes," he says. "People assume that there is oxygen in the world and that living things need it." In reality, Sowers says, probably half the microbes out there are anaerobes.

Sowers's particular passion: anaerobes that can break down polychlorinated biphenyls (PCBs), chlorinated compounds used heavily by the manufacturing industry. PCBs were banned in 1977 by the U.S. Environmental Protection Agency (EPA) because of their link to certain cancers, liver, and skin problems. Although similar in structure to the PCE and TCE studied by Jennifer Becker (see Of Microbes and Messes), PCBs do not linger in the groundwater. Instead they stick to sediment in rivers and streams and slowly work their way up the food chain — passing from worm to fish to bird — persisting for decades or more.

Tributaries with a history of industrial and shipping activities, such as the Patapsco and Anacostia Rivers, have contributed significantly to PCB contamination in the Chesapeake Bay. White perch from more than 20 of the estuary's river systems harbor PCBs in their body tissue, according to EPA's Chesapeake Bay Program, which monitors these fish as indicators of contaminant levels. In Baltimore's Patapsco River, levels of the compound in white perch exceed the zero meals-per-year advisory established by the state, which means that these fish should not be consumed at all.

After the EPA banned the use of PCBs, companies that discharged these compounds into waterways suddenly faced the prospect of cleaning them up. General Electric, which operated manufacturing plants on the Hudson River for roughly 30 years (1947-1977), bore responsibility for contaminating 200 miles of the Hudson River Estuary in New York � the removal of a dam had flushed 1.3 million cubic yards of PCB-laden sediment downstream. The company invested in research, spearheading an effort to prove that microbes would take care of the problem on their own.

But soon scientists realized that while the sediment seemed to contain some anaerobic bacteria that could break down PCBs, the process appeared to unfold very slowly and often not to an endpoint that completely detoxified the chemical. And although they had identified groups of bacteria (consortia) collectively responsible for degrading PCBs, they had not been able to catch the specific bugs in action — making the development of technologies for microbial bioremediation impossible. The EPA determined that dredging would be the only acceptable option for cleaning up PCBs. Today General Electric and the EPA are poised to begin a large-scale dredging operation in the Hudson River to the tune of 2.65 million cubic yards of sediment.

But "you can't dredge the entire Eastern coastline," says Sowers. "It will take a combination of technologies." Sowers has built his scientific career trying to understand anaerobes — especially those unique microbes that use chlorine atoms to breathe, don't need oxygen, and can take apart stubbornly recalcitrant PCBs. His persistence and patience have finally paid off.

To study PCB-breathing microbes that thrive in environments with no oxygen, UMBI microbiologist Kevin Sowers (top of page) uses an apparatus called a glove box (left), which creates an oxygen-free workspace. White perch (above) serve as biological indicators of PCB contamination in the Chesapeake Bay food chain. Drawing by Diane Rome Peebles. Photo by Erica Goldman

Sowers's preliminary efforts in Baltimore Harbor demonstrated that conventional microbiological techniques would not be sufficient to identify the specific microbe that could degrade PCBs. He suspected that part of the difficulty stemmed from the fact that other bacteria outnumbered the PCB-breathers (dehalogenators) in his samples.

Sowers used molecular probes to detect the bacteria's presence in his sediment samples, in collaboration with colleague Harold May, a microbiologist at the Medical University of South Carolina in Charleston who worked extensively in the Hudson River system. At first they found that the PCB-breather seemed to depend on the sediment to thrive. When Sowers and May removed the sediment, the microbe's activity would die out. After time, however, they could again measure microbial activity with molecular techniques. As the bacteria began to adapt to life without sediment, their activity grew stronger and stronger.

Nine months later, in 1996, Sowers and May finally isolated chlorine-breathing bacteria that did not require sediment. They called this bacterial strain DF-1 (Double-Flanked-1) for its ability to break down PCBs that have chlorines flanked by other chlorine atoms. This bug proved so small that even a fluorescent microscope could not detect it.

Now Sowers and his colleagues are beginning to find out what makes DF-1 tick. They know that DF-1 associates closely with the microbes in the genus Vibrio, bacteria that are very common in the marine environment. Understanding the nature of this association with Vibrio may prove critical. Sowers suspects that Vibrio may provide either nutrients or electrons to DF-1, although the basis of the association is not obvious. Sowers is now working to figure out what Vibrio supply to DF-1, information that will likely prove critical in putting DF-1 to work cleaning up PCBs (for more information about microbial associations, see Mutual Arrangements).

Technologically speaking, the potential to use microbes such as DF-1 to clean up PCBs is still a long way off. But with the identity of the bug in hand, Sowers and his colleagues can now craft a specific molecular probe to detect its presence in the environment. They can begin to ask the important questions. Where are these bacteria naturally present? Do they aggregate only in PCB-contaminated areas or can they thrive elsewhere? How can dehalogenation, the process by which the bacteria strip chlorine atoms off PCBs, be enhanced? One of the next steps will be to figure out what physical factors in the environment affect the ability of this newly isolated microbe (DF-1) to degrade PCBs.

PCBs tend to attach tenaciously to activated carbon, a dry, granular substance that can remove organic substances from water. If one could till large amounts of activated carbon into PCB-contaminated sediments, the PCBs would be effectively locked up and blocked from making their way up the food chain. But how long will the PCBs remain bound to the carbon? And will DF-1 still be able to degrade the contaminant if it is bound? Sowers is working to answer these questions, in collaboration with Upal Ghosh, an environmental engineer at the University of Maryland Baltimore County, and chemist Joel Baker at the Chesapeake Biological Laboratory, part of the University of Maryland Center for Environmental Science. This research, funded by the Department of Defense, offers one promising remediation strategy for PCBs in the marine environment.

For now, dredging may remain the only remediation option for PCBs accepted by the EPA. But Sowers and his colleagues are working hard to change the status quo.

This page was last modified July 07, 2006

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