Researchers have isolated a novel bacterium that flourishes as it destroys harmful chlorinated compounds in polluted environments, leaving behind environmentally benign end products.
The finding opens the door for designing more efficient and successful bioremediation strategies for thousands of contaminated sites that remain threats, despite years of cleanup work.
‘This organism might be useful for cleaning contaminated subsurface environments and restoring drinking-water reservoirs,’ Georgia Institute of Technology researchers report in the current issue of Nature.
Scientists and engineers have struggled for years with clean up of groundwater and subsurface environments contaminated decades ago by unregulated use of the common solvents tetrachloroethene (PCE) and trichloroethene (TCE). These toxic compounds are primarily used in dry cleaning operations and degreasing of metal components. Complicating the situation are natural biotic and abiotic processes that transform these solvents to intermediate substances, such as toxic dichloroethenes.
But in a step toward engineering better bioremediation strategies, Georgia Tech researchers have isolated a naturally occurring bacterium, designated Dehalococcoides strain BAV1, in a pure culture without other microbial species present in the sample. Though some progress was made in the past decade in understanding the bacteria involved in partial degradation of PCE and TCE, this study represents a significant advance, researchers said.
‘Isolating this bacterium will allow us to study the organism and the dechlorination process in more detail,’ said lead researcher Frank Loeffler, an assistant professor in the School of Civil and Environmental Engineering. ‘We can get a lot more information that we can then use to engineer systems in the environment so PCE and TCE degradation would not stop at the toxic intermediate stage, but rather would continue to be dechlorinated to a non-toxic end product, such as ethene.’
One site that appears likely to benefit from in-place bioremediation with this bacterium is the Bachman Road residential area contaminated with PCE by a former dry cleaning operation in Oscoda, Michigan. There, researchers recently used BAV1 in a successful pilot demonstration. At this contaminated site, PCE penetrated the water table and contaminated drinking-water wells in the area. The contaminants also migrated through the groundwater into nearby Lake Huron.
In 14-feet by 16-feet, 20-feet-deep test plots at the Bachman Road site, researchers compared a non-treated control section to two bioremediation approaches using BAV1, which already occurs at this site in low numbers. One strategy, called biostimulation, added lactate and nutrients to the contaminated plot. In another section, researchers injected a mixed culture containing high numbers of BAV1 along with nutrients in a strategy called bioaugmentation. This technique resulted in complete dechlorination of PCE to ethene within six weeks. Biostimulation, on the other hand, worked but took more time to accomplish detoxification.
‘Bioaugmentation had a relatively poor reputation,’ Loeffler said. ‘In cases targeting petroleum candidates, it didn’t help any more than less expensive strategies. Now, we have a good example of bioaugmentation at work.
‘It is a viable option, especially at sites with this type (chlorinated solvents) of contamination. So there’s a lot of excitement about this. People have spent a lot of money to clean up those sites without success. Now there’s new hope.’
Both laboratory and field work revealed that the growth of BAV1 depends strictly on the reduction of chlorinated compounds such as dichloroethenes and vinyl chloride to ethene and the presence of hydrogen as an electron donor. Also, genetic analyses, analytical chemistry techniques and high-resolution scanning electron microscopy yielded information about the organism’s appearance, makeup and behaviour. One peculiarity is filament-like appendages extending from BAV1 cells. Loeffler speculates that these appendages may allow the organisms to colonise contaminated subsurface environments.
Also, phylogenetic analysis revealed that BAV1 belongs to the only-recently discovered Dehalococcoides group, the only organisms documented to convert all chlorinated ethenes, including PCE, TCE, cis-1, 2-dichloroethene and vinyl chloride, to non-toxic ethene. The findings highlight the largely untapped reservoir of bacterial diversity, Loeffler added.
BAV1’s origin is unknown, though Loeffler believes it evolved long ago, deriving energy from naturally occurring chlorinated compounds, including chlorinated solvents, in the environment. He suggested that BAV1 in some natural areas survives by eating low concentrations of chlorinated compounds formed from volcanic, biologic and possibly ultraviolet light processes. Other scientists assert that BAV1 occurred in the environment long ago, but only developed its chlorinated compound-based metabolism in response to PCE and TCE pollution.
Georgia Tech researchers will continue to learn more about BAV1 as they conduct larger-scale studies, in which Loeffler admits researchers will have a more difficult job in monitoring the dechlorination process. Also, that process may not happen as quickly as it did in the smaller, pilot demonstration. In addition to future studies at the Michigan site, Loeffler is proposing similar research to the US Department of Defense at a TCE-contaminated military installation near Atlanta. There, he expects bioremediation to be complicated by a fractured rock layer beneath the water table.
In addition to opening the doors for further research on BAV1, the Georgia Tech study yielded a molecular technique that likely will be useful to scientists and engineers conducting similar research. The technique allows researchers to quantify the number of BAV1 organisms present at a contaminated site. An increasing number of organisms indicates a positive response to bioremediation efforts. This technique uses a real-time polymerase chain reaction (PCR) device to test field sites.
‘Organisms like BAV1 have an enormous potential to help detoxify chlorinated pollutants. But we’re just at the beginning of understanding their function, distribution and ecology in the environment,’ Loeffler concluded.