Image: Stephen H. Zinder; Science
In the disconcerting world of hazardous waste problems, few compounds are as troublesome as chlorinated solvents. These dense toxics tend to sink below the water table and aggregate in pools that slowly dissolve in groundwater, gradually spreading the region of contamination. "Bad actors," as they are sometimes called by chemists (see sidebar), resist degradation by microorganisms--and pumping the sullied water to the surface for treatment is a thankless endeavor, because the chemicals in the soil continue to seep into the water table.
Now a research team at Cornell University has discovered a promising tool to aid in the cleanup effort: a bacterium that can neutralize two of the most common chlorinated solvents, rendering them harmless. The finding, published in the June 6, 1997, issue of the journal Science, gives scientists insight into the biology of dechlorinating organisms--knowledge that can be used to formulate strategies for the biological cleanup (or "bioremediation") of a wide variety of environmental pollutants.
Trichloroethene (TCE) and tetrachloroethene --better known as perchloroethylene, or PCE--are the most widespread of the troublesome chlorinated compounds. Both PCE and TCE are toxic and are suspected carcinogens. Yet the compounds are ubiquitous because of their usefulness in industrial processes. PCE is a staple of the dry cleaning industry; both chemicals are routinely used for degreasing machinery and are constituents in many paint thinners and in antifreeze.
Until recently, no bacterium was known to degrade PCE completely (although at least one had been identified that could break down TCE). But the Cornell group found a microbe known only as Strain 195 in a mixed culture of microorganisms from a now abandoned sewage plant in Ithaca, N.Y.
The researchers discovered something very useful about Strain 195: it is an anaerobic bacterium (meaning that it does not require oxygen for respiration) that actually "breathes" chlorinated solvents--extracting energy from the breakdown of the chemical for its natural metabolism. Enzymes within Strain 195 catalyze a four-step series of reactions in which chlorine atoms are removed from PCE. The severing of the atomic bonds between carbon and chlorine within the PCE molecule releases energy that the bacterium utilizes to sustain itself. PCE assumes a role for the microbe analogous to that of oxygen in for humans.
Image: Adriana Rovers/Cornell
Strain 195 is not the first microorganism discovered to remove chlorine from organic chemicals. Other microbes, however, usually do not remove all the unwanted atoms. Rather, they produce intermediate reaction products that can be as bad or worse than the original toxic. Taking one chlorine atom off PCE, for instance, yields TCE. Stripping two produces cis-dichloroethene, a suspected carcinogen. Excising the third yields vinyl chloride, a known carcinogen.
Only removal of all four of the chlorine atoms from PCE leaves a truly innocuous by-product: ethene (ethylene), a simple hydrocarbon. The bonded chlorine and carbon atoms react with hydrogen gas produced by fermented organic matter to form a carbon-hydrogen bond and hydrochloric acid, which then reacts with the abundant metal ions in the ground to form harmless salts. Strain 195, which may eventually be dubbed Dehalococcoides ethenogenes, is the first microbe that can take noxious PCE all the way down to its safe components.
Despite its obvious promise, Strain 195 is not about to find an immediate home in the hazardous waste sites around the world. Perry L. McCarty, a professor of environmental engineering at Stanford University (and author of an accompanying article in Science), explains that these microbes do not generally flourish in the soil. Strain 195 and other dechlorinators can thrive only if they have access to a source of molecular hydrogen, which provides the electrons needed to break the chlorine-carbon bonds in the solvent molecules. But in the field, ordinary microorganisms capture most of the hydrogen-supplied electrons for their own metabolic needs.
Continuing research by one member of the Cornell team, James M. Gossett, focuses on how to alter the soil conditions at contaminated sites to increase the amount of hydrogen available for Strain 195 and its ilk--and to determine the optimum nutrient mix that those microbes require. In the coming year, Gossett will participate in a joint Air Force, Navy and EPA study that will place potential hydrogen sources in the soil at Fallon Naval Air Station in Nevada to encourage the growth of dechlorinators. If that approach does not work, the group may try adding into the groundwater there a mixed culture that includes Strain 195 along with other partner bacteria.
Strain 195 also raises some intriguing scientific puzzles. The microbe appears unrelated to any of the other genera in Eubacteria (the phylogenetic domain that includes most of the known bacteria). The unusual diet of Strain 195 also has the Cornell scientists wondering how it evolved. "The big question with a bug like this is, What was it doing before people were dumping PCE into the environment?" says Stephen H. Zinder, the microbiologist who is a co-author on the paper.
One of the difficulties with Strain 195 is that it is difficult to culture, so the researchers might one day consider cloning its valued dechlorinating genes and inserting them into an organism that would be easier to grow. That work is for the future, however. For now, Zinder is just trying to come to a basic knowledge of these enigmatic microorganisms. "When we started in this area a few years ago we compared it to wine making before Pasteur," he says. "We're really trying to get this from an art to a science--understanding who the players [bacteria] are and what they like."