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.