A MOLECULAR ASSAY FOR EVALUATING PERFORMANCE OF NATURAL AND ENGINEERED BIOREMEDIATION OF CHLORINATED SOLVENTS

Chlorinated solvents have impacted groundwater supplies at thousands of properties throughout the world. Chemicals such as tetrachloroethene (PCE) and trichloroethene (TCE) are recalcitrant to aerobic biodegradation and can persist in groundwater for decades. Under certain anaerobic groundwater environments, however, PCE and TCE may be biodegraded via reductive dechlorination. Microbially-mediated reductive dechlorination of chlorinated ethenes is a sequential process in which chlorine atoms are replaced with hydrogen atoms and the carbon atom on the ethene molecule is reduced. For PCE, reductive dechlorination follows the order of PCE => TCE => cis-1,2-dichloroethene (c-DCE) => vinyl chloride (VC) => ethene (ETH).

A variety of bacteria are capable of dechlorinating PCE to cDCE, and data indicate that these bacteria are ubiquitous in the subsurface. Transformation of cDCE and VC to ETH is less common, however, and only bacteria related to the Dehalococcoides genus have been shown to have this capability. Dehalococcoides has been implicated as a critical prerequisite for complete dechlorination of PCE, TCE, cDCE, and VC to ETH.

For most of the Dehalococcoides strains, dechlorination of VC is the rate-limiting step the controls the overall rate of ethene generation from PCE and TCE. VC degradation by strain 195 is a relatively slow, cometabolic (non-growth) reaction that is mediated by a TCE-reductive dehalogenase enzyme. In contrast, strain VS is able to degrade VC rapidly in a growth-linked (catabolic) reaction that appears to be mediated by a VC reductase enzyme.

Increasingly, quantitative molecular assays are being applied to screen chlorinated solvent sites for the presence of Dehalococcoides. Such assays are used to forecast the potential of natural biodegradation, and to assess the need for bioaugmentation with specialized Dehalococcoides cultures. The current state-of-the-science is focused on application of genetic probes for detecting Dehalococcoides strains that produce VC reductase. In this paper we present a molecular tool for quantifying Dehalococcoides in groundwater samples. Data will be presented that illustrate the link between Dehalococcoides, VC reductase, and the extent of PCE and TCE dechlorination in aquifer settings.