Research Brief 155Superfund Basic Research ProgramAssessing Bioremediation of Chloroethenes through Stable Carbon Isotope FractionationRelease Date: 11/07/2007 Background: Chloroethenes, such as tetrachloroethene (PCE), trichloroethene (TCE), isomers of dichloroethene (DCE), and vinyl chloride (VC), are among the most prevalent and problematic groundwater pollutants. These compounds pose serious health threats due to their toxicity and potential carcinogenicity. In situ bioremediation is often used at contaminated sites, but it is difficult to quantify the effectiveness of the biological processes because of changes that can result from physical processes, such as dissolution, volatilization, and sorption. Advances: Early fractionation studies performed with mixed bacterial cultures from field samples yielded highly variable isotopic fractionation effects. This suggests that different organisms utilizing different pathways to degrade chloroethene compounds may cause different isotopic shifts. Dr. Alvarez-Cohen's research group studied stable carbon isotope fractionation during the reduction of TCE using three test systems representing phylogenetically distinct organisms:
This is the first measurement of carbon isotope fractionation by Dehalococcoides isolates and of TCE transformation by S. multivorans and D. restrictus strain PER-K23. The extent of fractionation varied widely, yet within each culture constant enrichment factors were observed. Dechlorinating cultures exhibited a range of enrichment factors at each dechlorination step, and strains within the same genus or species generated significantly different enrichment factors. Additionally, isotope fractionation generated by a microbial community was quite different from that generated by isolates. These results offer an interesting comparison and insight into factors that affect biological fractionation. The large range of TCE enrichment factors observed, coupled with the reported similarities of the reductive dehalogenase enzymes (RDases), suggest that these biological fractionations are governed by a combination of the structure of the native enzyme and its cofactor, transport, and enzyme-substrate binding prior to the carbon-chlorine bond-breaking step. As a result of these findings, Dr. Alvarez-Cohen believes that general categorization of enrichment factors without specific measurements can result in misinterpretation and that caution should be exercised in selecting appropriate values for quantitative analysis to predict the extent of dechlorination. Significance: Compound-specific stable isotope analysis is a valuable tool for assessing in situ bioremediation at contaminated sites. However, as a result of wide variation in enrichment factors measured for both bacterial isolates and enriched communities, this study found that it is important to estimate site representative factors to optimize estimates of degradation rates. For More Information Contact: Lisa Alvarez-CohenCiv Engr/CEE Environmental 760 Davis Hall 94720-1710 Berkeley, CA 94720-1710 Tel: 510-643-5969 Email: To learn more about this research, please refer to the following sources:
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