Isotope analysis as a natural reaction probe to determine mechanisms of biodegradation of 1,2-dichloroethane

1,2-Dichloroethane (1,2-DCA), a chlorinated aliphatic hydrocarbon, is a well-known groundwater contaminant. In this study, fractionation of stable carbon isotope values of 1,2-DCA during biodegradation was used as a novel reaction probe to provide information about the mechanism of 1,2-DCA biodegradation under both aerobic (O2-reducing) and anaerobic (NO3-reducing) conditions. Under O2-reducing conditions, an isotopic enrichment value (epsilon) of -25.8 +/- 1.1 per thousand (+/-95% confidence intervals) was measured for the enrichment culture. Under NO3-reducing conditions, an epsilon-value of -25.8 +/- 3.5 per thousand was measured. The microbial culture produced isotopic enrichment values (epsilon) that are not only large and reproducible, but also are the same whether O2 or NO3 was used as an electron acceptor. Combining data measured under both O2- and NO3-reducing conditions, an isotopic enrichment value (epsilon) of -25.8 +/- 1.6 per thousand is measured for the microbial culture during 1,2-DCA degradation. The epsilon-value can be converted into a kinetic isotope effect (KIE) value to relate the observed isotopic fractionation to the mechanism of degradation. This KIE value (1.05) is consistent with degradation via hydrolytic dehalogenation under both electron-accepting conditions. This study demonstrates the added value of compound-specific isotope analysis not only as a technique to verify the occurrence and extent of biodegradation in the field, but also as a natural reaction probe to provide insight into the enzymatic mechanism of contaminant degradation.