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

Abstract

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.

C and Cl Isotope Fractionation of 1,2-Dichloroethane Displays Unique δ13C/δ37Cl Patterns for Pathway Identification and Reveals Surprising C–Cl Bond Involvement in Microbial Oxidation

“…This conclusion is in agreement with the Streitwieser limit for 13 C-KIE in C−Cl bonds (1.057) 44 and could explain in part the relatively narrow range of reported ε bulk C values (from −28.7 to −33.0‰) for both pure cultures using the haloalkane hydrolytic dehalogenase reaction ( Table 2). Hirschorn et al 9 measured a similar 13 C-AKIE (1.05) for 1,2-DCA in laboratory biodegradation experiments under nitrate reducing conditions by an enrichment culture from a contaminated site, which was interpreted as transformation via hydrolytic dehalogenation.…”

Proteomic and Transcriptomic Analyses Reveal Genes Upregulated bycis-Dichloroethene inPolaromonassp. Strain JS666

“…Stable isotope fractionation (i.e., a change in the 13 C/ 12 C ratio) during biodegradation occurs due to differences in the reaction rates and activation energies of heavy and light atoms present at a reacting bond, with lightisotope bonds in general reacting more quickly (16,46). The degree of fractionation depends strongly on the type of bond being cleaved and, therefore, the mechanism of the initial step in degradation (5,11,24,25). During biodegradation of many chlorinated ethenes and chlorinated ethanes, it has been found that the preferential breakage of the bond containing the lighter isotope (resulting in enrichment of the heavy isotope in the remaining substrate) can be described by the Rayleigh equation (4,25,27,53,57):…”

Stable isotope fractionation to investigate natural transformation mechanisms of organic contaminants: principles, prospects and limitations

“…14,25 The analysis was confirmed by carbon isotope fractionation measurements with pure strains of known transformation mechanisms, 72 and was subsequently used to probe for the mechanism under nitrate-reducing conditions. 73 Through a careful analysis in terms of position-specific isotope effects, the binomial distribution of enrichment factors could therefore be linked to different underlying transformation mechanisms! Almost simultaneously, the same approach made it possible to infer for the first time the transformation mechanism of anaerobic methyl tert-butyl ether (MTBE) degradation.…”