Triple-element compound-specific stable isotope analysis of 1,2-dichloroethane for characterization of the underlying dehalogenation reaction in two Dehalococcoides mccartyi strains

Abstract

Chlorinated ethanes belong to the most common groundwater and soil contaminants. Of these, 1,2-dichloroethane (1,2-DCA) is a man-made, persistent and toxic contaminant, released due to improper waste treatment at versatile production sites. This study investigated the anaerobic transformation of 1,2-DCA by Dehalococcoides mccartyi strain 195 and strain BTF08 using triple-element compound-specific stable isotope analysis of carbon, chlorine and hydrogen for the first time. Isotope fractionation patterns for carbon (εCBTF08 = -28.4 ± 3.7‰; εC195 = -30.9 ± 3.6‰) and chlorine (εClBTF08 = -4.6 ± 0.7‰; εCl195 = -4.2 ± 0.5‰) within both investigated D. mccartyi strains, as well as the dual-element analysis (ΛBTF08 = 6.9 ± 1.2; Λ195 = 7.1 ± 0.2), supported identical reaction mechanisms for dehalogenation of 1,2-DCA. Hydrogen isotope fractionation analysis revealed dihaloelimination as prevalent reaction mechanism. Vinyl chloride as major intermediate could be excluded by performing the experiment in deuterated aqueous media. Furthermore, evaluation of the derived apparent kinetic isotope effects (AKIECBTF08 = 1.029/AKIEC195 = 1.031; AKIEClBTF08 = 1.005/AKIECl195 = 1.004) pointed towards simultaneous abstraction of both involved chlorine-substituents in a concerted matter. It was shown that D. mccartyi strain BTF08 and strain 195 are capable of complete, direct dihaloelimination of 1,2-DCA to ethene.

Dual Element (C/Cl) Isotope Analysis Indicates Distinct Mechanisms of Reductive Dehalogenation of Chlorinated Ethenes and Dichloroethane in Dehalococcoides mccartyi Strain BTF08 With Defined Reductive Dehalogenase Inventories

“…Dehalococcoides mccartyi strain BTF08 was pre-cultivated for at least 4 transfers in 100 mL mineral medium (final pH 6.8) in 240 mL serum bottles and in total 1.2 mM PCE, cis -DCE, 1,2-DCA or VC as electron acceptor prior to transfer 0 ( Supplementary Figure S1A ) as described elsewhere ( Schmidt et al, 2014 ; Franke et al, 2017 ). Chlorinated solvents were added stepwise with initially approximately 200 μM, and subsequently 400 and 600 μM, added after the previous dose was converted.…”

Isotopic Characterization (2H, 13C, 37Cl, 81Br) of Abiotic Degradation of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies

“…Aerobic microbial degradation of 1,2dichloroethane, for instance, caused an ε Cl of at least −3.8 mUr 64 which was similar to the ε Cl reported for anaerobic microbial degradation (−4.2 mUr). 26 In contrast, CH 3 X in the troposphere mainly degrades via OH and Cl radical reactions 1 which cause a C−H bond dissociation. Consequently, reported hydrogen isotope effects for CH 3 Cl were large 28 (>−264 to −280 mUr) and carbon isotope effects were moderate (−10.2 to −11.2 mUr).…”

Isotopic Characterization (2H, 13C, 37Cl, 81Br) of the Abiotic Sinks of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies.

“…Large carbon isotope enrichment factors were reported for this biotic sink but rather small secondary hydrogen isotope enrichment factors due cleavage of the C-X bond (Nadalig et al, 2013). Halogen isotope effects for aerobic microbial degradation has not been measured yet but it is conceivable that they show a similarly large isotope fractionation as reported for anaerobic microorganisms (Franke et al, 2017) due to cleavage of the same bond. In contrast, reactions in the gas phase show very different isotope effects.…”

“…Despite these achievements, stable isotope measurements have not yet 5 contributed to better budget estimates of CH3Cl and CH3Br because the ranges for isotopic source signatures are often overlapping and coincide with the tropospheric δ 13 C of CH3X. Significant improvement may be expected if isotopic values of several elements are measured simultaneously for one compound and this technique has been successfully applied to describe the fate of groundwater contaminants (Kuder et al, 2013;Franke et al, 2017). For methyl chloride and methyl bromide no multi-element isotopic studies are available yet.…”