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

Abstract

This study investigated dual element isotope fractionation during aerobic biodegradation of 1,2-dichloroethane (1,2-DCA) via oxidative cleavage of a C-H bond (Pseudomonas sp. Strain DCA1) versus C-Cl bond cleavage by SN2 reaction (Xanthobacter autrophicus GJ10 and Ancylobacter aquaticus AD20). Compound-specific chlorine isotope analysis of 1,2-DCA was performed for the first time and isotope fractionation ε_bulk^Cl was determined by measurements of the same samples in three different laboratories using two GC-IRMS and one GC-quadrupole MS. Strongly pathway-dependent slopes (Δδ13C / Δδ37Cl), 0.78±0.03 (Oxidation) and 7.7±0.2 (SN2), delineate the potential of the dual isotope approach to identify 1,2-DCA degradation pathways in the field. In contrast to different ε_bulk^C values: -3.5±0.1¿ (Oxidation), -31.9±0.7¿ and -32.0±0.9¿ (SN2), the obtained ε_bulk^Cl values were surprisingly similar for the two pathways: -3.8±0.2¿ (Oxidation), -4.2±0.1¿ and -4.4±0.2¿ (SN2). Apparent kinetic isotope effects of 13C-AKIE=1.0070±0.0002 (Oxidation), 13C-AKIE=1.068±0.001 (SN2) and 37Cl-AKIE=1.0087±0.0002 (SN2) fell within expected ranges. In contrast, an unexpectedly large secondary 37Cl-AKIE of 1.0038±0.0002 reveal a hitherto unrecognized involvement of C-Cl bonds in microbial C-H bond oxidation. Our 2D isotope fractionation patterns enable for the first time reliable 1,2-DCA degradation pathway identification in the field, which unlocks the full potential of isotope applications for this important groundwater contaminant.