Comprehensive exploration of the anaerobic biotransformation of polychlorinated biphenyls in Dehalococcoides mccartyi CG1: Kinetics, enantioselectivity, and isotope fractionation

Anaerobic microbial transformation is a key pathway in the natural attenuation of polychlorinated biphenyls (PCBs). Much less is known about the transformation behaviors induced by pure organohalide-respiring bacteria, especially kinetic isotope effects. Therefore, the kinetics, pathways, enantioselectivity, and carbon and chlorine isotope fractionation of PCBs transformation by Dehalococcoides mccartyi CG1 were comprehensively explored. The results indicated that the PCBs were mainly dechlorinated via removing their double-flanked meta-chlorine, with their first-order kinetic constants following the order of PCB132 > PCB174 > PCB85 > PCB183 > PCB138. However, PCBs occurred great loss of stoichiometric mass balance during microbial transformation, suggesting the generation of other non-dehalogenation products and/or stable intermediates. The preferential transformation of (-)-atropisomers and generation of (+)-atropisomers were observed during PCB132 and PCB174 biotransformation with the enantiomeric enrichment factors of -0.8609 +/- 0.1077 and -0.4503 +/- 0.1334 (first half incubation times)/-0.1888 +/- 0.1354 (second half incubation times), respectively, whereas no enantioselectivity occurred during PCB183 biotransformation. More importantly, although there was no carbon and chlorine isotope fractionation occurring for studied substrates, the delta C-13 values of dechlorination products, including PCB47 (-28.15 +/- 0.35 parts per thousand similar to -27.77 +/- 0.20 parts per thousand), PCB91 (-36.36 +/- 0.09 parts per thousand similar to -34.71 +/- 0.49 parts per thousand), and PCB149 (-28.08 +/- 0.26 parts per thousand similar to -26.83 +/- 0.10 parts per thousand), were all significantly different from those of their corresponding substrates (PCB85: -30.81 +/- 0.02 parts per thousand similar to -30.22 +/- 0.21 parts per thousand, PCB132: -33.57 +/- 0.15 parts per thousand similar to -33.13 +/- 0.14 parts per thousand, and PCB174: -26.30 +/- 0.09 parts per thousand similar to -26.01 +/- 0.07 parts per thousand), which further supported the generation of other non-dehalogenation products and/or stable intermediates with enrichment or depletion of C-13. These findings provide deeper insights into the anaerobic microbial transformation behaviors of PCBs.