Insights into anaerobic biotransformation of Polychlorinated Biphenyls in Dehalococcoides mccartyi CG1 through Kinetic and Stable Isotopic Analysis

Abstract

Microbial degradation processes largely govern the fate of organic contaminants in the environment. Therefore, reliable evaluation of in situ biodegradation is essential for effective on-site contaminant management. Although compound-specific isotope analysis (CSIA) shows significant potential for assessing in situ attenuation and evaluating chemical and biodegradation mechanisms, empirical evidence supporting its application in the microbial degradation of polychlorinated biphenyls (PCBs) is still lacking. Microbial degradation of trace persistent organic pollutants is a multifaceted process influenced by various factors, with substrate concentration being a key factor affecting isotopic fractionation. Herein, to the best of our knowledge, for the first time, batch biodegradation experiments were conducted for analyzing the kinetics and carbon/chlorine isotope fractionation of chiral substrates (−)/(+)-PCB132 by Dehalococcoides mccartyi CG1 at varying substrate concentrations (0.3, 1.7, 2.4, 3.5, and 4.7 μM). The dechlorination of (−)/(+)-PCB132 was predominantly consistent with pseudo-first-order kinetics (k_obs) in most cases. However, when the ratio of substrate concentration to the density of functional microorganisms falls below a specific threshold (<5.3 × 10⁻³ μmol /(× 10¹⁰ CG1 cells)), a decline in observed k_obs is noted as degradation time increases, ultimately approaching the lower limit of bioavailability (k_obs = 0). Notably, substantial normal isotope fractionation was observed for the first time during the anaerobic degradation of (−)/(+)-PCB132, with the isotopic enrichment factor (Ɛ_C) varying from −1.27 ± 0.18‰ to −2.22 ± 0.01 for (−)/(+)-PCB132. Our findings indicate that, in addition to the effect of substrate concentration, the observed isotope fractionation of (−)/(+)-PCB132 was considerably affected by putative biodegradation activity. Enhanced activity within the anaerobic degradation system resulted in pronounced isotope masking. This study aims to contribute to a foundational understanding of bacterial reductive dehalogenation of PCBs at differing substrate concentrations while considering bioavailability.