Assessing Aerobic Biotransformation of Hexachlorocyclohexane Isomers by Compound-Specific Isotope Analysis

• Published in: Environmental science & technology Vol. 53; no. 13; pp. 7419 - 7431
• Main Authors: Schilling, Iris E, Bopp, Charlotte E, Lal, Rup, Kohler, Hans-Peter E, Hofstetter, Thomas B
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 02.07.2019
• Subjects: Aerobic biodegradation; Biodegradation; Biotransformation; Chemical reactions; Contaminants; Contamination; Dechlorination; Dehydrochlorination; Enzymes; Fractionation; gene expression; HCH (pesticide); Hexachlorocyclohexane; Isomers; Isotope composition; Isotope fractionation; Isotopes; Lewis bases; Sediments; Soil analysis; Soil contamination; Substrates
• ISSN: 0013-936X; 1520-5851; 1520-5851
• DOI: 10.1021/acs.est.9b01007

Contamination of soils and sediments with the highly persistent hexachlorocyclohexanes (HCHs) continues to be a threat for humans and the environment. Despite the existence of bacteria capable of biodegradation and cometabolic transformation of HCH isomers, such processes occur over time scales of decades and are thus challenging to assess. Here, we explored the use of compound-specific isotope analysis (CSIA) to track the aerobic biodegradation and biotransformation pathways of the most prominent isomers, namely, (−)-α-, (+)-α-, β-, γ-, and δ-HCH, through changes of their C and H isotope composition in assays of LinA2 and LinB enzymes. Dehydrochlorination of (+)-α-, γ-, and δ-HCH catalyzed by LinA2 was subject to substantial C and H isotope fraction with apparent 13C- and 2H-kinetic isotope effects (AKIEs) of up to 1.029 ± 0.001 and 6.7 ± 2.9, respectively, which are indicative of bimolecular eliminations. Hydrolytic dechlorination of δ-HCH by LinB exhibited even larger C but substantially smaller H isotope fractionation with 13C- and 2H-AKIEs of 1.073 ± 0.006 and 1.41 ± 0.04, respectively, which are typical for nucleophilic substitutions. The systematic evaluation of isomer-specific phenomena showed that, in addition to contaminant uptake limitations, diffusion-limited turnover ((−)-α-HCH), substrate dissolution (β-HCH), and potentially competing reactions catalyzed by constitutively expressed enzymes might bias the assessment of HCH biodegradation by CSIA at contaminated sites.