Trichloromethane dechlorination by a novel Dehalobacter sp. strain 8M reveals a third contrasting C and Cl isotope fractionation pattern within this genus

Ernest Marco-Urrea,Camelia Algora,Pilar Cortés,Clara Torrentó,Mònica Rosell,Teresa Vicent,Albert Soler,Kenneth Wasmund,Jesica M. Soder-Walz

doi:10.1016/j.scitotenv.2021.152659

Abstract

Trichloromethane (TCM) is a pollutant frequently detected in contaminated aquifers, and only four bacterial strains are known to respire it. Here, we obtained a novel Dehalobacter strain capable of transforming TCM to dichloromethane, which was denominated Dehalobacter sp. strain 8M. Besides TCM, strain 8M also completely transformed 1,1,2-trichloroethane to vinyl chloride and 1,2-dichloroethane. Quantitative PCR analysis for the 16S rRNA genes confirmed growth of Dehalobacter with TCM and 1,1,2-trichloroethane as electron acceptors. Carbon and chlorine isotope fractionation during TCM transformation was studied in cultured cells and in enzymatic assays with cell suspensions and crude protein extracts. TCM transformation in the three studied systems resulted in small but significant carbon (εC = −2.7 ± 0.1‰ for respiring cells, −3.1 ± 0.1‰ for cell suspensions, and − 4.1 ± 0.5‰ for crude protein extracts) and chlorine (εCl = −0.9 ± 0.1‰, −1.1 ± 0.1‰, and − 1.2 ± 0.2‰, respectively) isotope fractionation. A characteristic and consistent dual CCl isotope fractionation pattern was observed for the three systems (combined ΛC/Cl = 2.8 ± 0.3). This ΛC/Cl differed significantly from previously reported values for anaerobic dechlorination of TCM by the corrinoid cofactor vitamin B12 and other Dehalobacter strains. These findings widen our knowledge on the existence of different enzyme binding mechanisms underlying TCM-dechlorination within the genus Dehalobacter and demonstrates that dual isotope analysis could be a feasible tool to differentiate TCM degraders at field studies.

Highlights

Trichloromethane (TCM) is a pollutant frequently detected in contaminated aquifers, and only four bacterial strains are known to respire it
Three main biodegradation pathways have been reported for TCM: i) cometabolic reductive dechlorination, ii) hydrolysis followed by oxidation to CO2, and iii) organohalide respiration (Cappelletti et al, 2012)
Carbon isotope analyses were performed by headspace solid phase microextraction (HS-SPME) coupled to gas chromatography isotope ratio mass spectrometry (GC-IRMS), as explained elsewhere (Blázquez-Pallí et al, 2019) and detailed in the Supplementary Information (SMM4)

Summary

Introduction

Trichloromethane (TCM) is a pollutant frequently detected in contaminated aquifers, and only four bacterial strains are known to respire it. A characteristic and consistent dual CCl isotope fractionation pattern was observed for the three systems (combined ΛC/Cl = 2.8 ± 0.3) This ΛC/Cl differed significantly from previously reported values for anaerobic dechlorination of TCM by the corrinoid cofactor vitamin B12 and other Dehalobacter strains. The principle of CSIA is tied to measuring the underlying kinetic isotope effect (KIE) occurring during a transformation process whereby, in most cases, chemical bonds containing light isotopes (e.g., 12C, 35Cl) are preferentially cleaved compared with those containing one or more heavy isotopes (e.g., 13C, 37Cl) and, a relative enrichment of the remaining contaminant mass in molecules containing the heavier isotopes is expected (Elsner et al, 2005) These compound specific isotope effects can be evaluated according to a simplified version of the Rayleigh equation Eq (1): ln Rt 1⁄4 ε∙ ln ð f Þ (1).

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