δ¹³C and δ³⁷Cl Isotope Fractionation to Characterize Aerobic vs Anaerobic Degradation of Trichloroethylene

Trichloroethylene (TCE) is a carcinogenic organic chemical impacting water resources worldwide. Its breakdown by reductive vs oxidative degradation involves different types of chemical bonds. Hence, if distinct isotope effects are reflected in dual element (carbon and chlorine) isotope values, such trends could help distinguishing both processes in the environment. This work explored dual element isotope trends associated with TCE oxidation by two pure bacterial cultures: Pseudomonas putida F1 and Methylosinus trichosporium OB3b, where the latter expresses either soluble methane-monooxygenase (sMMO) or particulate methane-monooxygenase (pMMO). Carbon and chlorine isotope enrichment factors of TCE (ϵ¹³C = -11.5, -2.4, and -4.2‰ ϵ³⁷Cl = 0.3, -1.3, and -2.4‰, respectively) differed strongly between the strains. The dual element isotope trend for strain F1 (ϵ¹³C/ϵ³⁷Cl = -38) reflected, as expected, primary carbon and negligible chlorine isotope effects, whereas unexpectedly large chlorine isotope effects became apparent in the trend obtained with strain OB3b (ϵ¹³C/ϵ³⁷Cl = +1.7 for sMMO and pMMO). Therefore, although dual element isotope analysis partly reflects predicted differences in oxidative vs reductive (ϵ¹³C/ϵ³⁷Cl = 3.4-5.7) degradation, the unexpected OB3b fractionation data may challenge field interpretation.