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

Reductive Dechlorination of TCE by Chemical Model Systems in Comparison to Dehalogenating Bacteria: Insights from Dual Element Isotope Analysis (¹³C/¹²C, ³⁷Cl/³⁵Cl)

Chloroethenes like trichloroethene (TCE) are prevalent environmental contaminants, which may be degraded through reductive dechlorination. Chemical models such as cobalamine (vitamin B₁₂) and its simplified analogue cobaloxime have served to mimic microbial reductive dechlorination. To test whether in vitro and in vivo mechanisms agree, we combined carbon and chlorine isotope measurements of TCE. Degradation-associated enrichment factors ε_carbon and ε_chlorine (i.e., molecular-average isotope effects) were −12.2‰ ± 0.5‰ and −3.6‰ ± 0.1‰ with <i>Geobacter lovleyi</i> strain SZ; −9.1‰ ± 0.6‰ and −2.7‰ ± 0.6‰ with <i>Desulfitobacterium hafniense</i> Y51; −16.1‰ ± 0.9‰ and −4.0‰ ± 0.2‰ with the enzymatic cofactor cobalamin; −21.3‰ ± 0.5‰ and −3.5‰ ± 0.1‰ with cobaloxime. Dual element isotope slopes m = Δδ¹³C/ Δδ³⁷Cl ≈ ε_carbon/ε_chlorine of TCE showed strong agreement between biotransformations (3.4 to 3.8) and cobalamin (3.9), but differed markedly for cobaloxime (6.1). These results (i) suggest a similar biodegradation mechanism despite different microbial strains, (ii) indicate that transformation with isolated cobalamin resembles in vivo transformation and (iii) suggest a different mechanism with cobaloxime. This model reactant should therefore be used with caution. Our results demonstrate the power of two-dimensional isotope analyses to characterize and distinguish between reaction mechanisms in whole cell experiments and in vitro model systems