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

Chloroethenes like trichloroethene (TCE) are prevalent environmental contaminants, which may be degraded through reductive dechlorination. Chemical models such as cobalamine (vitamin B sub(12)) 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 epsilon sub(carbon) and epsilon sub(chlorine) (i.e., molecular-average isotope effects) were -12.2% plus or minus 0.5% and -3.6% plus or minus 0.1% with Geobacter lovleyi strain SZ; -9.1% plus or minus 0.6% and -2.7% plus or minus 0.6% with Desulfitobacterium hafniense Y51; -16.1% plus or minus 0.9% and -4.0% plus or minus 0.2% with the enzymatic cofactor cobalamin; -21.3% plus or minus 0.5% and -3.5% plus or minus 0.1% with cobaloxime. Dual element isotope slopes m = Delta delta super(13)C/ Delta delta super(37)Cl approximately epsilon sub(carbon)/ epsilon sub(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.