Mechanistic Dichotomy in Bacterial Trichloroethene Dechlorination Revealed by Carbon and Chlorine Isotope Effects

Tetrachloroethene (PCE) and trichloroethene (TCE) are significant groundwater contaminants. Microbial reductive dehalogenation at contaminated sites can produce nontoxic ethene but often stops at toxic cis-1,2-dichloroethene (cis-DCE) or vinyl chloride (VC). The magnitude of carbon relative to chlorine isotope effects (as expressed by I_C/Cl, the slope of ι³C versus γ⁷Cl regressions) was recently recognized to reveal different reduction mechanisms with vitamin B₁₂ as a model reactant for reductive dehalogenase activity. Large I_C/Cl values for cis-DCE reflected cob(I)alamin addition followed by protonation, whereas smaller I_C/Cl values for PCE evidenced cob(I)alamin addition followed by Cl^- elimination. This study addressed dehalogenation in actual microorganisms and observed identical large I_C/Cl values for cis-DCE (I_C/Cl = 10.0 to 17.8) that contrasted with identical smaller I_C/Cl for TCE and PCE (I_C/Cl = 2.3 to 3.8). For TCE, the trend of small I_C/Cl could even be reversed when mixed cultures were precultivated on VC or DCEs and subsequently confronted with TCE (I_C/Cl = 9.0 to 18.2). This observation provides explicit evidence that substrate adaptation must have selected for reductive dehalogenases with different mechanistic motifs. The patterns of I_C/Cl are consistent with practically all studies published to date, while the difference in reaction mechanisms offers a potential answer to the long-standing question of why bioremediation frequently stalls at cis-DCE.