Factors controlling the carbon isotope fractionation of tetra- and trichloroethene during reductive dechlorination by Sulfurospirillum ssp and Desulfitobacterium sp strain PCE-S

Carbon stable isotope fractionation of tetrachloroethene (PCE) and trichloroethene (TCE) was investigated during reductive dechlorination. Growing cells of Sulfurospirillum multivorans, Sulfurospirillum halorespirans, or Desulfitobacterium sp. strain PCE-S, the respective crude extracts and the abiotic reaction with cyanocobalamin (vitamin B-12) were used. Fractionation of TCE (alpha C=1.0132-1.0187) by S. multivorans was more than one order of magnitude higher than values previously observed for tetrachloroethene (PCE) (alpha C=1.00042-1.0017). Similar differences in fractionation were observed during reductive dehalogenation by the close relative S. halorespirans with alpha C=1.0046-1.032 and alpha C=1.0187-1.0229 for PCE and TCE respectively. TCE carbon isotope fractionation (alpha C=1.0150) by the purified PCE-reductive dehalogenase from S. multivorans was more than one order of magnitude higher than fractionation of PCE (alpha C=1.0017). Carbon isotope fractionation of TCE by Desulfitobacterium sp. strain PCE-S (alpha C=1.0109-1.0122) as well as during the abiotic reaction with cyanocobalamin (alpha C=1.0154) was in a similar range to previously reported values for fractionation by mixed microbial cultures. In contrast with previous results with PCE, no effects due to rate limitations, uptake or transport of the substrate to the reactive site could be observed during TCE dechlorination. Our results show that prior to a mechanistic interpretation of stable isotope fractionation factors it has to be carefully verified how other factors such as uptake or transport affect the isotope fractionation during degradation experiments with microbial cultures.