Evidence of substantial carbon isotope fractionation among substrate, inorganic carbon, and biomass during aerobic mineralization of 1,2-dichloroethane by Xanthobacter autotrophicus

Carbon isotope fractionation during aerobic mineralization of 1,2-dichloroe thane (1,2-DCA) by Xanthobacter autotrophicus GJ10 was investigated. A stro ng enrichment of C-13 in residual 1,2-DCA was observed, with a mean fractio nation factor ex a standard deviation of 0.968 +/- 0.0013 to 0.973 +/- 0.00 15. In addition, a large carbon isotope fractionation between biomass and i norganic carbon occurred. A mechanistic model that links the fractionation factor or to the rate constants of the first catabolic enzyme was developed . Based on the model, it was concluded that the strong enrichment of C-13 i n 1,2-DCA arises because the first irreversible step of the initial enzymat ic transformation of 1,2-DCA consists of an S(N)2 nucleophilic substitution . S(N)2 reactions are accompanied by a large kinetic isotope effect. The su bstantial carbon isotope fractionation between biomass and inorganic carbon could be explained by the kinetic isotope effect associated with the initi al 1,2-DCA transformation and by the metabolic pathway of 1,2-DCA degradati on. Carbon isotope fractionation during 1,2-DCA mineralization leads to 1,2 -DCA, inorganic carbon, and biomass with characteristic carbon isotope comp ositions, which may be used to trace the process in contaminated environmen ts.