Effects of Bacterial Growth Conditions on Carbon and Chlorine Isotope Fractionation Associated with TCE Biotransformation

Abstract

The variability of reported carbon and chlorine isotope enrichment factors (εC and εCl) for trichloroethene (TCE) respiration even for pure bacterial cultures limits the applicability of compound-specific isotope analysis (CSIA) for quantitative assessment of TCE biotransformation. As the growth and metabolism of bacteria in the environment can differ substantially from standardized laboratory conditions, we investigated the effects of environmentally relevant variables on microbial TCE isotope fractionation including adaptation period to TCE exposure, temperature, electron donor/acceptor concentration, and supplied cobalt species, as well as coculture effects. Desulfitobacterium hafniense strain Y51 was chosen as a model organism, as it contains only one reductive dehalogenase enzyme allowing for an unequivocal assignment of observed isotope fractionation variability to specific environmental factors. In 20 independent TCE transformation experiments, the εC values differed only slightly by 1‰ despite drastic variations in the growth conditions. These robust εC values reflect the intrinsic isotope fractionation of the TCE-catalyzing enzyme, as changes in both, the enzymatic reaction mechanism and rate-determining steps, can be excluded based on robust two-dimensional isotope analysis (δ 13C/δ 37Cl). Thus, growth conditions that cells, holding only one enzyme for TCE transformation, encounter in the environment are not expected to affect isotope fractionation during biotransformation and thus likely do not contribute to the reported variability of carbon and chlorine isotope enrichment factors for reductive dechlorination of TCE.