Enantioselective Carbon Stable Isotope Fractionation of Hexachlorocyclohexane during Aerobic Biodegradation by Sphingobium spp.

Carbon isotope fractionation was investigated for the biotransformation of gamma- and alpha- hexachlorocyclohexane (HCH) as well as enantiomers of alpha-HCH using two aerobic bacterial strains: Sphingobium indicum strain B90A and Sphingobium japonicum strain UT26. Carbon isotope enrichment factors (epsilon(c)) for gamma-HCH (epsilon(c) = -1.5 +/- 0.1 parts per thousand and -1.7 +/- 0.2 parts per thousand) and alpha-HCH (epsilon(c) = -1.0 +/- 0.2 parts per thousand and -1.6 +/- 0.3 parts per thousand) were similar for both aerobic strains, but lower in comparison with previously reported values for anaerobic gamma- and alpha-HCH degradation. Isotope fractionation of alpha-HCH enantiomers was higher for (+) a-HCH (epsilon(c) = -2.4 +/- 0.8 parts per thousand and -3.3 +/- 0.8 parts per thousand) in comparison to (-) alpha-HCH (epsilon(c) = -0.7 +/- 0.2 parts per thousand and -1.0 +/- 0.6 parts per thousand). The microbial fractionation between the a-HCH enantiomers was quantified by the Rayleigh equation and enantiomeric fractionation factors (epsilon(c)) for S. indicum strain B90A and S. japonicum strain UT26 were -42 +/- 16% and -22 +/- 6%, respectively. The extent and range of isomer and enantiomeric carbon isotope fractionation of HCHs with Sphingobium spp. suggests that aerobic biodegradation of HCHs can be monitored in situ by compound-specific stable isotope analysis (CSIA) and enantiomer-specific isotope analysis (ESIA). In addition, enantiomeric fractionation has the potential as a complementary approach to CSIA and ESIA for assessing the biodegradation of alpha-HCH at contaminated field sites.