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

Carbon isotope fractionation was investigated for the biotransformation of γ- and α- hexachlorocyclohexane (HCH) as well as enantiomers of α-HCH using two aerobic bacterial strains: Sphingobium indicum strain B90A and Sphingobium japonicum strain UT26. Carbon isotope enrichment factors (ε_c) for γ-HCH (ε_c = −1.5 ± 0.1‰ and −1.7 ± 0.2‰) and α-HCH (ε_c = −1.0 ± 0.2‰ and −1.6 ± 0.3‰) were similar for both aerobic strains, but lower in comparison with previously reported values for anaerobic γ- and α-HCH degradation. Isotope fractionation of α-HCH enantiomers was higher for (+) α-HCH (ε_c = −2.4 ± 0.8 ‰ and −3.3 ± 0.8 ‰) in comparison to (−) α-HCH (ε_c = −0.7 ± 0.2‰ and −1.0 ± 0.6‰). The microbial fractionation between the α-HCH enantiomers was quantified by the Rayleigh equation and enantiomeric fractionation factors (ε_e) 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 α-HCH at contaminated field sites.