Anaerobic degradation of lindane and other HCH isomers

Abstract

Lindane (γ-HCH) is a pesticide that has mainly been used in agriculture. Lindane and the other HCH isomers are highly chlorinated hydrocarbons. The presence of a large number of electron withdrawing chlorine groups makes some of the HCH isomers rather recalcitrant in oxic environments. Especially β-HCH is poorly degraded by aerobic bacteria. The chlorine groups make HCH isomers more accessible for an initial reductive attack, a common mechanism in anoxic environments. Among the HCH isomers, γ-HCH is degraded most easily while β-HCH is most persistent. Little is known about the diversity of the microorganisms involved in anaerobic HCH degradation. Thus far, species within the genera Clostridium and Bacillus, two Desulfovibrio species, and one species each of Desulfococcus, Desulfobacter, Citrobacter and Dehalobacter have been found to metabolize lindane and other HCH isomers. Benzene and monochlorobenzene are the end products of anaerobic degradation, while in some studies pentachlorocyclohexane, tetrachlorocyclohexene, chlorobenzenes and chlorophenols have been detected as intermediates. Enzymes and coding genes involved in the reductive dechlorination of HCH isomers are largely unknown. Recently, a metagenomic analysis has indicated the presence of numerous putative reductive dehalogenase genes in the genome of β-HCH degrading Dehalobacter sp. High-throughput omics techniques can help to explore the key players and enzymes involved in the reductive dehalogenation of lindane and other HCH isomers.