Biodegradation of hexachlorocyclohexane (HCH) by microorganisms

Abstract

The organochlorine pesticide Lindane is the gamma-isomer of hexachlorocyclohexane (HCH). Technical grade Lindane contains a mixture of HCH isomers which include not only gamma-HCH, but also large amounts of predominantly alpha-, beta- and delta-HCH. The physical properties and persistence of each isomer differ because of the different chlorine atom orientations on each molecule (axial or equatorial). However, all four isomers are considered toxic and recalcitrant worldwide pollutants. Biodegradation of HCH has been studied in soil, slurry and culture media but very little information exists on in situ bioremediation of the different isomers including Lindane itself, at full scale. Several soil microorganisms capable of degrading, and utilizing HCH as a carbon source, have been reported. In selected bacterial strains, the genes encoding the enzymes involved in the initial degradation of Lindane have been cloned, sequenced, expressed and the gene products characterized. HCH is biodegradable under both oxic and anoxic conditions, although mineralization is generally observed only in oxic systems. As is found for most organic compounds, HCH degradation in soil occurs at moderate temperatures and at near neutral pH. HCH biodegradation in soil has been reported at both low and high (saturated) moisture contents. Soil texture and organic matter appear to influence degradation presumably by sorption mechanisms and impact on moisture retention, bacterial growth and pH. Most studies report on the biodegradation of relatively low (< 500 mg/kg) concentrations of HCH in soil. Information on the effects of inorganic nutrients, organic carbon sources or other soil amendments is scattered and inconclusive. More in-depth assessments of amendment effects and evaluation of bioremediation protocols, on a large scale, using soil with high HCH concentrations, are needed.

Complete Genome Sequence of the Representative γ-Hexachlorocyclohexane-Degrading Bacterium Sphingobium japonicum UT26

“…Although only several decades have passed since the initial release of ␥-HCH into the environment, many ␥-HCHdegrading bacterial strains have been isolated (9), suggesting that ␥-HCH-degrading ability was acquired by such strains within a short period (14). Sphingobium japonicum UT26 was isolated from upland ␥-HCH-contaminated soil in Japan and utilizes ␥-HCH as its sole source of carbon and energy (8,16).…”

Application of an ultrasonic field for the removal of selected pesticides

“…A widespread reaction in the environment is heptachlor epoxidation to the more persistent heptachlor epoxide [17,18]. γ-HCH, known commercially as Lindane, is a highly chlorinated, recalcitrant pesticide that has been targeted for remediation given both its toxicity and its persistence [19].…”

The effect of distribution processes on the isomeric composition of hexachlorocyclohexane in a contaminated riverine system

“…Consequently, higher reactivity of a-HCH and c-HCH over b-HCH can be assumed. Biotic degradation of HCHs was observed under aerobic (aqueous-phase) and anaerobic (sediment-phase) conditions (Phillips et al 2005), and numerous pathways were proposed (Li et al 2011b). General isomeric tendency to degradation is similar to abiotic reactions.…”

“…Various studies have been performed on metabolization (Macrae et al 1967;Willett et al 1998;Rodriguez-Garrido et al 2004;Li et al 2011b) and isomerization (Huhnerfuss et al 1992;Wu et al 1997;Phillips et al 2005) of HCH isomers under different environmental conditions (redox environment, biotic/abiotic). Slightly different biotic transformations under aerobic and anaerobic conditions were proposed involving stepwise dechlorination to less chlorinated derivatives such as pentachlorocylohexene (e.g., Bachmann et al 1988;Rodriguez-Garrido et al 2004;Phillips et al 2005;Ricking and Schwarzbauer 2008;Bhatt et al 2009;Li et al 2011b). Hydrolysis was reported as the dominant abiotic degradation pathway for HCHs (Bhatt et al 2009).…”