Compound degraded:Beta hexachlorocyclohexane

General Description (About POP compound)

Beta-Hexachlorocyclohexane (?-HCH) is an organochloride which is one of the isomers of technical hexachlorocyclohexane (HCH). It is a byproduct of the production of the insecticide lindane (?-HCH) and it is typically still contained in commercial grade lindane used as insecticide (typically constitutes 5–14% of technical-grade lindane).

Biodegradation pathway

Publications

Abstract Title Authors Article Link
The factors identified to be important for the aerobic biodegradation of alpha-hexachlorocyclohexane (alpha-HCH) in a soil slurry are temperature, auxiliary carbon source, substrate concentration, and soil inhomogeneities. Temperatures in the range of 20 to 30 degrees C were determined to be most favorable for biodegradation of alpha-HCH. No alpha-HCH biodegradation was detected at temperatures below 4 degrees C and above 40 degrees C. The addition of auxiliary organic carbon compounds showed repressive effects on alpha-HCH biomineralization. Increased oxygen partial pressures reduced the repressive effects of added auxiliary organic carbon compounds. A linear relationship between alpha-HCH concentration and its conversion rate was found in a Lineweaver-Burk plot. Inhomogeneities such as clumping of alpha-HCH significantly affected its biodegradation. Inhomogeneity as an influence on biodegradation has not drawn sufficient attention in the past, even though it certainly has affected both laboratory studies and the application of biotechnological methods to clean up contaminated sites. On the basis of metabolites detected during degradation experiments, the initial steps of aerobic alpha-HCH bioconversion in a soil slurry are proposed. Aerobic biomineralization of alpha-hexachlorocyclohexane in contaminated soil Bachmann et al., 1988 Link
Aerobic conditions proved to be best for the microbiol conversion of alpha-hexachlorocyclohexane (alpha-HCH) in a soil slurry. The dry soil contained 400 mg of alpha-HCH per kg. This xenobiotic compound was mineralized within about 18 days at an initial rate of 23 mg/kg of soil per day by the mixed native microbial population of the soil. The only intermediate that was detected during breakdown was pentachlorocyclohexene, which was detected at very small concentrations. Alpha-HCH was also bioconverted under methanogenic conditions. However, a rather long acclimation period (about 30 days) was necessary before degradation started, at a rate of 13 mg/kg of soil per day. Mass balance calculations showed that about 85% of the initial alpha-HCH that was present was converted to monochlorobenzene, 3,5-dichlorophenol, and a trichlorophenol isomer, possibly 2,4,5-trichlorophenol. Under both denitrifying and sulfate-reducing conditions, no significant bioconversion of alpha-HCH was observed. The beta isomer of HCH was recalcitrant at all of the four redox conditions studied. We propose that the specific spatial chloride arrangement of the beta isomer is responsible for its stability. The results reported here with complex soil slurry systems showed that alpha-HCH is, in contrast to the existing data in the literature, best degraded biologically in the presence of oxygen. Biodegradation of alpha- and beta-hexachlorocyclohexane in a soil slurry under different redox conditions Bachmann et al., 1988 Link
The organochlorine pesticide Lindane is the ?-isomer of hexachlorocyclohexane (HCH). Technical grade Lindane contains a mixture of HCH isomers which include not only ?-HCH, but also large amounts of predominantly ?-, ?- and ?-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. Biodegradation of hexachlorocyclohexane (HCH) by microorganisms Phillips et al., 2005 Link
Mixed cultures degrading chlorinated benzenes, chlorinated phenols, or hexachlorocyclohexane (HCH) as the sole source of carbon and energy were obtained by enrichment from contaminated soil samples. Cultures which metabolized 3-chlorophenol (3-CP), 2,3-dichlorophenol (2,3-DCP), or 2,6-dichlorophenol (2,6-DCP) were able to utilize several other chlorinated compounds as substrates, whereas cultures enriched with 1,2,4,5-tetrachlorobenzene (1,2,4,5-TeCB), ?-HCH, or ?-HCH did not metabolize most of the other chlorinated congeners tested. Chloride release and growth rates with all four chlorinated phenols decreased with increasing initial substrate concentrations within the range of 30–250 ?mol liter?1. Maximum chloride release was 3.8 mg liter?1 corresponding to 35 ?mol liter?1 trichlorophenol within 7 weeks. In contrast, the rate of metabolism of the nonphenolic compounds 1,2,4,5-TeCB, ?-HCH, or ?-HCH increased with increasing substrate concentrations. Initial concentrations of 750 ?mol liter?1 ?-HCH or 1,2,4,5-TeCB were completely dechlorinated within 2 weeks. Because aqueous solubility and bioavailability of the chlorophenolic compounds is much higher than that of the nonphenolic compounds, it is suggested that the high bioavailability of the chlorophenolic compounds is the reason for the high toxicity of these substrates to the degrading cultures. In contrast, the low aqueous solubilities of the chlorinated benzenes and HCH-isomers caused consistently low concentrations in the medium, which were high enough to induce degradation but too low to damage the bacterial cells. Degradation by and toxicity to bacteria of chlorinated phenols and benzenes, and hexachlorocyclohexane isomers Lang and Viedt. 1994 Link
Gamma-hexachlorocyclohexane (?-HCH or lindane), one of the most commonly used insecticides, has been mainly used in agriculture. Organochloride compounds are known to be highly toxic and persistent, causing serious water and soil pollution. The objective of the present study is the evaluation of the anaerobic degradation of ?-, ?-, ?-, ?-HCH in liquid and slurry cultures. The slurry system with anaerobic sludge appears as an effective alternative in the detoxification of polluted soils with HCH, as total degradation of the four isomers was attained. While ?- and ?-HCH disappeared after 20–40 d, the most recalcitrant isomers: ?- and ?-HCH were only degraded after 102 d. Intermediate metabolites of HCH degradation as pentachlorocyclohexane (PCCH), tetrachlorocyclohexene (TCCH), tri-, di- and mono-chlorobenzenes were observed during degradation time. Anaerobic degradation of hexachlorocyclohexane isomers in liquid and soil slurry systems Quintero et al., 2005 Link
Lindane (?-hexachlorocyclohexane) is an organochlorine pesticide that has been widely used in agriculture over the last seven decades. The increasing residues of lindane in soil and water environments are toxic to humans and other organisms. Large-scale applications and residual toxicity in the environment require urgent lindane removal. Microbes, particularly Gram-negative bacteria, can transform lindane into non-toxic and environmentally safe metabolites. Aerobic and anaerobic microorganisms follow different metabolic pathways to degrade lindane. A variety of enzymes participate in lindane degradation pathways, including dehydrochlorinase (LinA), dehalogenase (LinB), dehydrogenase (LinC), and reductive dechlorinase (LinD). However, a limited number of reviews have been published regarding the biodegradation and bioremediation of lindane. This review summarizes the current knowledge regarding lindane-degrading microbes along with biodegradation mechanisms, metabolic pathways, and the microbial remediation of lindane-contaminated environments. The prospects of novel bioremediation technologies to provide insight between laboratory cultures and large-scale applications are also discussed. This review provides a theoretical foundation and practical basis to use lindane-degrading microorganisms for bioremediation. Insights Into the Biodegradation of Lindane (?-Hexachlorocyclohexane) Using a Microbial System Zhang et al., 2020 Link
Lindane (?-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 ?-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, ?-HCH is degraded most easily while ?-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 ?-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. Anaerobic Degradation of Lindane and Other HCH Isomers. Mehboob et al., 2013 Link
Lindane, the ?-isomer of hexachlorocyclohexane (HCH), is a potent insecticide. Purified lindane or unpurified mixtures of this and ?-, ?-, and ?-isomers of HCH were widely used as commercial insecticides in the last half of the 20th century. Large dumps of unused HCH isomers now constitute a major hazard because of their long residence times in soil and high nontarget toxicities. The major pathway for the aerobic degradation of HCH isomers in soil is the Lin pathway, and variants of this pathway will degrade all four of the HCH isomers although only slowly. Sequence differences in the primary LinA and LinB enzymes in the pathway play a key role in determining their ability to degrade the different isomers. LinA is a dehydrochlorinase, but little is known of its biochemistry. LinB is a hydrolytic dechlorinase that has been heterologously expressed and crystallized, and there is some understanding of the sequence-structure-function relationships underlying its substrate specificity and kinetics, although there are also some significant anomalies. The kinetics of some LinB variants are reported to be slow even for their preferred isomers. It is important to develop a better understanding of the biochemistries of the LinA and LinB variants and to use that knowledge to build better variants, because field trials of some bioremediation strategies based on the Lin pathway have yielded promising results but would not yet achieve economic levels of remediation. Biochemistry of Microbial Degradation of Hexachlorocyclohexane and Prospects for Bioremediation Lal et al., 2010 Link