Enhanced biodegradation of β- and δ-hexachlorocyclohexane in the presence of α- and α-isomers in contaminated soils

The chlorinated insecticide hexachlorocyclohexane (HCH) has been used extensively in the past, and contaminated sites are present throughout the world. Toward their bioremediation, we isolated a bacterium Pseudomonas aeruginosa ITRC-5 that mediates the degradation of all the four major isomers of HCH under aerobic conditions, both in liquid-culture and contaminated soils. In liquid-culture, the degradation of α- and γ-HCH is rapid and is accompanied with the release of 5.6 μmole chloride ions and 4.1 μmole CO₂mole-1 HCH-isomer. The degradation of β- and δ5-isomers is slow, accompanied with the release of 0.9 μmole chloride ions μmole^-1 HCH-isomer, and results in a transient metabolite 2,3,4,5,6-pentachlorocyclohexan-1-ol. The strain ITRC-5 also mediates the degradation of α-, β-, γ, and <5-isomers in contaminated soils, where degradation of otherwise persistent β- and <3-HCH is enhanced severalfold inthe presence of α- or γ-HCH. The degradation of soil-applied β- and <3-HCH under aerobic conditions has not been reported earlier. The isolate ITRC-5 therefore demonstrates potential for the bioremediation of HCH-wastes and contaminated soils.