Anaerobic dechlorination and degradation of hexachlorocyclohexane isomers by anaerobic and facultative anaerobic bacteria.

Screening studies with strict and facultative anaerobic bacteria showed that Clostridium app. and several other representatives of Bacillaceae and Enterobacteriaceae actively degraded gamma-hexachlorocyclohexane (gamma-HCH) under anaerobic conditions. Representatives of Lactobacillaceae and Propronibacterium were inactive. With 36Cl-labelled gamma-HCH a nearly complete dechlorination was shown to occur in 4--6 days by Clostridium butyricum, C. pasteurianum and Citrobacter freundii, while other facultative anaerobic species were less active. Aerobically grown facultative anaerobes also dechlorinated actively gamma-HCH during subsequent anaerobic incubation with glucose, pyruvate or formate as substrates. The alpha-, beta- and delta-HCH isomers were also, but more slowly, dechlorinated (gamma larger than alpha larger than beta larger than or equal to delta-HCH). All species active in anaerobic degradation of gamma-HCH formed gamma-tetrachlorocyclohexene (TCH) as the main intermediate metabolite and no gamma-pentachlorocyclohexene (PCH) or other isomers of TCH or PCH have been found. Small amounts of tri- and tetrachlorinated benzenes have been found too. The mechanism of dechlorination is discussed.