The degradation of uniformly 14C-labeled isomers (a, y , and P) of hexachlorocyclohexane (HCH), added separately to a mineral salts medium as a sole source of carbon, by a soil bacterium, Pseudomonas sp., under aerobic conditions was studied. During aerobic degradation, about 10-12% of the 14C in a-and y-HCH was accounted for as 14C02 as compared to 5% from P-HCH. Most of the 14C in a-and y-HCH accumulated as water-soluble products, while formation of water-soluble products from P-HCH was negligible. Almost all of the 14C in the three isomers of HCH was accounted for in different fractions (Con, chloroform-diethyl ether, water phase) following bacterial degradation under aerobic conditions. GC-MS analysis of P-HCH residues in the chloroformdiethyl ether extract suggested the formation of pentachlorocyclohexanol and tetrachlorocyclohexanediol as products of aerobic metabolism of P-HCH.

A Pseudomonas sp., isolated from sugarcane rhizosphere soil, readily metabolized not only alpha and gamma isomers of hexachlorocyclohexane, but also the thermodynamically more stable beta isomer, under aerobic conditions. Bacterial degradation of the three isomers led to the accumulation of a transitory metabolite and eventual release of covalently linked chlorine as chloride in stoichiometric amounts. Commercial formulations of hexachlorocyclohexane (HCH), a widely used insecticide in India and many tropical countries of the Third World, contain a mixture of isomers which may include alpha, beta, gamma, and other isomers. Until very recently, HCH isomers were considered to be highly persistent in aerobic environments, but they readily undergo rapid degradation in predominantly anaerobic ecosystems such as flooded soils and lake sediments (11, 13). A strict anaerobe, Clostridium sp., isolated from a flooded soil degraded alpha-and gamma-HCH, but not betaor delta-HCH, under anaerobic conditions (6, 10). Recently, aerobic biomineralization of alpha-HCH was demonstrated in a soil slurry from an HCH-contaminated site in the Netherlands (1, 2). In a long-term field experiment, gamma-HCH degraded slowly in the first 2 years after being applied to an upland soil, but in the third year more than 80% of the insecticide disappeared within 1 month (K.

No abstract

No abstract

No abstract