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.

A bacterium, tentatively identified as an Arthrobacter sp., was isolated from flooded soil that was incubated at 35°C and repeatedly treated with carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl N-methylcarbamate). This bacterium exhibited an exceptional capacity to completely mineralize the ring-labeled 14C in carbofuran to 14Co2 within 72 to 120 h in a mineral salts medium as a sole source of carbon and nitrogen under aerobic conditions. Mineralization was more rapid at 35°C than at 20°C. No degradation of carbofuran occurred even after prolonged incubation under anaerobic conditions. The predicted metabolites of carbofuran, 7-phenol (2,3-dihydro-2,2-dimethyl-7-benzofuranol) and 3-hydroxycarbofuran, were also metabolized rapidly. 7-Phenol, although formed during carbofuran degradation, never accumulated in large amounts, evidently because of its further metabolism through ring cleavage. The bacterium readily hydrolyzed carbaryl (1-naphthyl N-methylcarbamate), but its hydrolysis product, 1-naphthol, resisted further degradation by this bacterium. Carbamate pesticides have been developed as a biodegradable and short-lived alternative to highly stable organochlorines (1, 7). Among the carbamate pesticides, carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl N-methylcarbamate) is widely used in rice culture for controlling the brown planthopper (Nilapari'ata ligens St'al) and other insect pests of rice. Carbofuran is chemically hydrolyzed under alkaline conditions (5), but microorganisms have been implicated in its degradation in near-neutral soil and water environments (11). An Achromobacteer sp., which was isolated from carbofuran-treated soils, exhibited an exceptional capacity to hydrolyze almost all the added carbofuran within 42 h as a sole source of nitrogen in a medium with glucose as an additional carbon source (6); 7-phenol was not metabolized further. Recently, we found that repeated additions of carbofuran to flooded soil incubated at 35°C yielded a soil enrichment culture with an exceptional capacity to hydrolyze carbofuran and to mineralize the 7-phenol to CO, in 4 to 5 days (14). Here we report the degradation of carbofuran by a bacterium isolated from this enrichment culture. MATERIALS AND METHODS Insecticides and their metabolites. Analytical grade carbofuran (99.4% purity), uniformly ring-labeled [14C]carbofuran (specific activity, 39.4 mCi/mmol; 98% purity), carbonyl-1

Soil enrichment cultures were prepared by repeated additions of methyl parathion to flooded alluvial and laterite soils incubated at 35 °C. These cultures were tested for their ability to degrade methyl parathion in a mineral salts medium in the presence and absence of yeast extract. Addition of yeast extract (0.05% w/v) accelerated the degradation of methyl parathion by both enriched cultures. Methyl parathion was degraded by the enrichment culture from alluvial soil essentially by hydrolysis in the absence of yeast extract and by nitro group reduction in its presence. The enrichment culture from laterite soil degraded methyl parathion (by hydrolysis) only in the presence of yeast extract. A Bacillus sp., isolated from laterite soil, degraded methyl parathion essentially by hydrolysis in the presence of a concentration (w/v) of yeast extract of 0.05%, by both hydrolysis and nitro group reduction at 0.1 and 0.25%, and exclusively by nitro group reduction at 0.5%. A similar trend was also noticed with parathion. However, fenitrothion was degraded by Bacillus sp. mainly by hydrolysis at all concentrations of yeast extract, whereas diazinon was not degraded.Key words: organophosphorothioates, biodegradation, yeast extract dependent pathway.

No abstract

No abstract