The anaerobic metabolism of chlorinated benzenes and toluenes was evaluated in soil slurry microcosms under methanogenic conditions. A mixture of hexachlorobenzene, pentachlorobenzene, and 1,2,4-trichlorobenzene (TCB) in soil slurries was biotransformed through sequential reductive dechlorination to chlorobenzene (CB). The metabolic pathway for hexachlorobenzene and pentachlorobenzene decay proceeded via 1,2,3,4tetrachlorobenzene (TTCB)-1,2,3-TCB + 1,2,4-TCB 1,2-dichlorobenzene (DCB) + 1,4-DCB-CB. In a mineral salts medium, the CB-adapted soil microorganisms dehalogenated individual 1,2,4,5-TTCB, 1,2,3,4-TTCB, 1,2,3-TCB, and 1,2,4-TCB but not 1,2,3,5-TTCB or 1,3,5-TCB. Similarly, a mixture of 2,3,6-trichlorotoluene (TCT), 2,5-dichlorotoluene (DCT), and 3,4-DCT was reductively dechlorinated in soil slurries to predominantly toluene and small amounts of 2-, 3-, and 4-chlorotoluene (CT). Toluene was further degraded. When tested individually in a mineral salts medium, the CT-adapted soil microorganisms dechlorinated several TCT and DCT isomers. Key metabolic routes for TCTs followed: 2,3,6-TCT-+ 2,5-DCT-2-CT-+ toluene; 2,4,5-TCT-2,5-DCT + 3,4-DCT-3-CT + 4-CT-> toluene. Among DCTs tested, 2,4-DCT and 3,4-DCT were dechlorinated via the removal of o-and m-chlorine, respectively, to 4-CT and subsequently to toluene via p-chlorine removal. Likewise, 2,5-DCT was dechlorinated via 2-CT to toluene. Evidently, microorganisms capable of removing o-, m-, and p-chlorines are present in the soil system, as reflected by the dechlorination of different isomers of CBs and CTs to CB and toluene, respectively. These findings help clarify the metabolic fate of chlorinated benzenes and toluenes in anaerobic environments.

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

The anaerobic biodegradation of m-cresol was observed in anoxic aquifer, slurries kept under both sulfate-reducing and nitrate-reducing but not methanogenic conditions. More than 85% of the parent substrate (300 ,uM) was consumed in less than 6 days in slurries kept under the former two conditions. No appreciable loss of the compound from the corresponding autoclaved controls was measurable. A bacterial consortium was enriched from the slurries for its ability to metabolize m-cresol under sulfate-reducing conditions. Metabolism in this enrichment culture was inhibited in the presence of oxygen or molybdate (500 ,iM) and in the absence of sulfate but was unaffected by bromoethanesulfonic acid. The consortium consumed 3.63 mol of sulfate per mol of m-cresol degraded. This stoichiometry is about 87% of that theoretically expected and suggests that m-cresol was largely mineralized. Resting-cell experiments demonstrated that the degradation of m-cresol proceeded only in the presence of bicarbonate. 4-Hydroxy-2-methylbenzoic acid and acetate were detected as transient intermediates. Thus, the parent substrate was initially carboxylated as the primary degradative event. The sulfate-reducing consortium could also decarboxylate p-but not m-hydroxybenzoate to near stoichiometric amounts of phenol, but this reaction was not sulfate dependent. The presence of p-hydroxybenzoate in the medium temporarily inhibited m-cresol metabolism such that the former compound was metabolized prior to the latter and phenol was degraded in a sequential manner. These findings help clarify the fate of a common groundwater contaminant under sulfate-reducing conditions.

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.

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