Microbial Reduction of Phorate Sulfoxide to Phorate in a Soil-Lake Mud-Water Microcosm1

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Abstract
Pesticide contaminated agricultural soils can be transported with water into rivers and lakes, where in most cases they end up as bottom deposits. By utilizing a loam-lake mud-water microcosm, the reduction of phorate sulfoxide, an oxidation product of the insecticide phorate, was demonstrated. With flooded phorate sulfoxide-treated loam soil, only small amounts of phorate were produced. The addition of lake mud, however, dramatically increased reduction of phorate sulfoxide to phorate, which after 2 wk of incubation accounted for 44% of the totally recovered residues. Exposure of Drosophila melanogaster Meigen to dry residues of extracts of phorate sulfoxide-treated loam soil, deposited on top of lake mud under flooded conditions, indicated an increase in toxicity due to the production of phorate. This reduction of phorate sulfoxide to phorate was the result of the activity of microorganisms, which in turn was dependent on the supply of organic nutrient sources. Thus, addition of glucose to this system further enhanced the reduction of phorate sulfoxide to phorate. Phorate sulfone, another oxidative product of phorate in aerobic agricultural soils, was not reduced to phorate sulfoxide or to phorate.

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