Dimethyl phosphorodithioate (DMDTP)is an initial breakdown product of organophosphorus pesticides in fields. DMDTP is also released to natural environments by pesticide manufacturers. DMDTP-degradingmicroorganisms were not known. Weisolated two bacteria from activated sludge. One of them, strain TK-1 identified as Thiobacillus thioparus, utilized DMDTP as a sole energy source and produced dimethyl phosphate (DMP) and sulfate. The other, strain AK-2identified as Pseudomonas sp., utilized DMPas a sole energy and carbon source and degraded DMPto inorganic orthophosphate (P;). DMDTP was degraded to P; by the coaction of the two bacteria.

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O,O-Dimethyl phosphorodithioate (DMDTP) is the raw material for such organophosphorus pesticides as malathion, dimethoate and cidial, and is abundant in wastewater of pesticide manufacturers. The activated sludge was acclimated to DMDTP by the fill-and-draw method. The acclimated, activated sludge degraded 500mg/liter of DMDTP in 7 hr and as a result , produced 260 mg/liter of inorganic orthophosphate and 510mg/liter of sulfate. The pH of the mixed liquor was adjusted to degrade DMDTP, and the optimum pH was from 6.5 to 7.0. The activated sludge acclimated to DMDTP degraded dimethyl phosphorothioate , dimethyl phosphate, diethyl phosphorodithioate and diethyl phosphate.

O,O-Dimethyl phosphorodithioate (DMDTP) is an initial breakdown product of organophosphorus pesticides in fields. DMDTP is also released to natural environments by pesticide manufacturers. DMDTP-degrading microorganisms were not known. We isolated two bacteria from activated sludge. One of them, strain TK-l identified as Thiobacillus thioparus, utilized DMDTP as a sole energy source and produced dimethyl phosphate (DMP) and sulfate. The other, strain AK-2 identified as Pseudomonas sp., utilized DMP as a sole energy and carbon source and degraded DMP to inorganic orthophosphate (PJ DMDTP was degraded to Pi by the coaction of the two bacteria.O,O-Dimethyl phosphorodithioate (DM-DTP) is an initial breakdown product of such organophosphorus pesticides as malathion, dimethoate, cidial, formothion, and others in fields. Many investigators have dealt with the degradation of organophosphorus pesticides and have found all pesticides susceptible to hydrolysis. Organophosphorus pesticides are regarded as nonpersistent. However, ionic dialkyl phosphates and sulfur analogs thereof which are the initial products of pesticide breakdown have been reported to be unsusceptible to hydrolysis.!) Some of the breakdown products are toxi<;; to fish.2) Organophosphorus pesticides cannot be regarded as nonpersistent if the toxic breakdown products remain unaltered. The only reported instances of utilization of ionic dialkyl phosphates or sulfur analogs thereof by microorganisms have occurred when the compounds were the sole phosphorus sources. 3 ,4) These instances do not prove that the compounds will be degraded in natural environments, especially in the presence of available orthophosphate.These ionic compounds are also released to natural environments by pesticide manufacturers because these compounds are precursors in the production of pesticides and are abundant in wastewater.In a previous paper,S) we showed that DMDTP was degraded to orthophosphate by activated sludge. The present study was designed to isolate DMDTP-degrading bacteria from the activated sludge. We isolated two bacteria, Thiobacillus thioparus and Pseudomonas sp. By a symbiont of these two bacteria, DMDTP was utilized as a sole energy and carbon source and was degraded to inorganic orthophosphate. MATERIALS AND METHODSChemicals. The organophosphorus compounds used and abbreviations are listed in Table 1. DMDTP ammonium salt was obtained from Aldrich Chemical Co., Milwaukee, U.S.A. DMTP sodium salt was kindly supplied by Sumitomo Chemical Co., Osaka, Japan. The mixture of DMP and MP was obtained from ICN K&K Laboratories, N.Y., U.S.A. All other chemicals were obtained from commercial sources.DMDTP used in degradation tests was purified by preparative layer chromatography as follows. One milliliter of 50% DMDTP was streaked across the width of a chromatoplate of a 2-mm layer of silica gel 60 F 254 (Merck, Art. 5717). After development in a solvent system

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