Diazinon microbial degradation

Diazinon (phosphorothioic acid 0,0-diethyl 0-(6-mcthyl-2-(l-mcthylcthyl)-4-pyrimidinyl) ester) is an organophosphorus compound with an anticholinesterase mode of action. It is used extensively to control hies, lice, insect pests of ornamental plants and food crops, as well as nematodes and soil insects in lawns and croplands. Diazinon degrades rapidly in the environment, with half-time persistence usually less than 14 days. But under conditions of low temperature, low moisture, high alkalinity, and lack of suitable microbial degraders, diazinon may remain biologically active in soils for 6 months or longer. 

Ester hydrolysis is the most important microbial degradation of diazinon, whereas mammals produce glutathione conjugates and oxidation products. 

Diazinon. 2-Isopropyl-6-methyl-4-hydroxypyrimidine, the hydrolysis product of diazinon, did not condition the soils for enhanced degradation of diazinon (Table II). Despite the low microbial toxicity and high availability (discussed elsewhere in this chapter), the hydroxypyrimidine metabolite did not predispose soils for rapid degradation of diazinon. Enhanced biodegradation of diazinon in rice soils has been previously reported (1). Evidently, the soil we studied did not contain microbes capable of adapting for diazinon enhanced degradation. 

Biological. Sethimathan and Yoshida (1973a) isolated a Flavobacterium sp. (ATCC 27551) from rice paddy water that metabolized diazinon as the sole caibon source. Diazinonwas readily hydrolyzed to 2-isopropyl-4-methyl-6-ltydroxypyrimidine under aerobic conditions but less rapidly under anaerobic conditions. This bacterium as well as enrichment cultures isolated from a diazinon-treated rice field mineralized the hydrolysis product to carbon dioxide (Sethunathan and Pathak, 1971 Sethunathan and Yoshida, 1973). Rosenberg and Alexander (1979) demonstrated that two strains of Pseudomonas grew on diazinon and produced diethyl phosphorothioate as the major end product. The rate of microbial degradation increased in the presence of an enzyme (parathion hydrolase), produced by a mixed culture of Pseudomonas sp. (Honeycutt et al., 1984). 

Gunner HB, Zuckerman BM. 1968. Degradation of diazinon by synergistic microbial action. Nature 217 1183-1184. 

Soil temperature influences chemical degradation, microbial decomposition, and volatilization. For example, no aldrin or heptachlor was lost from frozen soils, but at 6°C, 16-27% of the dose applied to soil was lost in 56 days at 26°C, 51-55% disappeared and at 46°C, 86-98% was lost. Diazinon was also degraded faster at higher temperatures than at low ones (Edwards, 1973b). Temperature also influences the adsorption of pesticide in soils because adsorption is a exothermic process, so that increased temperatures decrease adsorption and release pesticides. 

Any factors that stimulate the growth of soil microorganisms or that increase the availability of pesticides in soil will enhance the degradation of the chemical. Felsot et al. (1981) found that the persistence of carbofuran was inversely correlated with microbial activity in corn-cropped soils. Tu and Miles (1976) reported that 88% of parathion was lost from soil in 7 months diazinon, 92% lost in 20 weeks paraoxon, 100% hydrolyzed in 12 hr mala-thion, 50-90% lost in 24 hr and carbofuran, 50% lost in 3-50 weeks. 

The hydroxypyrimidine hydrolysis product of diazinon is more readily available in all soils tested and is mineralized by microbes (33). Our Microtox studies have demonstrated its low toxicity to bacteria. Availability, low microbial toxicity, and susceptibility to microbial metabolism of this hydrolysis product may favor enhanced degradation of its parent compound in soils with populations of degrading microorganisms, but no adaptation was noted in our laboratory studies. 

In our studies, diazinon was applied to autoclaved and nonautoclaved samples of an acid soil (Luisiana clay) and two neutral soils (Maahas clay and a clay loam), and the soils were flooded. The insecticide disappeared from both autoclaved and nonautoclaved acid soils, indicating that degradation was nonbiological. Degradation was more rapid in the nonautoclaved sample than in the autoclaved sample of the other two soils (2), The half-lives of diazinon in the nonautoclaved and autoclaved Maahas clay were 8.8 and 33.8 days, respectively the corresponding values for the clay loam were 17.4 and 43.8. The increased breakdown of diazinon in nonautoclaved soils of neutral pH can be attributed to microbial participation, but some of the insecticide was lost from the autoclaved lots of these soils as well as a result of chemical degradation and volatilization. 

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