Ferdag KAZANICI
Kahramanmaraş Sütçü İmam University, Faculty of Arts & Science, Department of Microbiology, 46045-Kahramanmarat-TURKEY
ABSTRACT
In this study the effect of some organophosphorus insecticides [ Isofenfos (=Oftanol DS 40), Phorate (=Thimet 10G and Foratox 10-G) and Fonofos] were investigated on soil microbiota (Total viable bacteria, Actinomycetes, Anaerobic bacteria, Aerob endospore-forming, Proteolytic bacteria, Cellülolytic microorganisms and Yeast-mold).
It was determined that in the isofenfos-treated soil sample the total viable bacteria number was found to be excessive than that of the control groups during the incubation. Moreover it was observed that this treatment had no inhibitory effect on the development of the other microorganism groups.
It was determined that in soil sample used fonofos and phorate pesticides were had no inhibitory effect on the development of the soil microorganism groups.
KEY WORDS: Organophosphorus insecticide, Soil microorganisms
The wide use of organophosphorus pesticides has created several problems, including the pollution of environmental by pesticide manufacturers. Organophosphorus insecticides (e.g. isofenphos, chlorpyrifos, diazinon, phorate, ethoprop, terbufos, phosalone, pirimiphos methyl) were registered for use against soil-dwelling pests of corn such as larval corn rootworms (Diabrotica spp.) and cutworms (Agrotis spp.). Organophosphorus pesticides are regarded as non persistent (1).
In recent years, the role of soil microorganisms in affecting the persistence of agricultural pesticides has gained interest from two major viewpoints. One is to achieve a rapid disappearance of highly persistent or toxic chemicals. The second is associated with reduced pesticide efficacy attributed to enhanced biodegradation, particularly of chemicals applied under a continuous cropping program. A streptomycete bacterium was isolated from a field soil sample previously treated with the insecticide isofenphos and found to be capable of growing on several commercial carbamate and organophosphate insecticides (2). Environmental stability, bioaccumulation, and toxicity to non-target species have brought about restricted usage of some organochlorine insecticides; these properties have also initiated many studies on their microbiological breakdown which have been reviewed by Lal and Saxena (3) and Motosugi and Soda (4).
Catabolism and detoxification metabolism occur when a soil microorganism uses the pesticide as a carbon and energy source. The latter process is facilitated by resistant microorganisms (5). Reduced persistence of organophosphate insecticides attributed to soil microorganisms has been described Chapman and Harris (6), Gorder et al. (7) and Sharmila et al. (8). The degradation of xenobiotic compounds by members of the soil microflora is an important means by which these compounds are removed from the environment, thus preventing from becoming pollution problems. Much work has been directed towards understanding the complexity of pesticide-microbial interactions in soil. Many studies have employed pure cultures of soil isolates or agar plate counts of soil populations (9,10). Microbial communities composed of several numerous species are more likely to be responsible for pesticide biodegradation in soil and rhizosphere environments than are single species. Pesticides applied to soil at planting should persist during the development of plant roots. Therefore, a portion of the pesticide likely interacts with microorganisms in the soil and rhizosphere (11). Biodegradation of organophosphorus insecticides by microorganisms in soil has been widely reported (1,8,12,13) however, effects of organophosphorus pesticides on soil microorganisms has received less attention. This paper reports the effects of three organophosphorus insecticides (isofenphos, fonofos and phorate) on soil microbial activities in silt loam soils.
MATERIALS AND METHODS
In this study we examined the response of microbial populations in soils after incorporation of several organophosphorus insecticides. The chemical configurations of these insecticides are shown in Fig 1. The soil samples were collected from experimental field at the Kahramanmaraş Sütçü İmam University on April 12, 1998.
The silt loam soil was collected from the upper 10 cm of the soil profile on an experimental field that had no history of insecticide application. The soil had the following properties: pHS (1:1 0.01M CaCl2), 6.78; cation exchange capacity, 16.7 meq/100 g; organic matter, 3.6% and total nitrogen, 0.15%.
In the laboratory, plant material and soil macrofauna were removed, and soil samples sieved (<2 mm) and mixed. Each soil received one of the following insecticides selected to represent important chemical groups; isofenfos at the rate of 175 g/100 kg of seeds, phorate at the rate of 2.5 kg a.i. ha-1 and fonofos at the rate of 4.48 kg a.i. ha-1. Controls, with soil only, were included within all tests. The mixed and control samples were set up in 1000 ml-bottles (Beher glass) and incubated at 28± 1°C for 20 days. Measurements were done of the number of actinomycete, spore-forming aerobes, anaerobic bacteria, proteolytic, cellulolytic and total microorganisms and yeast-mold using the dilution method (14,15). In the different days (0, 5, 10, 15, and 20th) of incubation period, 20 each grams of pesticided and control samples was taken. Soil samples (20 g soil) were suspended in 180 ml strength Ringer’s solution. Dilutions (of 10-7) were prepared by using sterilized Ringer’s solution from the soil samples and dispersed with a top drive macerator for 5 min (16). The soil samples taken from suitable dilution were sawed into or on the solid feeding medium relying on the purpose. The number of actinomycetes was determined on Bacto-Actinomycete Isolation agar (Difco) with added rifampicin and cycloheximide (50 mg Litter-1) (17); plate count agar was used to estimate total aerobic bacteria and spore-forming aerobes (14); the roll tube method (18) was used to estimate anaerobic bacteria, using N2-purged plate count agar. The number of cellulolytic microorganisms was determined in the Skinner medium (19), gelatin medium for proteolytic bacteria, and malt extract agar for yeast-molds (14). The results were evaluated as the number of microorganism in 1 g oven-dried soil. The study was performed in three parallel group and the averages of the results obtained were recorded.
RESULTS AND DISCUSSION
Results of the microbial number determination (Table 1) indicate that soils in which enhanced isofenphos application has occurred contain a population of microorganisms capable of metabolizing this insecticide. In control soil, decrease of microorganisms has occurred during the incubation periods. Racke and Coats (20) reported that a bacterial strain was isolated from isofenphos-treated culture medium, and it proved capable of using isofenphos as a carbon source. This bacterium was identified as a species of Pseudomonas. In addition, the isofenfos-degrading was Arhrobacter sp. isolated in the current investigation was able to rapidly metabolize this compound. Several Pseudomonas spp. have been isolated from soil that metabolize organophosphorus and carbamate insecticides (21,22). The response of soil microorganisms to pesticides in pure culture was variable with no consistent relationship evident between the source of the microorganisms and the observed response (20).
Effect of fonofos on the number of the microbial groups per gram of dry soil are presented in Table 2. As can clearly be seen from this table, normal doses of fonofos have only slight effects on soil microflora. The number of isolates which is able to grow on pesticide substrates alone was the highest also for the bacteria. Representative Actinomycetes isolates grew on the fonofos insecticide compounds. The fungal isolates were less inhibited by the fonofos pesticide.
Some organophosphorus insecticides such as diazinon, chlorpyrifos, ethion, parathion, fonofos, malathion or gusathion are susceptible to microbial hydrolysis and may serve as carbon sources for the growth of pure and mixed cultures to Flavobacterium sp., Pseudomonas sp. and Arthrobacter sp. (13,23). Gauger et al. (2) reported that the Streptomyces pilosus was capable of growing on several insecticides (carbofuran, cloethocarb, trimethacarb, isofenphos, fonofos and phorate); growth on turbefos was nonexistent. The microbiological degradation of pesticides in soil is strongly affected by climatic factors: in the finnish climate, the low temperature and the shortness of the summer are the most limiting factors. Soil respiration at 5°C is less than half that to 15°C, and during winter, when the soil is frozen, respiratory is very low (19,24).
Effect of phorate on the number of the microbial groups per gram of dry soil are presented in Table 3. In phorate-treated soil counts of microorganisms that the spore-forming aerobes, anaerobic, proteolytic and cellulolytic bacteria grew on phorate. In control soil, decrease of total microorganisms had occurred during the 5. day incubation period, while the number of fungi remained the same.
Microbial metabolism is an important process for degrading pesticides in the soil environment. Biochemical research on the pesticide metabolism over the last three decades has primarily been aimed at identifying the microorganisms, metabolites, and enzymes associated with a specific pesticidal compound. With advances in modern microbial genetics, some insight is developing into the evolutionary events that occur during the pesticide adaptation process.
The bacterial isolates were able to degrade carbaryl and carbofuran, especially in the absence of an additional nitrogen source (NH4)2HPO4 (25). The pesticide treatments were observed in many experiments to have significant effects on the microbial activities but the microorganisms recovered rapidly. These effects were not drastic but minor in nature. There is little evidence to suggest that these pesticide treatments have any prolonged deleterious effect on the soil microbial activities. The pesticide treatments were observed in many experiments to have significant effects on the microbial activities but the microorganisms recovered rapidly. These effects were not drastic but minor in nature. There is little evidence to suggest that these pesticide treatments have any prolonged deleterious effect on the soil microbial activities. Finally, it was observed that generally pesticide treatment had no inhibitory effect on the microorganism groups.
Acknowledgement: I thank Merve Zirai Mücadele İlaçları Bayii-Kahramanmaraş for help in containing of the pesticides and M.Hakkı ALMA (Kahramanmaraş Sütçü İmam University, Faculty of Forestry, Department of Industrial Engineering of Forestry) for critically reviewing this manuscript.
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