Date Published: June 24, 2016
Publisher: Springer Berlin Heidelberg
Author(s): M. Supreeth, M. A. Chandrashekar, N. Sachin, N. S. Raju.
The application of pesticides in agricultural fields not only reaches the target pests but also with soil where it interacts with soil microorganisms resulting in change of microbial diversity. Chlorpyrifos (CP) is one such organophosphorous insecticide most widely used against various insects, termites, and beetles throughout the globe. In the present work, the effect of CP on soil microbial population was assessed by the cultivable method. The fertile soil which does not have a history of any pesticide application was treated with 100 and 200 µg/g of CP along with control which received only sterile water and incubated for 1, 7, and 14 days. The soil amended with the insecticide showed decrease in the number of colony forming units (CFU) of bacteria and fungi. However, Streptomyces sp. HP-11 which tolerated high concentration and also inhibited fungal population was further selected for biodegradation studies. After 14 days of incubation in Mineral salt media (MSM), the strain HP-11 biotransformed CP into 3, 5, 6-trichloro-2-pyridinol (TCP) and Diethyl Phosphorothioate (DETP), and its formation was confirmed by the m/z peak of LC–MS analysis, which was later metabolized to unknown polar metabolites. The results obtained highlights that the application of chlorpyrifos favored the Actinomycete growth in the soil, thereby inhibiting other microorganisms and the strain HP-11 harbors metabolic pathway for detoxification of CP and its hydrolysis product TCP into polar metabolites, thus suggesting the strain HP-11 will be a potential bioaugmenting agent for the bioremediation of chlorpyrifos contaminated soil and water.
Pesticides are the synthetic compounds used to protect agricultural crops from disease causing pests. The applied pesticide will reach target pests by only 1 % and the remaining will come into contact with soil, where they undergo a variety of transformations that provide a complex pattern of metabolites (Andreu and Pico 2004). Fertility of soil is dependent on the soil microbial richness and diversity. As a natural decomposers, microorganisms enrich the soil nutrients, and improve soil texture and water holding capacity. Researchers have shown that the pesticides are always having their effect on the soil microorganisms. Some pesticides stimulate the growth of soil microorganisms and some have depressive effects or no effects. However, the relationship of different structures of pesticides on the growth of soil microorganisms is not easily predictable (Lo 2010).
The applied insecticide persists in the soil for long periods and has negative impacts on soil microbial flora, killing or inhibiting certain specific groups of microorganisms (Araujo et al. 2003). In the present work, the response of soil microorganisms to Chlorpyrifos treatment at 100 and 200 µg/g concentrations was variable in alteration in microbial diversity of dominant Actinomycete species. The large numbers of microorganisms inhibited were fungi, due to the formation of e TCP through hydrolysis by Streptomyces sp. HP-11. This is to our knowledge is the first report that describes the effect of chlorpyrifos on soil fungal diversity. The application of chlorpyrifos favored Actinomycete growth, thereby inhibiting other microorganisms. However, Martinez-Toledo et al. (1992) had shown that Chlorpyrifos at concentration 10–300 µg/g significantly decreased aerobic dinitrogen fixing bacteria and also dinitrogen fixation but no effect on fungi and denitrifying bacteria. This may be due to pesticides, application results in decrease in certain groups of soil microorganisms and also increase in the population of certain resistant and dominant microorganisms, Kalia and Gosal (2011).
The applied insecticide persists in the soil for a long period and has negative impacts on soil microbial flora, resulting in the change of microbial diversity. In the present work, the effect of organophosphorous insecticide chlorpyrifos (CP) on soil microbial population was assessed by cultivable method. A number of microorganisms, such as fungi and bacteria, were inhibited by CP. The application of CP favored the Actinomycete growth in the soil, thereby inhibiting other microorganisms. Biotransformation studies showed that Streptomyces sp. HP-11 could degrade both CP and TCP, which can be used to clean up chlorpyrifos contaminated sites.