Effect of pesticides and biotransformation of pesticides by soil microbes
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The extensive use of pesticides in agriculture to eliminate pest, such as grasshoppers, has resulted in the accumulation of pesticide residues in soil and has altered the soil microbial communities by favouring the growth of those pesticide degrading organisms. Due to the xenobiotic features of pesticides, pesticides in soil can be persistent in the environment and eventually enter the food chains, which cause reproductive failure in birds and even cancer in humans (Arias-Estevez et al. 2008). Pesticides can inhibit the growth of certain microorganisms by interfering with enzymatic activity. For example, one type of organophosphate pesticide can inhibit nitrogenase activity involved in nitrogen fixation (Hussain et al. 2009). The inhibition of nitrogenase activity can reduce the amount of nitrogen available to plants, thus reducing crop yields. The abundance of pesticide applied to agricultural soils (5.1 billion pounds/ annually in US) has created selective pressures for microorganisms that have the ability to use pesticides in soil as sources of energy and nutrients. This can be useful in bioremediation to detoxify contaminated soil.
Factors affecting the bioavailability of pesticides
The bioavailability of pesticides to microorganisms is determined by several environmental factors and chemical properties of pesticides. Volatilization, leaching, plant uptake, run-off and sorption of pesticides can influence the transport of pesticides from soil to air, from soil to groundwater, or within soil, thus affecting the availability of pesticides for degradation by soil microbes (Arias-Estevez et al. 2008). Sorption of pesticides is the most important factor that can be influenced by soil pH and organic matter and clay contents in soil (Arias-Estevez et al. 2008). For example, higher pH indicates that the soil is more acidic, thus attracting basic and neutral pesticides. Organic matter and clay possess weak negative charges and, therefore, attract weakly acidic and neutral pesticides. Although increase in sorption can limit the amount of pesticides available to soil microbes, the degradation of pesticides by soil microbes does not decrease dramatically (Arias-Estevez et al. 2008).
Recently, a few bacterial strains have been observed to have the ability to degrade one of the very toxic pesticides, organochlorine. Organochlorine pesticides are lipophilic compounds that tend to bio-magnify along the food chains through accumulation in body fats (Cruz et al. 2003). Hexachlorocyclohexane (HCH), one type of organochlorines, has four isomers, α, β, γ, and δ (Gupta et al. 2000). The insecticide, γ- HCH, has been mostly used among the four isomers due to its fast degradation rate under aerobic condition (Chaudhary et al. 2006). Under aerobic condition, Sphingomonas paucimobilis has all the enzymes necessary to degrade HCH to acetyl-CoA (see Figure 2) (Camacho-Pérez et al. 2012). Therefore, S. paucimobilis is able to use Hexachlorocyclohexane as a source of energy and nutrients.
Parathion is one of the most studied organophosphorous pesticides (Singh and Walker 2006). Organophophorous pesticides can inhibit the activity of enzyme acetylcholine esterase, involved in nervous system that disrupts the transmission of nerve impulse (Kanekar et al. 2004). Pseudomonas sp. is proposed to hydrolyze parathion to yield p-nitrophenol and diethyl thiophosphate (Singh and Walker 2006). Furthermore, these microorganisms can degrade p-nitrophenol to p-benzoquinone by using monooxygenase (Singh and Walker 2006). Further degradation of p-benzoquinone produces unidentified intermediates that can enter TCA cycle; therefore, parathion can act as a source of carbon and nitrogen for Pseudomonas sp. (Singh and Walker 2006). Pseudomonas stutzeri is able to degrade parathion co-metabolically to diethyl thiophosphate and p-nitrophenol (Kanekar et al. 2004); however, it cannot use p-nitrophenol as a source of energy. Interestingly, Pseudomonas aeruginosa can use p-nitrophenol made by P. stutzeri as a source of carbon and energy (Kanekar et al. 2004).
Hexachlorocyclohexane Degrading Bacteria
Several bacteria that are able to degrade hexachlorocyclohexane under aerobic condition have been isolated and characterized from the HCH contaminated soil (Gupta et al. 2000). Only some organisms have fully proposed mechanisms that describe the process of HCH degradation (Camacho-Pérez et al. 2012). • Bacillus circulans • Bacillus brevis • Sphingomonas paucimobilis
Parathion Degrading Bacteria
Pseudomonas sp. is one major organism found to degrade and use parathion as a source of energy and nutrients (Singh and Walker 2006). • Pseudomonas stutzeri • Pseudomonas aeruginosa
Conditions for degrading parathion
Different conditions are examined to determine the best environment for increasing the efficiency of p-nitrophenol degradation by a common parathion degrader, Pseudomonas aeruginosa (Zheng et al. 2009). The addition of ammonium chloride as a nitrogen source increases the p-nitrophenol degradation, but the addition of glucose as a carbon source decreases the p-nitrophenol degradation (Zheng et al. 2009).
Effect of pesticide on microbial community composition
Different pesticides have been shown to promote or inhibit the growth of certain soil microorganisms in either aerobic or anaerobic conditions (Lo 2010). This may be helpful to identify possible pesticide degraders and the efficiency of the degradation of pesticides.
 Arias-Este´vez, M., Lo´pez-Periago, E., Martı´nez-Carballo, E., Simal-Ga´ndara, J., Mejuto, J., and Garcı´a-Rı´o, L. “The mobility and degradation of pesticides in soils and the pollution of groundwater resources.” Agriculture, Ecosystems and Environment, 2008, 123: 247–260
 Camacho-Pérez, B., Ríos-Leal, E., Rinderknecht-Seijas, N., and Poggi-Varaldo, H.M. “Enzymes involved in the biodegradation of hexachlorocyclohexane: A mini review.” Journal of Environmental Management, 2012, 95: 5306-5318
 Chaudhary, P., Kumar, M., Khangarot, B.S., and Kumar, A. “Degradation and detoxification of hexachlorocyclohexane isomers by Pseudomonas aeruginosa ITRC-5.” International Biodeterioration & Biodegradation, 2006, 57:107–113
 Cruz, S., Lino, C., and Silveira, M. I. “Evaluation of organochlorine pesticide residues in human serum from an urban and two rural populations in Portugal.” The Science of the Total Environment, 2003, 317:23-35
 Gupta, A,. Kaushik, C.P., and Kaushik, A. “Degradation of hexachlorocyclohexane (HCH; a, b, g and d) by Bacillus circulans and Bacillus brevis isolated from soil contaminated with HCH.” Soil Biology & Biochemistry, 2000, 32:1803-1805
Hussain, S., Siddique, T., Siddique, M., Arshad, M., and Khalid, A. “Impact of Pesticides on Soil Microbial Diversity, Enzymes, and Biochemical Reactions.” Advances in Agronomy, 2009, 102: 159-200
 Kanekar, P.P., Bhadbhade, B., Deshpande, M.N., and Sarnaik, S.S. “Biodegradation of organophosphorous pesticides.” Indian National Science Academy, 2004, 70: 57-70
 Singh, B.K., and Walker, A. “Microbial degradation of organophosphorus compounds.” Federation of European Microbiological Societies, 2006, 30: 428-471
 Zheng, Y., Liu, D., Xu, S., Yuan, Y., and Xiong, Li. “Kinetics and mechanisms of p-nitrophenol biodegradation by Pseudomonas aeruginosa HS-D38.” Journal of Environmental Sciences, 2009, 21: 1194-1199.
 Lo, C. C. “Effect of pesticides on soil microbial community.” Journal of Environmental Science and Health, 2010, 45: 348-359.