Bioremediation of Pesticides with Microbes: Methods, Techniques and Practices
Introduction: Pesticides in the environment
The pesticide has empowered human civilization not only to secure agricultural produces including food, fodder, and fibre during crop cultivation, but also over the storage period. The application of pesticides has played a significant role to secure the supply of food for the growing population of this planet by controlling the pests. Ina broad definition, the term ‘pest’ includes all biotic sources harming agricuhural products either during production or storing phase and, generally it covers various insects, pathogens, and weed along with some other damage-causing organisms like nematodes, rodents, etc. It has been estimated that the major loss of agricultural products is nearly 14, 13, and 13%, which is contributed by insects, pathogens, and weeds, respectively. Non-application of pesticides may increase loss in agricultural production up to 78, 54, and 42% for fruits, vegetables, and cereals, respectively (UN 2015). Further, global wanning under the scenario of climate change is assumed to add momentum in the frequency and quantum of pests and subsequently a higher loss of agricultural produce. It has been reported that a one-degree increase in temperature may result in 10-25% more loss in major crops like rice, wheat, and maize (Deutsch et al. 2018). Hence, pesticides have gained attention to protect agricultural produce fr om various pests, and pesticide sciences have gone through various changes to develop pesticide molecules of better performance against pests. Pesticides can be grouped mto five generations, which started with first- generation products (before 1940), mostly broad-spectrum inorganic compounds and few botanical extracts, followed by 2nd generation (during 1940-1960) products covering organochlorines (DDT, HCH, Mirex, Toxaphene, cyclodienes, etc.), organophosphates and carbamates, 3rd generation (after the 1970s) candidates including molting hormones, chitin synthesis inhibitors, juvenile hormones, etc., 4th generation products of antifeedants and pheromones and 5tli generation members like novel natural products, brain hormone antagonists, etc. Without pesticide applications, the loss in production of major crops like wheat, maize, rice, soybeans, potato, and cotton, etc. may go to the extent of 20 to 40% (Oerke 2006). Now, the application of pesticides has become a mandatory practice in the modem intensive agricultural system. Presently, the total pesticide production across the world has increased from 0.2 million tonnes to 5 million tonnes with an annual growth rate of 11%. As per one report, Asia leads in the average pesticide usage (3.62 kg/lia), followed by America (3.39 kg/ha), Europe (1.67 kg/ha), Oceania (1.17 kg/lia) and Africa (0.31 kg/lia) (FAOSTAT 2017). Out of various pesticides, the share of synthetic pesticides from the 2nd and 3rd generation is still more than 4th or 5th generation pesticides. Indiscriminate and injudicious applications of pesticides have polluted the entire ecosystem.
Researchers have found that only 7-10% of applied pesticides reach target/pest, and more than 90% of pesticides reach mostly soil (Figure 1). Soil acts as a sink for received pesticides and pesticides undergo various transformation processes like sorption, volatilisation, degradation (photo, chemical, and microbial) and leaching to groundwater (Ghosh and Singh 2013). Some portions of soil-sorbed pesticide reach open water bodies by surface runoff, soil erosion, or subsurface drainage, and may come back again to the same/new soil site while irrigating the crops with contaminated water (Carvalho et al. 2003). Hence, open water bodies act as the second sink of pesticide residues, after soil. Some part of pesticides which are prone to volatilisation/fumigation may travel long distances and contaminate new areas by condensation/precipitation process. Researchers have tracked toxapliene residues in Canada’s Great Lake which traveled from South USA (Li and Jin 2013) or residues of chlorpyriplios in Ar ctic ice which were sprayed in Central America (Garbarino et al. 2002). Indiscriminate use of pesticides has resulted in contamination of various components of the environment and several reports across the world have drawn attention. Pesticides may adversely affect the population dynamics, sex ratio, size at birth, swimming pattern and physiology of phytoplankton (Ikram and Shoaib 2018) and zooplanktons (Hanazato 2001), morphological and behavioural abnormalities in invertebrates dwelling in soil (Frampton et al. 2006) and water
Figure 1. Schematic presentation of pesticide dynamics in the environment.
(Schafer et al. 2012), various morphological defects in vertebrates including fishes (Allison et al. 1964, Robinson et al. 2015), buds (Goulsou 2014), reptiles (Khan and Law 2005) including humans. In humans, pesticides may pose risks of respiratory diseases, cancer, tumor, neurotoxicity, child development, fertility, reproduction (Osman 2011, Mostafalou and Abdollahi 2013), etc. This is the result of severe contamination of soil, air and water, and various trophic levels of the food chain with pesticide residue (Burnett and Welford 2007, Chourasiya et al. 2015, Witczak and Abdel-Gawad 2014, Nag and Raikwar 2011). Lately. Bliaduri et al. (2018) have also comprehensively reviewed the common and potential bioindicators under soils polluted by pesticides.
Two pre-emergence herbicides, pendimethalin and oxyfluorfen, were found to stimulate soil microbial biomass carbon, fluorescein diacetate hydrolysing activity, alkaline phosphatase and ammonification rates in peanut grown soil, while dehydrogenase activity, acid phosphatase, nitrification rate and available phosphorous was adversely affected (Saha et al. 2015). Further, increased soil alkaline phosphatase and decreased acid phosphatase activities was observed after applying two post-emergence herbicides (imazethapyr and quizalofop-p-ethyl) while stimulated soil ammonification and nitrification rates indicates that these herbicides had distinct effects on nitrogen and phosphorus dynamics in soil both soil process (Saha et al. 2016a). In another study, tebuconazole application at field rate (FR) and 2FR resulted in a short-lived and transitory toxic effect while the disturbance was persistent at higher rate (10FR). It showed stimulating effect on soil ammonification and nitrification rates, and microbial biomass C, but was more toxic to soil ergosterol which is an indicator of the presence of viable fungi (Saha et al. 2016b).
