Biodegradation of pesticide by microbial isolates from cotton growing agricultural soil

摘要

Pesticide consumption for good agronomical husbandry is increasing at an unprecedented rate in the world, but unmanaged use of pesticides in the third world countries including Pakistan is creating serious environmental concerns. Pesticides are often persistent in soils, enter into food chain, and ultimately reach to humans causing various illnesses. So pesticide remediation bears prime importance. At the current time, biostimulation and bioaugmentation are considered as the most reliable techniques of pesticide biodegradation being eco-friendly. The present study deals with the biodegradation of chlorpyrifos (CP), an organophosphate insecticide. In the 1st phase of experimentation, chlorpyrifos resistant 56 microbial strains from cotton growing agricultural soils, and 24 from sludge of industrial wastewater drain (carrying effluents of pesticide factory) were isolated. These isolates were resistant at 175mgL-1 ndudof CP. In the 2 phase of study, growth potential of these isolates was tested, and 3 isolates (Ct3, Ct27 and WW7) were selected based upon their highest level of CP resistance and growth potential. These isolates were identified as Bacillus cereus (Ct3), Klebsiella oxytoca. (Ct27) and Pseudomonas aeruginosa. (WW7), based on 16S rRNA (ribotyping). Thirdly, biodegradation potential of these isolates was investigated in liquid media and soil. Number of factors like, CP concentration, temperature, pH, carbon sources and inoculum densities were optimized, in order to enhance the percentage and rate of biodegradation. In liquid media, best degradation was exhibited by Bacillus sp. The order of degradation ability was as follows: Bacillus cereus > Pseudomonas aeruginosa > Klebsiella oxytoca udBacillus sp. degradaed 84% of 300mgL-1 CP with inoculum density of 106 CFUml-1 at 30oC and 8.5 pH in 6 days. By increasing inoculum density up to 108 CFUml-1, 100% degradation was achieved in the same time. Whereas, maximum degradation by Pseudomonas sp. was 81% in 15 days (initial concentration, 300mgL-1). Optimum conditions for Pseudomonas sp. were 8 pH, and 30oC. Conversely, maximum degradation efficiency in Klebsiella sp. was up to 79% in 15 days. Among all the 3 carbon sources, glucose proved to be the best in enhancing CP degradation. The order of effectiveness of tested carbon sources is as follows: udGlucose > yeast extract > starch ? no added supplement udThe CP degradation ability of selected isolates was also tested in soil to explore insitu bioremediation possibilities. It was noted that the strain which was potent in liquid media also showed good result in soil. The degradation percentages were 93, 86 and 84 by Bacillus sp., Pseudomonas sp. and Klebsiella sp., respectively. Enhancement potential of different organic amendments was studied and the results revealed the following order: udFarmyard manure > green compost > rice husk > no added organic amendment udLastly, the biodegradation kinetics in liquid media and in soil was calculated. The kinetic data revealed that all the 3 selected strains have the potential for bioremediation and can be used for rapid CP degradation. The present study was thus innovative and highly successful as it provided the eco-friendly solution using indigenous bacteria for countering CP pollution. These strains (Bacillus sp., Pseudomonas sp. and Klebsiella sp.) can be used for soil and ecological restoration.