Soil bacteria showing a potential of chlorpyrifos degradation and plant growth enhancement

Abstract

Background: Since 1960s, the organophosphate pesticide chlorpyrifos has been widely used for the purpose of pest control. However, given its persistence and toxicity towards life forms, the elimination of chlorpyrifos from contaminated sites has become an urgent issue. For this process bioremediation is the method of choice.

Results: Two bacterial strains, JCp4 and FCp1, exhibiting chlorpyrifos-degradation potential were isolated from pesticide contaminated agricultural fields. These isolates were able to degrade 84.4% and 78.6% of the initial concentration of chlorpyrifos (100mgL(-1)) within a period of only 10 days. Based on 16S rRNA sequence analysis, these strains were identified as Achromobacter xylosoxidans (JCp4) and Ochrobactrum sp. (FCp1). These strains exhibited the ability to degrade chlorpyrifos in sterilized as well as non-sterilized soils, and were able to degrade 93-100% of the input concentration (200mgkg(-1)) within 42 days. The rate of degradation in inoculated soils ranged from 4.40 to 4.76mg(-1)kg(-1)d(-1) with rate constants varying between 0.047 and 0.069d(-1). These strains also displayed substantial plant growth promoting traits such as phosphate solubilization, indole acetic acid production and ammonia production both in absence as well as in the presence of chlorpyrifos. However, presence of chlorpyrifos (100 and 200mgL(-1)) was found to have a negative effect on indole acetic acid production and phosphate solubilization with percentage reduction values ranging between 2.65-10.6% and 4.5-17.6%, respectively. Plant growth experiment demonstrated that chlorpyrifos has a negative effect on plant growth and causes a decrease in parameters such as percentage germination, plant height and biomass. Inoculation of soil with chlorpyrifos-degrading strains was found to enhance plant growth significantly in terms of plant length and weight. Moreover, it was noted that these strains degraded chlorpyrifos at an increased rate (5.69mg(-1)kg(-1)d(-1)) in planted soil.

Conclusion: The results of this study clearly demonstrate that the chlorpyrifos-degrading strains have the potential to develop into promising candidates for raising the productivity of crops in pesticide contaminated soils.

Keywords: Achromobacter; Bioremediation; Chlorpyrifos; IAA; Phosphate solubilization.

Fig. 1

Degradation of chlorpyrifos (•-•) during growth (▴-▴) of bacterial strains in mineral salt medium at concentration of 100 mg L⁻¹. Error bars represent SD, and values are means of three replicates.

Fig. 2

Degradation dynamics of chlorpyrifos in different soil treatments with initial CP concentration of 200 mg kg⁻¹ soil. Symbols: (◆) sterilized soil (SS) or SS inoculated with CP degrading strains, (•) non-sterilized soil (nSS) or nSS inoculated with CP degrading strains, and (▴) planted soil (PS) or PS inoculated with CP degrading strains. Values are the means of three replicates and error bars represent SD.

Fig. 3

Growth experiment with V. unguiculata in soil supplemented with CP (200 mg kg⁻¹). Control, without CP and bacterial inoculation; CP, no inoculum; CP + JCp4, inoculated with A. xylosoxidans JCp4 and CP + FCp1, inoculated with Ochrobactrum sp. FCp1.