Bioremediation of Thiamethoxam by Plant Growth Promoting Rhizobacterium, Pseudomonas fluorescens in Combination with Biostimulants and Effect on Soil Enzymatic Activity

Thiamethoxam, a systemic novel insecticide, is a foliar and seed treatment formulation. However, studies on the potential microorganisms for its bioremediation are least explored. Similarly, the sublethal ill effects of thiamethoxam on native soil microflora have also not been studied. Hence, Pseudomonas fluorescens, a plant growth-promoting rhizobacterium, was evaluated for its ability to bioremediate thiamethoxam, known for its relatively longer half-life in soil. Survivability of P. fluorescens was determined in the minimal salt and half-strength nutrient broth media contaminated with thiamethoxam in the range of 5-100 mg/L for 21 days, and maximum growth in terms of optical density was observed on 3rd day after inoculation at 10 mg/L (0.86-1.00 AU) in half-strength nutrient broth. The bacterium's ability to bioremediate thiamethoxam in the presence of biostimulants such as vermicompost and vesicular-arbuscular mycorrhiza (VAM) was evaluated through soil microcosms studies conducted under laboratory conditions for 21 days. Soil microcosms fortified with thiamethoxam at 10 mg/L were treated with P. fluorescens alone and in combination with vermicompost and VAM, and the half-life of thiamethoxam was calculated from residual concentration in soil extracts on 0, 3, 7, 14, and 21days. The results indicated that the combined application of VAM with P. fluorescens exhibited the shortest halflife of thiamethoxam (31.5 days) compared to natural attenuation (86.6 days). The impact of thiamethoxam on soil microbial activity was assessed by the enzymatic activity of acid/alkaline phosphatases, dehydrogenases, and β-D glucosidases in soil, and the results revealed the adverse effect of thiamethoxam on soil microflora with relatively lower measured values in contaminated soils for all the enzymes under the study compared to uncontaminated control.. KEYWORDS :Bioremediation, Biostimulants, Enzyme activity, PGPR, Pseudomonas, fluorescens, Thiamethoxam