Biosynthesis of auxin under cr (VI) stress conditions by thermophilic Brevibacillus borstelensis SSAU-3T.

Rapid industrialization, technological advancement, excessive use of chemical fertilizers, and ongoing climatic fluctuations have collectively imposed severe stress on global agricultural productivity. Among the various environmental pollutants, hexavalent chromium (Cr (VI)) poses a substantial threat to plant growth and soil fertility due to its high toxicity, mobility, and bioavailability. Cr (VI) disrupts the biosynthetic pathways of essential phytohormones, particularly auxins, by inhibiting enzymatic activities and cellular signaling mechanisms, ultimately leading to reduced crop yield and impaired physiological functions. To mitigate such effects, the exploration of robust microbial candidates capable of tolerating heavy metal stress while promoting plant growth is imperative. In the present study, a novel thermophilic bacterium, Brevibacillus borstelensis SSAU-3T, was isolated and characterized for its dual ability to withstand Cr (VI) toxicity and synthesize auxins under elevated temperature conditions. The strain demonstrated significant auxin production in the presence of Cr (VI) concentrations up to 20 ppm, beyond which a gradual decline was observed. Optimization studies revealed that maximum auxin synthesis occurred at pH 7.0, temperature 55 °C, tryptophan concentration of 1%, and an incubation period of six days, while salinity exhibited negligible effects up to 100 g/L. Among the various nutrient sources tested, lactose and tryptone were identified as the most effective carbon and nitrogen sources, respectively, for optimal auxin synthesis. The auxins produced were extracted using solvent partitioning and analyzed via Thin Layer Chromatography (TLC), which revealed variation in auxin profiles depending on the nutritional composition of the growth medium. Further confirmation and structural elucidation were achieved using Gas Chromatography-Mass Spectrometry (GC-MS) and Fourier Transform Infrared Spectroscopy (FTIR), validating the synthesis of indole-based auxin derivatives and associated metabolites. This study highlights the biotechnological potential of B. borstelensis SSAU-3T as a thermophilic, Cr (VI)-tolerant, auxin-producing microorganism with direct applications in sustainable agriculture. Its use as a bioinoculant in chromium-contaminated or high-temperature soils offers an eco-friendly and resilient strategy for restoring soil health and enhancing crop productivity in stress-prone agroecosystems.