Exploring the Phosphate Solubilizing Potential and Acid-Alkaline Phosphatase Activity of Two Multitrait Heavy Metal Tolerant Bacterial Strains Isolated from North Gujarat, India

A study was conducted to isolate and characterize phosphate solubilizing bacteria (PSB) from soils of various locations in North Gujarat. A total of 40 PSB isolates were obtained, exhibiting a phosphate solubilizing index (PSI) ranging from 2.11 to 4.33. Interestingly, among 40 PSB isolates, a subset of 16 displayed an additional trait of potassium solubilization, as evidenced by their potassium solubilizing index (KSI) values falling within the range of 2.10 to 3.10. These 16 PSB isolates were screened for tolerance to heavy metal stressors, namely CdCl2, HgCl2, Pb(NO3)2, and ZnSO4, spanning a concentration gradient from 0.1 mM to 10 mM. Remarkably, among the tested isolates, two, namely, D4 and G2, exhibited exceptional resilience against the heavy metal stressors, as indicated by their minimum inhibitory concentration (MIC). Specifically, isolate D4 showed MIC values of 5 mM, 0.2 mM, 18 mM, and 12 mM, respectively, against CdCl2, HgCl2, Pb(NO3)2, and ZnSO4. For isolate G2, the corresponding MIC values were 6 mM, 0.2 mM, 21 mM, and 11 mM. The bacterial isolates were molecularly identified as strains Klebsiella pneumoniae PHmD4 (abbreviated as D4) and Enterobacter chuandaensis PHmG2 (abbreviated as G2), based on sequence homology obtained from 16S rDNA gene sequencing and subsequent BLAST analysis. In Pikovskaya’s broth, K. pneumoniae PHmD4 and E. chuandaensis PHmG2 had phosphate solubilization values of 4.30 μ moles mL⁻¹ and 3.51 μmoles mL⁻¹, respectively, with terminal pH values of 3.06 and 4.07. Enzymatic activity assays conducted after 96 hours of incubation in Pikovskaya’s broth revealed that the highest acid phosphatase activity was 21.62 U mL⁻¹ for K. pneumoniae PHmD4 and 21.09 U mL⁻¹ for E. chuandaensis PHmG2. The highest alkaline phosphatase activity was 5.85 U mL⁻¹ and 6.38 U mL⁻¹, respectively. In the tomato seed germination study, both non-bacterized and bacterized tomato seeds achieved a 100% germination rate. However, increasing metal concentrations diminished the germination percentage of heavy metal-treated seeds. Interestingly, bacterization with K. pneumoniae PHmD4 and E. chuandaensis PHmG2 ameliorated germination under cadmium stress but had no mitigative effect under mercury and zinc stress conditions.