Bioprocess Engineering for Enhanced Zinc Solubilization Through Response Surface Methodology (RSM) by Newly Isolated Rhizobacteria

Zinc (Zn) is an essential micronutrient for optimum plant growth; however, Zn deficiency is widespread in agricultural soils due to the low availability of plant-accessible or soluble forms of Zn. Therefore, harnessing microorganisms’ ability to solubilize Zn can be a significant, environmentally beneficial contribution to sustainable agriculture. This study isolated Zn-solubilizing bacteria (ZnSB) from rhizosphere soil and assessed their capacity to solubilize Zn minerals (ZnO, ZnCO3). Based on qualitative and quantitative evaluation, the highest solubilization efficiency (345%) and 954 ± 46 mg Zn L−1, respectively, were observed in the AEYTU11 strain and were associated with a strong pH decrease in the culture medium. Based on phenotypic characterization and 16S rRNA sequence analysis, strain AEYTU11 was identified as Serratia marcescens. The in vitro solubilization potential of microbes is significantly influenced by process parameters, emphasizing the importance of optimizing the culture conditions to maximize microbial solubilization efficiency. A Box–Behnken design (BBD) experiment with 17 combinations of varying levels of insoluble Zn compound (0.5–1.5 g/L), pH (5–9), and incubation time (3–5 days) was used to model their effects on Serratia marcescens AEYTU11 strain-mediated Zn release from ZnCO3. Following process optimization, the solubilization efficiency of the AEYTU11 strain increased, achieving a maximum Zn solubilization capacity of 1608 ± 51 mg/L. The resulting model had high predictive ability (R2 = 0.9099) and showed that ZnCO3 concentrations were the most significant factor for the amounts of Zn released. In addition to Zn solubilization, AEYTU11 possess potassium (K) solubilization and extracellular enzyme production for protease and cellulose activity. The optimization findings highlight the value of statistical optimization tools for maximizing microbial Zn-solubilizing activity and improving biofertilizer development.