Qualitative and quantitative evaluation of phosphate-solubilizing ability of Lysinibacillus macroides: a climate-resilient biofertilizer candidate for sustainable crop nutrition.

Phosphorus (P) is a critical macronutrient governing plant productivity, however its bioavailability in agricultural soils remains severely constrained due to fixation in insoluble mineral complexes, particularly calcium-bound phosphates in alkaline systems. The low use efficiency of chemical fertilizers, coupled with escalating environmental concerns, necessitates biologically driven strategies for sustainable phosphorus management. In this study, rhizospheric bacteria associated with chili (Capsicum annuum L.) were systematically investigated to elucidate the phosphate-solubilizing potential of Lysinibacillus macroides, an underexplored spore-forming plant growth-promoting rhizobacterium (PGPR). A total of 23 isolates were characterized using integrated qualitative and quantitative assays. All isolates exhibited halo formation on Pikovskaya's agar (1.26-4.70 mm), while NBRIP broth analysis revealed substantial tricalcium phosphate solubilization, reaching up to 30.03 μg mL-1. This activity was consistently associated with pronounced acidification, with pH declining from 7.0 to 3.2-4.1, indicating an acidification-driven solubilization mechanism. A strong inverse correlation between soluble phosphate and pH (r = -0.91, p < 0.01), supported by multivariate analysis, suggests that phosphate mobilization is governed by metabolically regulated acidification dynamics, likely mediated through organic acid production and proton extrusion. Among the isolates, 2.B (L. macroides) and 4.1 (L. fusiformis) exhibited superior solubilization efficiency and were distinctly separated through clustering analysis. Functional validation through seed biopriming demonstrated significant enhancement in germination (96.66%) and Seedling Vigor Index (766.88), establishing a direct linkage between microbial phosphorus mobilization and early plant development. Lysinibacillus macroides maintained significant growth across a broad range of pH (4-10), temperature (15-45°C), and salinity (0.01-0.1% NaCl), demonstrating its inherent tolerance to multiple abiotic stresses and confirming its potential as a climate-resilient plant growth-promoting bacterium. This study provides a quantitative and multivariate demonstration of acidification-coupled phosphate solubilization in Lysinibacillus macroides, integrating biochemical, statistical, and plant-based validation to elucidate a consistent mechanism of phosphorus mobilization. Collectively, the findings establish L. macroides as a resilient biofertilizer candidate for enhancing phosphorus-use efficiency and sustainable nutrient management in chili and other crops.