Isolation, Solubilization of Inorganic Phosphate, and Production of Organic Acids by Individual and Co-inoculated Microorganisms

Phosphate solubilizing microorganisms (PSMs) mineralize phosphate (PO₄³⁻) to make phosphorous available for plant uptake. This study is designed to isolate the PSMs from the subtropical environmental samples. Total 35 bacterial and 18 fungal isolates were found to be effective P solubilizers depend on their ability of P solubilization that were determined qualitatively and quantitatively using National Botanical Research Institute Phosphorus media. Efficient PSMs that were solubilizing phosphate in maximum amount were identified as Pseudomonas aeruginosa, Serratia marcescens, Pseudomonas mosselii, Bacillus licheniformis, Bacillus subtilis, Bacillus species, Aspergillus flavus, Aspergillus foetidus, Aspergillus niger, Aspergillus tubingensis, and Penicillium chrysogenum. Among them, A. tubingensis was observed as the most efficient phosphate solubilizing strain. For the first time, combined phosphate solubilization activity of A. tubingensis with other efficient phosphate solubilizers was checked and found that A. tubingensis and P. aeruginosa acted synergistically and solubilized phosphate in increased amount. High-performance liquid chromatography results revealed that A. tubingensis produced variety of organic acids including succinic, gluconic, oxalic, and citric acids. Scanning electron microscopy results confirmed the presence of firmly attached large fungal hyphae on tri-calcium phosphate (Ca₃(PO₄)²). Thus, the results suggest that the use of these PSMs either single or combined as bioinoculants will help to increase the availability of soluble phosphorous.