Isolation and Characterization of Thermo-tolerant Phosphate-Solubilizing Bacteria From a Phosphate Mine and Their Rock Phosphate Solubilizing Abilities

A study of rock phosphate (RP) mines, rich in R2O3 (Fe2O3 and Al2O3) contents, was done to explore the possibilities of isolating and characterizing thermo-tolerant phosphate solubilizing bacteria (PSB) having the ability to solubilize ferric phosphate (Fe-P) and aluminium phosphate (Al-P) efficiently. One hundred and ten thermo-tolerant PSB were isolated employing Pikovskaya (PVK) broth amended with RP, and six of these isolates were selected for further study. The phosphate solubilizing efficiency (PSE) was determined in PVK broth containing tri-calcium phosphate (TCP), hydroxyapatite (Hy-P), Fe-P, Al-P or RP. These isolates showed solubilization of all the phosphorus (P) sources used in this study, particularly Fe-P, Al-P and RP. The main mechanism of P-solubilization seemed to be mediated by the production of organic acids leading to medium acidification. HPLC analysis revealed production of multiple organic acids (oxalic, citric, malic, succinic, formic, acetic and gluconic acids) in the culture supernatant along with some unknown acids. Based on the partial sequence analysis of 16S rRNA genes, these thermo-tolerant isolates were found to belong to two bacterial genera, Brevibacillus and Bacillus.