Isolation and Characterization of Phosphate Solubilizing Bacteria from Phosphate Solid Sludge of the Moroccan Phosphate Mines

Although phosphorus (P) reserves in the soil are important, orthophosphate ions are the only forms of P that can be used by plants. The specificity of phosphate solubilizing bacteria (PSB) to convert insoluble forms of P to an available form Pi is an important trait to reactivate immobilized phosphorus. Therefore, the use of PSB as inoculants increases the P uptake by plants.

The present study aims at isolating and selecting the phosphate solubilizing bacteria from Phosphate Solid Sludge (PSS) in order to improve the agronomic efficacy of P fertilizers.

The strains isolated from 7 samples of phosphate sludge were tested for their ability to solubilize phosphate in NBRIP medium with Tricalcium Phosphate (TCP) as the sole source of phosphorus, strains with a high solubility index (PSI) were selected. The isolates obtained have been studied for their ability to solubilize TCP quantitatively, and their phosphate solubilizing activity was estimated by the vanadate-molybdate method. The production of Indol-1-Acetic Acid (IAA), Siderophore, and Hydrogen Cyanide (HCN) by the isolates was tested. The best strains have been identified based on 16S rRNA gene sequence comparisons.

27 strains were isolated on selective medium NBRIP from Phosphate solid sludge. They all showed significant different abilities to solubilize P. CB19 was the most efficient strain in NBRIP agar with SI= 4.79; BM11 was the most efficient strain in NBRIP liquid medium releasing 174.33μg/ml soluble P. Based on 16S rRNA gene sequence comparisons, four genera were identified: Pseudomonas, Serratia, Pantoea and Enterobacter.

The PSB isolates varied in their Phosphate solubilizing efficiency, IAA production, Siderophore, and HCN.