Solubilization of inorganic phosphate by Pseudomonas strains isolated from rice rhizosphere

Abstract

The use of phosphate solubilizing bacteria to mobilize large reserves of insoluble P in soils instead of phosphate fertilizers is an ecologically safe and economically reasonable alternative to assure a sustainable agriculture. In this work, the mineral phosphate solubilization activity was evaluated for 4 Pseudomonas strains isolated from rice rhizosphere. For qualitative tests, different media were used; NBRIP, YED and Pikovskaya (PVK) amended with 0.5% Ca3(PO4)2 and modified PVK medium containing CaHPO4, Ca5HO13P3, FePO4 and AlPO4 as P source. All bacteria were able to dissolve calcium phosphates, while none was surrounded by transparent halos when FePO4 and AlPO4 were the P source. Quantitative test was assessed in PVK substituting glucose by sucrose and galactose. The highest levels of soluble P were recorded in the presence of glucose, while it didn’t exceed 43.64 μg / ml and 19.19 µg / ml in the presence of galactose and sucrose, respectively. Therefore, glucose has been chosen as a carbon source to assay the effect of carbon concentration on P solubilization ability. The level of dissolved P was enhanced with increasing amount of glucose (1%, 2% and 3% (w/v)). In all cases, the P biosolubilisation was accompanied by a significant pH decrease of the liquid media. A significant inverse correlation was found between P concentrations and pH reduction. Thus, acidification of the media seems to be the main mechanism for calcium phosphates solubilization in this study, and these test strains could serve as efficient biofertilizer candidates for improving crop production in low P soils.