Optimization of culture conditions impacting phosphate solubilization efficiency of bacterial isolates

Abstract

Phosphate solubilizing bacteria (PSB) solubilize phosphorus through secretion of low molecular weight organic acids. This P solubilization is dependent on culture as well as environmental growth conditions. In the present study, five bacteria (Enterobacter cloacae BHUAS1, Bacillus cereus BHUAS2, Bacillus megaterium IESDBHUAS3, Bacillus megaterium IESDBHUAS4, and Bacillus megaterium IESDBHUAS5) were tested to solubilize inorganic P under variable culture conditions comprising different levels of salinity, pH, different sources of inorganic P, variable incubation temperatures and varying initial inoculum sizes under in vitro conditions. The bacterial strains were also checked for simultaneous production of Indole 3- acetic acid (IAA) and P solubilization to test for expression of multifarious beneficial traits under similar conditions. The results revealed that in different sources of insoluble inorganic P, calcium P was preferred source by all the bacterial strains where maximum phosphate solubilization efficiency (PSE) was observed. An exposure of 250 mM salinity was observed to increase PSE for Enterobacter cloacae BHUAS1 and Bacillus cereus BHUAS2 while it lowered the PSE of other isolates. A growth temperature of 30 °C was found suitable for all the isolates to express their highest PSE. In case of pH, Enterobacter cloacae BHUAS1 performed best at pH 7, Bacillus cereus BHUAS2 favoured pH 8, while Bacillus megaterium IESDBHUAS3, Bacillus megaterium IESDBHUAS4, and Bacillus megaterium IESDBHUAS5 preferred pH 9 for expression of their highest PSE. Considering inoculum size a cell density of 3 × 107 cells/mL was found optimum for maximum PSE by the bacterial isolates after 72 h of incubation. HPLC analysis of culture supernatant revealed the presence of several organic acids being secreted by each bacteria revealing their putative mechanism of P solubilization. A simultaneous production of significant amounts of IAA and substantial quantities of soluble P was observed under tested growth conditions. Taken together these results put forward the potential of these isolates to be applied as biofertilizers for P and their consortium based bioformulations may be tested under different agroclimatic conditions. © The Author(s), under exclusive licence to Springer Nature B.V. 2025.