Effects of pyrroloquinoline quinone on phosphate solubilization by the symbiotic nitrogen-fixing bacterium Bradyrhizobium diazoefficiens

The ability ofRhizobium andBradyrhizobium strains to solubilize phosphate from hydroxyapatite was determined in a medium containing NH4Cl or KNO3. The presence of NH4 + in the medium resulted in higher solubilization of phosphate as compared to the presence of KNO3, with the exception ofR. leguminosarium bv. viceae strain TAL 1236 and 1402 which solubilized comparable amounts of phosphate in a medium containing either KNO3 or NH4Cl. These results suggest that the strains employ two different mechanisms for phosphate solubilization, one depending on the presence of NH4 +, the other not requiring its presence. Temperature and aeration (O2 demand) optima were 30°C and 4.2 Hz (shaking frequency), respectively. In nonsterile soil the tested strain (R. meliloti TAL 1236) was very effective in solubilizing rock phosphate.