SOLUBILIZATION OF INORGANIC-PHOSPHATE BY RHIZOBIUM

A large number of strains of Rhizobium were able to solubilize the insoluble phosphate compound, hydroxyapatite, in liquid culture. Solubilization of hydroxyapatite by Rhizobium was not mediated by an enzyme but acidity developed in the cultures was involved in the process. An inverse relationship between the level of soluble phosphate and medium pH was evident. The ability to solubilize hydroxyapatite varied among the strains. In a medium without NH4+, some of the strains showed better activity than when NH4+ was present, suggesting involvement of different mechanisms for phosphate solubilization. R. meliloti SU 47 produced 2-ketogluconic acid along with an unidentified acid in the medium containing NH4+. 2-Ketogluconic acid was identified as the major factor in inorganic phosphate solubilization. Initial presence of soluble phosphate in the medium had no discernible influence on the extent of hydroxyapatite solubilization. Initial presence of calcium reduced solubilization of phosphate and addition of EDTA to stationary phase cultures caused an increase in the level of soluble phosphate.