CONICET | Buscador de Institutos y Recursos Humanos

Phosphorus (P) is one of the major essential macronutrients for biological growth and development. Most agriculturalsoils contain large reserves of P, a considerable part of which accumulates as a consequence of regular applications of P fertilizers. However, a greater part of soil phosphorus is present in the form of insoluble phosphates and hence cannot be utilized by plants. Phosphate solubilizing microorganisms (PSM) play an important role in supplementing phosphorus to the plants, allowing a sustainable use of phosphate fertilizers. In the case of calcium phosphates, a significant body of evidence has been developed to show that Gram negative bacteria exhibiting superior mineral phosphate solubilizing capabilities utilize the direct oxidative pathway. This pathway (also called nonphosphorylating oxidation) produces gluconic acid and 2-ketogluconic acid directly in the periplasmic space. These strong organic acids can dissolve poorly soluble calcium phosphates such as hydroxyapatite. G. diazotrophicus strains have been reported to posses the ability to solubilize insoluble phosphates. It has been reported that this microorganism posses a pyrroloquinoline quinone (PQQ)-linked glucose dehydrogenase responsible for the periplasmic conversion of glucose into gluconate. However the relationship between poorly soluble mineral phosphates solubilization and gluconic acid production are not yet completely understood. In the present work it was checked the ability of G. diazotrophicus to solubilize insoluble inorganic phosphates. Batch cultures of G. diazotrophicus were carried out in erlenmeyer flasks using a defined minimal medium with 1.32 g.l-1 (NH4)2SO4 and glucose (20 g.l-1) as C-source. When the organism was grown under N2-fixing conditions (BNF) the (NH4)2SO4 concentration was diminished to 0.132 g.l-1. Soluble calcium phosphates, Ca3(PO4)2, hydroxyapatite and rock phosphate (20 % P2O5) were employed as P sources. It is shown that soluble phosphate concentration in the medium was significantly increased by the presence of organic acids of bacterial origin. These acids solubilized not only insoluble phosphates but also hydroxyapatite. Phosphate solubilization was directly related to the pH decrease caused by growth of G. diazotrophicusin the medium containing glucose as carbon source.