The Amount of Phosphate Solubilization Depends on the Strain, C-Source, Organic Acids and Type of Phosphate

Although production of organic acids (OAs) is usually mentioned as the main mechanism of phosphate solubilization, the relationship between carbon sources (C-sources) and OAs produced during phosphate-solubilization by microorganisms is still poorly understood. We evaluated the influence of different C-sources on FePO4 center dot 2H(2)O and Ca-3(PO4)(2) solubilization by bacteria and on the identity/quantity of the OAs produced. Our results showed that the amount of phosphate solubilization depends on the strain, C-source, OAs, and type of phosphate. Among the five strains under study isolated from cowpea nodules (Rhizobium tropici strain UFLA 03-08, Acinetobacter sp. strain UFLA 03-09, Paenibacillus kribbensis strain UFLA 03-10, P. kribbensis strain UFLA 03-106, and Paenibacillus sp. strain UFLA 03-116), three of them solubilized Ca-3(PO4)(2) in all C-sources. The influence of C-sources on Ca-3(PO4)(2)-solubilization increased in the following order: cellulose < lactose < mannitol < glucose. A significant positive correlation between the amount of phosphorus solubilized from Ca-3(PO4)(2) and the concentration of total OAs in the presence of glucose and mannitol was observed for these three strains. In the presence of glucose, the highest solubilization rates are associated with high concentrations of tartaric acid, and in the presence of mannitol, are associated with maleic acid. Only one strain produced OAs in the medium with lactose and Ca-3(PO4)(2), but there was no OAs in the medium containing cellulose. Despite the production of OAs, albeit in small concentrations, in all the C-sources investigated, FePO4 center dot 2H(2)O-solubilization was not observed. Thus, a relationship among C-sources, OAs, and phosphate solubilization was not always verified.