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Pakistan Journal of Biological Sciences
Year: 2004  |  Volume: 7  |  Issue: 2  |  Page No.: 187 - 196

Organic Acids Production and Phosphate Solubilization by Phosphate Solubilizing Microorganisms (PSM) Under in vitro Conditions

Maliha Rashid, Samina Khalil, Najma Ayub, Sadia Alam and Farooq Latif    

Abstract: After screening a large number of phosphate solubilizing microorganisms (PSM) isolated from rice rhizosphere on the basis of solubilization index (SI) and pH drop ten best bacterial strains (10RB, 54RB, 57RB, 20RB, 21RB, 23RB, 22RB, 13RB, 20RB, 40RB) and three best fungal strains {(Aspergillus flavus(1RF), Penicillium canescens(4RF), A. niger (8RF)}were further analyzed for their acid production ability using HPLC technique. SI of 13 best isolates ranged from 2.16-6.23. SI and change of pH of these isolates showed abrupt changes during seven days of incubation in Pikovskayas agar and broth medium respectively. This drop of pH clearly indicated the production of organic acids. HPLC results confirmed that few PSM isolates produced gluconic, fumaric, succinic, acetic and some unknown organic acids in smaller concentrations while oxalic and citric acids in larger concentrations in their broth medium. Among bacterial strains 10RB produced largest amount of organic acids (2.4747 g-l) while among fungal isolates Aspergillus flavus produced greatest amount of acids (1.835 g-l) than the other two fungal strain. The concentration of glucose when analyzed during HPLC was found less in all the cultural samples as compared to the control (13 g-l) that was without any PSM culture. This shows that all the PSM strains utilized the carbon of glucose for the production of organic acids. The abilities of these efficient strains for phosphorus (P) solubilization and immobilization were also quantitatively investigated in Pikovskayas broth medium. The results showed that all the strains solubilized (0.04-0.1%) and immobilized (0.1-0.4%) P but fungi immobilized more P than bacteria. Correlation among all the parameters showed a positive(r = 0.2, P<0.1) correlation between the organic acid excretion and P solubilization and a negative correlation was found between pH and P solubilization (r =-0.4, P<0.1) and immobilization (r=-0.5, P<0.05).

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