Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 6 4 2014 08 15 285 295 EN Rahim Nosrati Molecular Microbiology Research Center (MMRC), Shahed University, Tehran, I.R. Iran AND Department of Biology, Faculty of Science, Shahed University, Tehran, I.R. Iran. Parviz Owlia Molecular Microbiology Research Center (MMRC), Shahed University, Tehran, I.R. Iran. Horieh Saderi Molecular Microbiology Research Center (MMRC), Shahed University, Tehran, I.R. Iran. Iraj Rasooli Molecular Microbiology Research Center (MMRC), Shahed University, Tehran, I.R. Iran AND Department of Biology, Faculty of Science, Shahed University, Tehran, I.R. Iran. Mohammad AliMalboobi National Institute of Genetic Engineering and Biotechnology (NIGEB), Tehran, I.R. Iran. 2015 10 16 Background and Objectives: Azotobacter is a diazotroph bacterium reported to possess various plant growth-promoting characteristics.The aim of this study was to isolate Azotobacter strains capable of fixing nitrogen and effectively hydrolyzing both organic and inorganic Pi compounds. Materials and Methods: In this study, soil samples collected from a diverse range of slightly alkaline soil types were screened for Azotobacter isolates. The inorganic and organic phosphate solubilization potentials of twenty competent phosphate solubilizing Azotobacter isolates were assessed.Variations were noted in the solubilization potentials. Result: Three isolates, identified as Azotobacter vinelandii strains O2, O4 and O6, were able to fix atmospheric N2 effectively. The nitrogenase activity of these isolates ranged between 158.6 and 326.4 C2H4h(-1)vial(-1) (ethylene). Bacterial growth rates and phosphate solubilization activities were measured quantitatively under various environmental conditions. A close association was evident between phosphate solubilizing ability and growth rate as an indicator of active metabolism. All three phosphate solubilizing bacteria (PSB) were able to withstand temperature as high as 45°C, high concentration of NaCl (upto 5%) and a wide range of initial pH from 5 to 10 while hydrolyzing phosphate compounds actively. Conclusion: Azotobacter vinelandii strains O2, O4 and O6 are superior candidates for biofertilizers that may result in the reduction of chemical nitrogen and phosphate fertilizers leading to increase crop production. https://ijm.tums.ac.ir/index.php/ijm/article/view/385 https://ijm.tums.ac.ir/index.php/ijm/article/download/385/162