Acid phosphatase secreted from Ustilago sp. is able to hydrolyze organic phosphorus. These soil yeast microorganisms were isolated from rice roots grown in acid sulphate soil that generally contains high amount of aluminum (Al), iron (Fe) and manganese (Mn) ions. Therefore, the objectives of this study were to examine the effect of pH and some cations on acid phosphatase activity. Two isolates of Ustilago sp., AR101 and AR102, were cultured in 100 mL of modified Pikovskaya's broth containing Na-phytate, pH 4, and acid phosphatase activity was determined at pH 2.0-7.0. Effect of Al, Fe, and Mn, including calcium (Ca) ions, on growth of AR101 and AR102, secreted acid phosphatase activity, and the ability of acid phosphatase on the phosphorus release from Na-phytate by Ustilago sp. were investigated. It was found that the optimum pH for acid phosphatase activity was 3.5-4.5. The activity of acid phosphatase secreted from AR101 (3,690 nmol min-1 mL-1) was remarkably higher than that from AR102 (956 nmol min-1 mL-1). Aluminum, iron, manganese and calcium ions in the medium did not affect the growth of either isolate. The activity of secreted acid phosphatase of AR101 was inhibited by Al and Ca ion, and synthesis of acid phosphatase of Ustilago sp. AR102 was possibly stimulated by Fe ion. Both AR101 and AR102 solubilized Na-phytate, resulting in the release of P. However, some amount of released P was then precipitated with Al and Fe ions as the highly insoluble Fe- or Al- phosphate.
