Aspergillus aculeatus as a rock phosphate solubilizer

Aspergillus aculeatus, a rhizosphere isolate of gram (Cicer ariatenum) invariably solubilized both foreign (China and Senegal) and Indian (Hirapur, Udaipur, Sonrai) rock phosphates (RP). Arabinose proved superior to glucose among 11 carbon compounds tested. All the test carbon sources, except lactose, supported good growth regardless of their phosphate solubilization. Ammonium sulphate was the best nitrogen source followed by urea and asparagine. Nitrates, while supporting good growth, did not support good phosphate solubilization. Maximum phosphate solubilization (PS), growth and acidity of the medium never occurred simultaneously with all C-to-N ratios (8 to 120). The chelator nitrilotriacetic acid increased RP solubilization at 2 mg ml−1 while diethylenetriaminepentaacetic acid enhanced PS only at 6 mg ml−1, while ethylenediaminetetraacetic acid, aluminon and oxine inhibited PS at all concentrations tested. Although increasing concentration of RP in the culture medium favoured mycelium growth and a decrease in pH, soluble phosphate concentrations were reduced, probably owing to consumption by the rapidly growing fungus. Acidic pH accompanied PS in all the experiments. However, no specific correlation could be established between maximum PS, growth or pH.