Phosphate solubilization in vitro by isolated Aspergillus niger and Aspergillus carbonarius

Two strains of Aspergillus, A. niger and A. carbonarius, were isolated from agricultural soil and a lake, respectively, in China. The two isolates could effectively release soluble phosphate in NBRIP medium containing Ca3(PO4)2 as the sole phosphorus source. Acidification of the broth seemed to be the major mechanism for phosphate solubilization by the isolates, and this had a significant correlation with a drop in pH. High-pressure liquid chromatography analysis indi- cated the participation of various organic acids, with gluconic acid as the principal component in the process of acidification in the broth. The isolates displayed trends of soluble phosphate release that closely matched the substantial increases in glu- conic acid concentration. Abiotic incubation study using organic or inorganic acid to solubilize Ca3(PO4)2 indicated that the content of soluble phosphate released was significantly lower than that of the broth inoculated with the isolates. Higher release of soluble phosphate and pH reduction have occurred when ammonium rather than nitrate served as the sole source of nitrogen with Ca3(PO4)2.