Tri-Calcium and Zinc Phosphates Solubilization by Aspergillus niger and Its Relation to Organic Acids Production

In recent years, numerous reports have characterized fungi not only to increase the efficiency of phosphate fertilizers but also to increase the availability of phosphorus from insoluble sources of phosphate. Aspergillus niger, as one of the ubiquitous organic acid-producing soil fungus, was screened for the ability to solubilize different concentrations of tricalcium phosphate (TCP) and zinc phosphate (ZP) ranging from 100 to 400 ppm. A. niger was able to solubilize TCP and ZP with solubilization index ranging from 2.10 to 3.5 and from 2.06 to 2.26, respectively. A drop in pH from 6.5 to 3.0 and 3.2 as one mechanism of phosphorus solubilization was observed in medium inoculated and supplemented with 400 ppm of TCP and ZP, respectively. A. niger released phosphorus from TCP (23.01 ppm) increasing its concentrations up to 400 ppm associated with increasing calcium releasing. Phosphorus releasing increased (42.16 ppm) with increasing ZP up to 200 ppm, and then decreased with increasing ZP up to 400 ppm associated with the releasing of zinc. HPLC results indicate that organic acids were released in high concentration when TCP and ZP were used with different concentrations compared with the control. For example, oxalic, malic, succinic, citric, and fumaric acid concentrations were 3.61, 3.13, 4.93, 3.17, and 2.11 at 100 ppm of TCP; and 3.09, 5.50, 5.08, 3.80, and 2.35 μg/ml at ZP compared with their concentrations 2.46, 1.83, 2.61, 3.16, and 1.37 μg/ml, respectively, in control.