Objectives Penicillium chrysogenum (P. chrysogenum) is a typical phosphate solubilizing fungi used in microbial strains. We studied the capacity and mechanism of P. chrysogenum in dissolving P from ferric, aluminum and tricalcium phosphate (Fe-P, Al-P and Ca-P).

Methods The tested P. chrysogenum was selected in our laboratory, and was isolated into Fe-P, Al-P and Ca-P suspension, respectively. After incubation of 1, 3, and 5 days, the suspension was filtered using a no-P filter paper, the filtrate was used for determination of soluble P, organic acids concentration, and enzyme activities. The filter residue were used for observation of morphology of phosphates and fungi mycelium using attenuated total reflectance infrared spectroscopy (ATR-IR) and scanning electron microscopy-energy dispersive spectrometer (SEM-EDS).

Results The dissolve amount of P from the insoluble phosphates increased with the elongation of incubation days. P. chrysogenum dissolved more P from Ca-P (P<0.05) than from Al-P and Fe-P. After five days of incubation, the P content in Fe-P, Al-P and Ca-P was 153.8, 215.9, and 569.4 mg/L, respectively; and the filtrate pH of the Fe-P, Al-P, and Ca-P treatments decreased to 2.0, 2.3, and 5.0, respectively. The Fe-P filtrate had (P<0.05) higher pyruvate dehydrogenase and citrate synthase activities than the Ca-P and Al-P filtrates. The oxalic acid and citric acid concentration in the Fe-P filtrate was 1086.6 and 806.4 mg/L, respectively. While only oxalic acid was detected in Al-P and Ca-P filtrates, with concentration of 261.1 and 201.3 mg/L, respectively. The ATR-IR spectra showed that the vibration absorption peaks of PO₄³⁻ in the Fe-P, Al-P, and Ca-P treatments became significantly weaker, indicating the release of P from the tested phosphates. The SEM-EDS analysis showed that the oxalic acid was chelated with Ca-P and formed calcium oxalate (CaC₂O₄). Compared with citric acid, oxalic acid was more efficient in the release of P from Fe-P, Al-P and Ca-P, with release rates of 14.8%, 32.4% and 39.6%, which was 37, 3 and 2 times higher than citric acid, respectively.

Conclusions The capacity of P. chrysogenum in dissolving P from different insoluble phosphates is in the order of Ca-P>​Al-P>Fe-P. When exposure to Fe-P, P. chrysogenum enhances the activity of pyruvate dehydrogenase and citrate synthasesecretes to secrete more oxalic acid and citric acid. Although oxalic acid is more capable of dissolving insoluble phosphates than citric acid, its chelating with carbonates may offset its capacity. So addition of ferric phosphate might help increase the efficiency of P. chrysogenum to dissolve insoluble phosphates via the secretion of citric and oxalic acid in soil.