Nutritional effects on oxalic acid production and solubilization of gypsumby Aspergillus niger

The effects of varying carbon, nitrogen, phosphate and sulphate source on s olubilization of natural gypsum by Aspergillus niger were assessed. The fun gus was grown on Czapek Dox agar amended with 0.5% (w/v) gypsum. Solubiliza tion activity was monitored by measuring the clear zone formed underneath a nd around the growing colonies. On different concentrations of glucose, nit rate, ammonium, urea, phosphate and sulphate, linear growth rate [R-g] was not significantly correlated with gypsum solubilization rate [R-s], but the solubilization ratio [R-s/R-g] was increased by increasing the concentrati on of glucose, nitrate, and urea. On ammonium nitrogen, the mycelial dry we ight was negatively correlated with linear growth, and gypsum solubilizatio n activity was markedly lower than that on nitrate or urea. Gypsum solubili zation was strongly correlated with both biomass dry weight and oxalic acid production. The fungus was unable to grow on carbonate, as a carbon source , but the addition of sucrose alleviated the effect of alkalinity caused by sodium carbonate. The optimum C:N ratio for gypsum solubilization was 50:1 , and the importance of phosphate, rather than sulphate, was also shown. Im plication of critic acid in the solubilization process was also suggested. This work emphasizes the importance of organic acids, particularly oxalic a cid, production by fungi in gypsum solubilization and revealed the optimum nutritional conditions for the solubilization process.