Influence of the carbon, nitrogen and phosphorus source on the solubilization of insoluble metal compounds by Aspergillus niger

The effects of varying carbon (glucose), nitrogen ((NH4),SO4, KNO3) and phosphate (KH2PO4) source on solubilization of insoluble Co-3(PO4)(2).8H(2)O, Zn-3(PO4)(2).2H(2)O and ZnO by the soil fungus Aspergillus niger were assessed. Solubilization activity was quantified by measuring the clear zones produced around colonies of A. niger growing on solidified mineral salts medium amended with the insoluble metal compounds. Effects of nutrient variation on solubilizing properties were compared using ratios of colony growth rate on the metal compounds (R-m) to control growth rate (R-e) and the rate of extension of the zone of solubilization (R-s) compared to the colony growth rate on the metal compound (R-m), i.e. R-m:R-c and R-s:R-m. Ratios of solubilization rate to growth rate (R-s:R-m) on all the compounds decreased with decreasing glucose concentration; there was no solubilization of ZnO below 60 mM glucose and no solubilization of the metal phosphates below 6 mM glucose. Reducing the concentration of ammonium sulphate in the growth medium decreased R-s:R-m but these values were increased when the nitrogen source was nitrate. Reducing the phosphate concentration increased solubilization of Co-3(PO4)(2) but reduced solubilization of Zn-3(PO4)(2). These findings demonstrate that manipulation of carbon, nitrogen and phosphate sources in the growth medium, and variation of the form of the nutrient source, can be used to alter the solubilizing ability of A. niger. Whilst, in the natural environment, this response to different nutrient sources allows optimal exploitation of resources, the potential to manipulate nutrients for maximum solubilizing ability may prove beneficial for the optimization of the solubilization of metal compounds with respect to the bioremediation of metal-contaminated wastes and polluted ecosystems. It could also prove useful in other biotechnological applications such as metal recycling and extraction of metals from low-grade ores.