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) an d 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 n iger were assessed. Solubilization activity was quantified by measurin g the clear zones produced around colonies of A. niger growing on soli dified mineral salts medium amended with the insoluble metal compounds . Effects of nutrient variation on solubilizing properties were compar ed 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 sol ubilization (R-s) compared to the colony growth rate on the metal comp ound (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 gluco se. Reducing the concentration of ammonium sulphate in the growth medi um decreased R-s:R-m but these values were increased when the nitrogen source was nitrate. Reducing the phosphate concentration increased so lubilization of Co-3(PO4)(2) but reduced solubilization of Zn-3(PO4)(2 ). These findings demonstrate that manipulation of carbon, nitrogen an d 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 differ ent nutrient sources allows optimal exploitation of resources, the pot ential to manipulate nutrients for maximum solubilizing ability may pr ove beneficial for the optimization of the solubilization of metal com pounds with respect to the bioremediation of metal-contaminated wastes and polluted ecosystems. It could also prove useful in other biotechn ological applications such as metal recycling and extraction of metals from low-grade ores.