The effects of varying carbon (glucose), nitrogen
((NH4)2SO4, KNO3) and phosphate
(KH2PO4) source on solubilization of insoluble
Co3(PO4)2.8H2O,
Zn3(PO4)2.2H2O 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 (Rm) to control growth rate
(Rc) and the rate of extension of the zone of solubilization
(Rs) compared to
the colony growth rate on the metal compound (Rm),
i.e. Rm[ratio ]Rc and
Rs[ratio ]Rm. Ratios of
solubilization rate to growth rate (Rs[ratio ]Rm)
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 Rs[ratio ]Rm
but these values were increased when the nitrogen source was nitrate. Reducing
the phosphate
concentration increased solubilization of Co3(PO4)2
but reduced solubilization of Zn3(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.