A. niger, a laboratory stock culture, originally isolated from a
heavy metal-contaminated soil, was cultured according to the procedure
of Bosshard et al. [13]. Adaptation of the fungus was carried
out through a series of sub-cultures after exposure to the soil used
in the study. For inoculum preparation, the adapted A. niger was
incubated three times on potato dextrose agar (PDA) slants using a
sterile platinum loop at 30 ◦C for 5 days. Five-day-old conidia were
harvested from potato dextrose agar surface using sterile distilled
water. The number of spores was counted using a haemocytometer
and standardised to approximately 3.5×106 sporesmL−1
of spore suspension. For the bioleaching experiments, 1mL of
spore suspension was added to 100mL of sucrose medium with
the composition: Sucrose 100 g L−1; NaNO3 1.5gL−1; KH2PO4
0.5gL−1; MgSO4·7H2O 0.025 g L−1; KCl 0.025 g L−1; yeast extract
1.6gL−1.