Cadmium uptake by Aureobasidium pullulans

Yeast-like cells, mycelium and melanin-pigmented chlamydospores of Aureobasidium pullulans accumulated cadmium from cadmium-containing medium. With chlamydospores, uptake was rapid and independent of temperature and the presence of glucose, and corresponded to binding of cadmium to cell surfaces. With yeast-like cells and mycelium, the initial rapid surface adsorption of cadmium was up to six times lower than that of chlamydospores, and was followed by metabolism-dependent transport of cadmium into the cytoplasm. The second phase of uptake was slower and followed Michaelis-Menten kinetics. Concomitant with the intracellular accumulation of cadmium was an efflux of potassium, two K+ ions being released for each Cd2+ ion accumulated. Potassium loss was low in comparison to the cell potassium levels and there was no loss of viability by any cell type at cadmium concentrations up to 0.5 mM. In yeast-like cells and mycelium, Ca2+ ions were found to inhibit cadmium uptake competitively. Work with the ionophore valinomycin confirmed that potassium efflux and cadmium influx occurred at different sites on the plasma membrane of yeast-like cells. Intracellular cadmium could be actively excreted from cells.