Tolerance of yeasts to zinc: distinction between cell growth and cell longevity

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Abstract
Torulopsis homii and several other yeasts grew well in a synthetic medium containing 1.0 μmolar zinc. Additional zinc increased neither the growth rate nor total growth. In concentrations up to 10 μmolar, essentially 100% of the zinc added to a culture of T. homii was accumulated during growth by the cells; soon after reaching the stationary phase, most of the metal was eliminated into the medium in the ionic state. The release of zinc was correlated with the lowering of the pH of the medium by the fermentation products.In contrast to the lack of effect on growth, the concentration of zinc markedly influenced the viability of yeast cultures. The presence of 10 to 100 μmolar zinc at the time of inoculation greatly increased the death rate of stationary phase cultures. Likewise, subfungistatic concentrations of cadmium or copper, but not iron or manganese, decreased culture longevity. This action of zinc, cadmium, and copper is attributed to the ability of these metals to suppress fungal secondary metabolism.

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