Influence of Zn2+ on yeast-mycelium dimorphism and exopolysaccharide production by the fungus Aureobasidium pullulans grown in a defined medium in continuous culture.

The yeast-mycelium dimorphism of Aureobasidium pullulans was studied in continuous culture in a defined medium. At a constant dilution rate (0.08 h-1) the morphological status of the culture could be controlled by the input concentration of Zn2+. As the input concentration of Zn2+ was increased (in intervals from 0 to 7.6 microM) the culture shifted from a zinc-limited to a carbon-limited state. In this interval the culture gradually passed through three growth regimes based on morphology and concentration of exopolysaccharide and biomass. The first growth regime was found when the input concentration of Zn2+ was kept below 0.45 microM. Growth in this regime was zinc-limited and more than 90% of the biomass was in the yeast growth form. An increase in the input concentration of Zn2+ in this growth regime led to a proportional increase in both the biomass and the concentration of exopolysaccharide. When the input concentration of Zn2+ was varied between 0.45 microM and 0.80 microM a second growth regime could be detected where simultaneous limitations in two nutrients were recognized. Although the carbon source (glucose) was exhausted an increase in the input concentration of Zn2+ led to a proportional increase in the steady-state biomass concentration. The increase in biomass concentration was at the expense of exopolysaccharide production, which gradually decreased. The culture, still being primarily limited by Zn2+, remained in the yeast growth form. In a third growth regime (input concentration of Zn2+ above 0.80 microM) no increase in the steady-state biomass was seen when the input concentration of Zn2+ was increased.(ABSTRACT TRUNCATED AT 250 WORDS)