The response of 31 ascomycetous yeasts to a reduction in water activity (a,) adjusted with D-glucose or NaCl was investigated. The growth of most yeasts was more tolerant to glucose than to NaCl at equivalent a,. Zygosaccharomyces rouxii was the most osmotolerant yeast examined. Natural abundance 13C-NMR spectroscopy and HPLC analyses of eight yeasts indicated that glycerol and arabitol or mannitol were accumulated intracellularly in response to a, reduction. Pichia sorbitophila, Candida cacaoi, Candida magnoliae and Zygosaccharomyces bisporus responded to reduced a, by a decrease in specific growth rate and cell volume, and an accumulation of glycerol. The other polyol accumulated did not increase in concentration with a, reduction to the same degree as glycerol. A polyol concentration ratio (intra/extracellular) as high as 800-fold was attained across the membrane. Greater amounts of polyols were produced at equivalent a, values when adjusted with glucose than with NaCl. The ability to accumulate high concentrations of polyols appears to be the most important criterion in determining osmotolerance.
The responseoftheyeast Hansenulaanomala toareduction inwater activity (a,)from0.998 to0.925 (adjusted with glucose or NaC1) was monitored. Natural abundance 3C NMR spectroscopy and HPLC analysis revealed that the type of carbon source determined which polyols were present intracellularly at 0.998 a,. At 0.95 a, (NaCl), glycerol was accumulated in all instances irrespective of the type of carbon source indicating the primary role of glycerol in osmoregulation. The carbon source had a bearing only on which other polyol(s) were accumulated. During growth on glucose at 0-95 a, (NaCl or glucose), glycerol was accumulated intracellularly in the exponential growth phase with a concentration ratio (intra-/extracellular) as high as 10000-fold whereas during the stationary phase arabitol accumulation occurred to a lower concentration ratio while the glycerol concentration decreased. The specific growth rate and cell volume decreased with increasing NaCl or glucose concentrations. This indicated that the yeast had no specific requirement for these compounds for optimum growth but tolerated high concentrations. Reducing the a, to 0.95 resulted in increasing intracellular concentrations of glycerol and arabitol whereas below 0.95 a, (NaC1 or glucose), the intracellular polyol concentration decreased while the polyol concentration ratios across the cell membrane increased. During the early exponential growth phase at 0-95 a,, glycerol was accumulated in sufficiently high concentrations to achieve an osmotic balance across the membrane whereas in the stationary phase the arabitol and glycerol concentration was insufficient to maintain the osmotic balance. t Present address: South
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