Summary: The response of the yeast to a reduction in water activity ( ) from 0·998 to 0·925 (adjusted with glucose or NaCl) was monitored. Natural abundance 1C NMR spectroscopy and HPLC analysis revealed that the type of carbon source determined which polyols were present intracellularly at 0·998 . At 0·95 (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 (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 to 0·95 resulted in increasing intracellular concentrations of glycerol and arabitol whereas below 0·95 (NaCl 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 , 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.


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