Osmotic adjustment and the accumulation of organic solutes in whole cells and protoplasts of Saccharomyces cerevisiae

In the presence of a suitable carbon source, whole cells and protoplasts of Saccharomyces cerevisiae synthesized glycerol as a compatible organic solute in response to increased external osmotic pressure. Boyle-van't Hoff plots showed that protoplasts, and non-turgid cells, exhibited a linear relationship between volume and the external osmotic pressure (i.e. they behaved as near-ideal osmometers), and that both protoplasts and cells have a component which is not osmotically responsive - the non-osmotic volume (NOV). Glycerol levels in whole cells and protoplasts were elevated by increased external osmotic pressure over a similar time-scale to the period of exponential cell growth, reaching a maximum value at 6-12 h and declining thereafter. This suggests that the restoration of turgor pressure in whole cells was not the sole regulator of glycerol accumulation. Stationary phase whole cells had negligible levels of intracellular glycerol after growth in a medium of raised osmotic pressure. However, intracellular trehalose synthesis in these cells began earlier and reached a higher maximum level than in basal medium. Once exponential growth had stopped, cell turgor and internal osmotic pressure decreased somewhat. These new, lower values may be determined by the extent of trehalose accumulation in stationary phase cells.