Physiological responses of Zygosaccharomyces rouxii to osmotic stress

When cell suspensions of Zygosaccharomyces rouxii were subjected to osmotic shock with NaCl, the cell volume decreased sharply and plasmolysis was observed. The cell subsequently recovered and volumes similar to those of cells growing at the respective water activity (a_w) values were found. Cycloheximide prevented cell recovery, indicating the involvement of protein synthesis in the recovery process. The intracellular glycerol concentration of Z. rouxii incubated in the presence of ¹⁴C]glycerol increased from 13 to 96 mmol/l during the initial 20 min after an upshock from 0.998 a_w to 0.96 a_w. All the intracellular glycerol was labelled and therefore derived from the medium. Labelled glycerol was subsequently utilized and replaced by unlabelled glycerol produced by the cell within 90 min. The initial increase in glycerol concentration following the upshock was confirmed by ¹³C-nuclear magnetic resonance (NMR) spectroscopic studies of cell extracts. The combined dihydroxyacetone and dihydroxyacetone phosphate concentrations fluctuated during this period, whereas glycerol-3-phosphate initially increased and then remained constant. This indicates that the production of glycerol is regulated. Decreases in ATP and polyphosphate levels were observed following osmotic upshock and may reflect a greater demand for ATP during the period of adjustment to decreased a_w. The changes in cell volume and in ATP concentration following osmotic upshock may serve as osmoregulatory signals in Z. rouxii, as suggested previously for other microorganisms. Correspondence to: S. G. Kilian