Biotechnology Letters Vol 12 No 5 361-366 (1990) Received 12th March ADAPTATION OF ZYGOSACCHAROMYCES ROUXII TO CHANGES IN WATERACTIVITY IN TRANSIENT CONTINUOUS CULTURE P.J. van ZyI* and B.A. Prior Department of Microbiology and Biochemistry, University of the Orange Free State, P.O. Box 339, Bloemfontein 9300, Republic of South Africa SUMMARY. The polyols glycerol and arabitol were accumulated against an increasing concentration ratio in response to a. reduction. Glycerol was the main osmoregulatory solute accumulated Wand arabitol accumulation only occurred during the initial transitory phase. The polyols were accumulated to concentrations less than that required to maintain equilibrium across the membrane. INTRODUCTION Microorganisms respond to reduction in a w by the intracellular accumulation of solutes which are compatible with cellular functions {Brown , 1978; Yancey et al., 1982). The type of solute accumulated differs between microorganisms. Halophilic bacteria accumulate potassium and betaine (Imhoff and Rodriguez-Valera, 1984), while amino acids are found in nonhalophilic bacteria (Measures, 1975). Glycerol is the main solute accumulated in eucaryotic microorganisms such as halophilic algae and osmotolerant yeasts although other solutes may also be accumulated (Brown, 1978; Brown, 1974; Brown et al., 1986~ Onishi, 1963). During balanced growth of Z. rouxii at reduced aw, glycerol is the principal osmoregulatory solute (Brown et al., 1986) although arabitol may also be accumulated during growth in media containing certain sugars and metabolic compounds (Nobre and Da Costa, 1985). Environmental alterations to microorganisms in balanced growth induce metabolic responses of the organism. During these metabolic changes the organism is in a transient stage until adaptation to the new environment is obtained (and once more steady-state) (Koplove and Cooney, 1978). The purpose of this study is to investigate the 361