Arch. Microbiol. 115, 79- 84 (1977) Archives of Hicrobiology 9 by Springer-Verlag 1977 The Influence of Different Carbon Sources and Medium Osmolarity on the Potassium Requirements of Candida utilis NCYC 321, Growing in Continuous Culture H. AIKING, G. J. VAN HOLST, and D. W. TEMPEST Laboratorium voor Microbiologie, Universiteit van Amsterdam, Plantage Muidergracht 14, NL-1018 TV Amsterdam, The Netherlands Abstract. In order to study the influence of different carbon sources on the K+-requirements of Candida utilis NCYC 321, this yeast was grown at several different dilution rates in potassium-limited continu- ous cultures with either glucose, glycerol, ethanol, citrate "or lactate serving as the carbon and energy source. It was found that the nature of the carbon source profoundly influenced the cellular potassium content, especially at low dilution rates, but that these differ- ences could not be correlated with any differences in relative growth rate (i.e., #/Pmax). And although small amounts of potassium seemingly were needed to serve in osmoregulation and in the cotransport of some acidic carbon sources (lactate and citrate), these requirements were negligible. Independent of carbon source, a strong correlation existed between the intracellular potassium concen- tration and the yield value on oxygen (Yo), and be- tween cellular potassium concentration and growth rate. From these two correlations it was concluded that potassium probably was involved mainly in processes associated with ATP synthesis in this yeast. Finally the effect of the addition of NaC1 to the medium was tested with glucose-containing cultures that were either carbon- or potassium-limited. Up to a concentration of 20 g/l, NaC1 was without influence on Yo, Ygl ..... and qo2, but effected a slight increase in the cellular potassium content of the potassium- limited cells and a decrease in that of the glucose- limited cells. Key words." Candida utilis - Potassium-limitation - Continuous culture - Oxidative phosphorylation - Yield values - Sodium chloride. Potassium is an essential nutrient in the growth of probably all living cells, and may account for 5 ~ or more of the dry weight of some organisms (Tempest, 1969). But in spite of being present in amounts which make it a major cellular component, the precise role(s) that this cation plays in the structure and functioning of various cells is largely unknown. Indeed, of the many functions in which potassium has been implicat- ed (like enzyme activation, glucose uptake and ribo- somal functioning in protein synthesis), only its role in osmoregulation would account for the high concen- trations routinely found in the growing microbial cell. In previous studies of the yeast Candida utilis (Aiking and Tempest, 1976; Aiking et al., 1977a) we found that the cellular potassium content varied markedly with the growth conditions, but correlated closely with the efficiency with which the organisms utilized glucose and oxygen for growth. Thus, there appeared to be a close relationship between the cell- bound potassium and both the ~l ..... and Yo values with potassium-limited as well as potassium-sufficient cultures. This was interpreted as indicating an involve- ment of potassium in the generation of ATP (i.e., by oxidative phosphorylation) and/or in its utilization for growth. However, all these data, and the conclu- sions drawn from them, were derived from cultures that had been grown with glucose as the sole carbon and energy source. And although C. utilis is a Crabtree- negative yeast (De Deken, 1966) and therefore should not be greatly influenced by the presence of excess glucose in the culture fluids (as would, say, Saccharo- myces cerevisiae), nevertheless the possibility remained that the observations that had been made were markedly, and uniquely, dependent on the presence of glucose in the culture. Hence we undertook a study of the influence of different carbon and energy sources on the requirement for potassium of C. utilis.