Regulation of the Potassium to Sodium Ratio and of the Osmotic Potential in Relation to Salt Tolerance in Yeasts

<jats:p> By using the isotope pairs <jats:sup>22</jats:sup> Na- <jats:sup>24</jats:sup> Na and <jats:sup>42</jats:sup> K- <jats:sup>86</jats:sup> Rb, the uptake and retention of Na and K was studied in the salt-tolerant <jats:italic>Debaryomyces hansenii</jats:italic> and in the less tolerant <jats:italic>Saccharomyces cerevisiae</jats:italic> at NaCl levels of 4 m <jats:sc>m</jats:sc> and 0.68, 1.35, and 2.7 <jats:sc>m</jats:sc> in the medium. The ratio of K to Na is much higher in the cells than in the media, and higher in <jats:italic>D. hansenii</jats:italic> than in <jats:italic>S. cerevisiae</jats:italic> under comparable conditions. The difference between the two species is due to a better Na extrusion and a better uptake of K in <jats:italic>D. hansenii</jats:italic> . The kinetics of ion transport show that at about the time when extrusion of Na could be demonstrated in <jats:italic>D. hansenii</jats:italic> , K-Rb previously lost to an easily washable compartment of the cells was reabsorbed in both organisms. More H <jats:sup>+</jats:sup> was given off from <jats:italic>S. cerevisiae</jats:italic> than from <jats:italic>D. hansenii</jats:italic> in the course of these events. The findings fit the working hypothesis tested, which regards salt tolerance as partly dependent on the ability to mobilize energy to extrude Na from the cells and to take up K. The volume changes in <jats:italic>S. cerevisiae</jats:italic> are greater and are more slowly overcome than those in <jats:italic>D. hansenii</jats:italic> . The total salt level of the cells is not sufficient to counteract the osmotic potential of the medium, so that additional osmoregulatory mechanisms must be involved in determining halotolerance. </jats:p>