When grown in a defined simple salts medium, plus vitamins, Candida utilis displayed an absolute requirement for potassium. But the potassium content of this yeast was exceedingly variable and, with aerobic chemostat cultures (grown at a dilution rate of 0.1h-l; 30~ pH5.5), was low (< 0.2~, w/w) when they were potassium-limited and high (> 2~, w/w) when glucose-limited. With potassium-limited cultures, the cell-bound potassium content also varied markedly with growth rate, though hardly at all with glucose-limited cultures; magnesiumand phosphate-limited cultures gave intermediate responses.
Changes in cell-bound potassium content correlated only weakly with changes in the cellular contents of magnesium, phosphate and RNA, but strongly with changes in both the Ygl . . . . . and Yo values, indicating an involvement of potassium in the generation of energy by oxidative phosphorylation reactions and/or the utilization of this energy for growth processes.
Studies with isolated mitochondria revealed that potassium-limited organisms had a changed content of cytochrome b relative to cytochrome a, and lacked coupling at either site 2 or site 3 of the respiratory chain. ~ These results are discussed in relation to the reported functions of potassium in the growth of micro-organisms, and the organizational differences between prokaryotic and eukaryotic cells.