Replacement of cellular potassium by caesium in Chlorella emersonii: Differential sensitivity of photoautotrophic and chemoheterotrophic growth

Photoautotrophic growth of Chlorella emersonii in the presence of 1 mM-CsCl resulted in a 34 % increase in cell doubling time and an 83 % reduction in the final cell yield as compared to growth in the absence of Cs⁺. In contrast, the presence of 1 mM-Cs⁺ had no effect on chemoheterotrophic growth in the dark with glucose. These observations were correlated with the stage of growth at which Cs⁺-induced cellular K⁺ loss was evident. For photoautotrophically growing cells this occurred during the exponential growth phase (after 4 d); for chemoheterotrophically growing cells this was during stationary phase (after 10 d). Inhibition of chemoheterotrophic cell division occurred after 2 d in 50 mM-Cs⁺ or 5 d in 20 mM-Cs⁺, and coincided with a decline in intracellular K⁺ to ~ 2 nmol (10⁶ cells)^-1. Accumulation of Cs⁺ ceased after 2 d in both of these cases. Cell doubling times during chemoheterotrophic growth remained approximately constant at internal K⁺ levels between 7 and 28 nmol (10⁶ cells)^-1. In contrast, a decline in intracellular K⁺, from 42 to 19 nmol (10⁶ cells)^-1, after 4 d photoautotrophic growth in the presence of Cs⁺, was concurrent with the 34% increase in cell doubling times. Photosynthesizing cells accumulated approximately 2-fold more Cs⁺ than respiring cells after incubation for 12 h in HEPES buffer, pH 8. Culture age and intracellular K⁺ levels had little effect on the ability of C. emersonii to accumulate Cs⁺. Externally supplied K⁺ inhibited Cs⁺ accumulation to a greater extent in photosynthesizing cells (75% inhibition at 10 mM-K⁺, 1mM-Cs⁺) than in respiring cells (50% inhibition at 1OmM-K⁺, 1mM-Cs⁺). Greatly elevated Na⁺ concentrations (50 mM) were required to inhibit Cs⁺ accumulation in both cases. Incubation of cells in buffer in the light in the absence and presence of 10 mM-Cs⁺ resulted in decreases in cellular K⁺ of 44% and 77% respectively and a concomitant 66% reduction in the rate of photosynthesis in both cases. A Cs⁺-induced K⁺ loss of 71% from respiring cells had no effect on the rate of respiration. These results indicate that it is not the presence of Cs⁺ in cells that is growth inhibitory, but rather the resulting loss of K⁺ and that photosynthesis and photoautotrophic growth are more sensitive to this K⁺ loss than respiration and chemoheterotrophic growth.