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

Summary: 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+, 1 mM-Cs+) than in respiring cells (50% inhibition at 10 mM-K+, 1 mM-Cs+). Greatly elevated Na+ light in the absence and presence of 10 mM-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.