Summary: Photoautotrophic growth of in the presence of 1 m-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 m-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 m-Cs or 5 d in 20 m-Cs, and coincided with a decline in intracellular K to ∼2 nmol (10 cells). 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 (10 cells). In contrast, a decline in intracellular K, from 42 to 19 nmol (10 cells), 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 to accumulate Cs. Externally supplied K inhibited Cs accumulation to a greater extent in photosynthesizing cells (75% inhibition at 10 m- K, 1 m-Cs) than in respiring cells (50% inhibition at 10 m-K, 1 m-Cs). Greatly elevated Na light in the absence and presence of 10 m-Cs accumulation in both cases. Incubation of cells in buffer in the light in the absence and presence of 10 m-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.


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