SUMMARY: Growth of PCC 6803 in BG-11 medium supplemented with 1 mM-CsCl resulted in intracellular accumulation of Cs to a final level of approximately 510 nmol (10 cells) after incubation for 10 d. The doubling time was increased by 64% and the final cell yield was decreased by 70% during growth in the presence of Cs as compared to growth in control BG-11 medium. When the total monovalent cation concentration of the medium was doubled by adding either K or Na, levels of accumulated Cs were decreased by approximately 50% to 220 and 270 nmol (10 cells), respectively, after 28 d with little inhibition of growth being apparent. Short-term experiments revealed that extracellular K and Na inhibited Cs accumulation to a similar extent, with 90% inhibition of Cs accumulation occurring at the highest concentrations used (50 mM-K or Na; 1 mM-Cs). In all experiments, Cs accumulation resulted in a reduction in intracellular K, except when cells were grown in K-depleted medium, although a stoichiometric relationship was not apparent, the amount of Cs accumulated generally being greater than the amount of K released. Cs accumulation had no discernible effect on intracellular Na. When K, Na, Rb, Li or Tl were supplied at equimolar (1 mM) concentrations to Cs, only Tl significantly reduced Cs accumulation. However, an approximately 50% inhibition of Cs accumulation resulted when concentrations of K, Na, Rb or Li were increased to 10 mM, which suggests that Cs may have a higher affinity for the monovalent cation transport system than K, Rb and TI also caused a decrease in intracellular K, whereas Na and Li stimulated K uptake. Cs accumulation was dependent on the external Cs concentration and showed a linear relationship to external Cs concentrations≤2 mM over 12 h incubation. However, prolonged incubation in external Cs concentrations≥ 0·8 mM resulted in Cs release from the cells and after 48 h, similar amounts of Cs and K were present in cells incubated at these higher concentrations. Cs accumulation was energy- and pH-dependent. Incubation in the light at 4 °C, or in the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), or at 22 °C in the dark resulted in decreased Cs accumulation and decreased K release from the cells. Increased amounts of Cs were accumulated as the pH of the external medium was increased, with maximal accumulation [approximately 1330 nmol Cs (10 cells) after 24 h incubation] occurring at pH 10, the highest pH value used. It is suggested that an important mechanism of Cs toxicity in PCC 6803 arises through replacement of cellular K by Cs. The possible role of primary producers such as cyanobacteria in the mobilization of this radionuclide in aquatic habitats is discussed.


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