Caesium accumulation and interactions with other monovalent cations in the cyanobacterium Synechocystis PCC 6803

Abstract

Summary Growth of Synechocystis 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 (109 cells)-1 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 (109 cells)-1, 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+ (109 cells)-1 after 24 h incubation] occurring at pH 10, the highest pH value used. It is suggested that an important mechanism of Cs+ toxicity in Synechocystis 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.