Characterisation of caesium transport in the microalga Chlorella salina.

Caesium displays similar chemical properties to biologically essential K+ and can be accumulated intracellularly by microorganisms via monovalent cation transport systems, which under normal circumstances mediate the uptake of K+ (or Na+) [ 1.21. As well as the ecological importance of such interactions in the environment, where they may involve radioactive caesium, there is also a potential application for 137Cs as a tracer of K+ fluxes in cells [2]. However, only a few investigations have sought to characterise the kinetics of Cs+ transport in microorganisms. In this study, infludefflux kinetics, transport selectivity and subcellular location of Cs+ are characterised for the euryhaline microalga Chlorella salina. Rates of Cs+ uptake by C. salina in Taps buffer (pH K O ) , determined using 137Cs+ as a tracer of Cs+ movement, declined with time and intracellular Cs+ reached a maximum after about 15 h. Semi-log plots of the data were linear, indicating first order Cs+ uptake kinetics and that Cs+ accumulation probably occurred into a single intracellular compartment. First-order uptake kinetics may arise when the vacuolar membrane is much more permeable than the plasma membrane; under these conditions the plasma membrane is the rate limiting membrane [3]. Suspension of Cs+ (137Cs)-loaded cells in buffer with stable CsCl or KCI resulted in two phases of Cs+ efflux from C. salina. These corresponded to rapid release from the cell wall and a slow single rate of release from the cytoplasm + vacuole; extrapolation of plots revealed that approximately 1 1% and 89% of total cellular Cs+ was associated with these pools, respectively.