TRANSPORT KINETICS, CATION INHIBITION AND INTRACELLULAR LOCATION OF ACCUMULATED CESIUM IN THE GREEN MICROALGA CHLORELLA-SALINA

Caesium accumulation by Chlorella salina, from buffer (pH 8.0) supplem ented with 50 muM-CsCl and Cs-137 continued for approximately 15 h and displayed first-order kinetics, indicating a single rate-limiting tra nsport process. Efflux of Cs+ from Cs+-loaded cells occurred in two di stinct phases: a rapid initial loss, representing approximately 11 % o f total cellular Cs+, corresponded to release from the cell surface, w hereas a second, slower, phase of efflux corresponded to loss from the cytoplasm and vacuole. Analysis of subcellular Cs+ compartmentation r evealed that most Cs+ was accumulated into the vacuole of C. salina, w ith lesser amounts being associated with the cell surface or located i n the cytoplasm. Uptake of Cs+ into the vacuole was correlated with a stoichiometric exchange for K+. However, no loss of K+ from the cell s urface or cytoplasm was evident nor was Cs+ or K+ associated with inso luble intracellular components. Calculated values for the Cs+ flux acr oss the vacuolar membrane were approximately equal to, or higher than, values for total cellular influx. Cs+ influx obeyed Michaelis-Menten kinetics over the lower range of external Cs+ concentrations examined (0.01-0.25 mm) and a single transport system with a K(m) approximately 0.5 mM was evident. The effects of other monovalent cations on Cs+ in flux implied that K+ and Rb+ were competitive, and NH4+ non-competitiv e/uncompetitive inhibitors of Cs+ uptake. The order of inhibition was Rb+ > K+ > NH4+. We propose that a single, relatively non-selective, r ate-limiting transport system for Cs+ influx is located on the cytopla smic membrane of C. salina, while a more permeable vacuolar membrane f acilitates transport of Cs+ into the vacuole.