Transport kinetics, cation inhibition and intracellular location of accumulated caesium in the green microalga Chlorella salina

Caesium accumulation by Chlorella salina, from buffer (pH 8.0) supplemented with 50 μM-CSCl and ¹³⁷Cs, continued for approximately 15 h and displayed first-order kinetics, indicating a single rate-limiting transport process. Efflux of Cs⁺ from Cs⁺-loaded cells occurred in two distinct phases: a rapid initial loss, representing approximately 11% of total cellular Cs⁺, corresponded to release from the cell surface, whereas a second, slower, phase of efflux corresponded to loss from the cytoplasm and vacuole. Analysis of subcellular Cs⁺ compartmentation revealed that most Cs⁺ was accumulated into the vacuole of C. salina, with lesser amounts being associated with the cell surface or located in the cytoplasm. Uptake of Cs⁺ into the vacuole was correlated with a stoichiometric exchange for K⁺. However, no loss of K⁺ from the cell surface or cytoplasm was evident nor was Cs⁺ or K⁺ associated with insoluble intracellular components. Calculated values for the Cs⁺ flux across 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) ~ 0.5 mM was evident. The effects of other monovalent cations on Cs⁺ influx implied that K⁺ and Rb⁺ were competitive, and NH₄⁺ non-competitive/uncompetitive inhibitors of Cs⁺ uptake. The order of inhibition was Rb⁺ > K⁺ > NH₄⁺. We propose that a single, relatively non-selective, rate-limiting transport system for Cs⁺ influx is located on the cytoplasmic membrane of C. salina, while a more permeable vacuolar membrane facilitates transport of Cs⁺ into the vacuole.