Ion Absorption and Retention by Chlorella Pyrenoidosa. II. Permeability of the Cell to Sodium and Rubidium

Abstract

The Na and Rb permeability of Chlorella pyrenoidosa were estimated from the rates of radioisotope self-diffusion.The isotopic exchange in absence of net ionic movements followed first order kinetics. This suggested that for sodium, which reached isotopic equilibrium in approximately 90 minutes, the cell behaved as 1 compartment with respect to isotopic exchange. Rubidium in 180 minutes approached isotopic equilibrium by 67%; thus, the existence of a single compartment for Rb has not been demonstrated. Net fluxes, calculated from the isotope exchange data, and expressed on a dry weight and surface area base showed that Na fluxes were approximately 7 times larger than Rb fluxes. Net Na fluxes of 90 milli-equivalents per 100 g dry weight per hour were far in excess of the observed maximum net accumulation of Na. However, Rb fluxes of 13 milliequivalents per 100 g dry weight per hour were of similar magnitude as the rate of Rb accumulation. Thus, permeability could be a limiting factor for Rb but not for Na accumulation. Sodium and Rb fluxes in absence of net ionic movements were inhibited by low temperature, dark air and dark N(2) conditions. This change in flux rates was explained mainly on the basis of metabolically dependent changes in the cell surface layers.Isotope fluxes of Rb were drastically reduced in dark air and dark N(2) in the absence or presence of net cation movements. Dark N(2) essentially eliminated net cation accumulation, whereas dark air had relatively little effect on the net K and Rb accumulation by Chlorella. Thus the 2 major factors involved in net cation accumulation in the Chlorella cell, permeability and processes leading to cation retention, respond differently to metabolic inhibition permitting a separation of these 2 important aspects of cation accumulation.