[The influence of sodium and potassium ions on the uptake of phosphate by Ankistrodesmus braunii]

Abstract

The uptake of phosphate as influenced by sodium and potassium ions was investigated in the light and in the dark. It was found to be a function of the external phosphate concentration. At a low concentration (up to 10(-5) mol/l) in the presence of Na(+) phosphate is quickly absorbed and hence phosphate is the limiting factor for further labelling. In the presence of K(+) phosphate uptake is constant over a long period.The enhancement of phosphate uptake by Na(+) is also found when the external concentration of P is raised up to 10(-4) mol/l. Then the gross uptake proceeds over six hours, with the greatest Na(+)-dependent increase occurring in the label of the TCA-insoluble phosphate fraction (Pu).The phosphate uptake is strongly dependent on the pH of the reaction mixture. In the presence of Na(+) it is highest between pH 5.6 and 7. As the uptake in the presence of K(+) parallels the dissociation curve of the dihydrogen form H2PO 4 (-) , the Na(+)-enhancement is optimal in the alkaline pH range (pH 8).On the basis of a comparison between the pH-dependence of phosphate uptake and the dependence of the uptake on the external phosphate concentration analysed by a method of enzyme kinetics, it is suggested that Ankistrodesmus metabolically transports H2PO 4 (-) but not HPO 4 (=) . Moreover, it is concluded from the absence of light stimulation and the weak inhibition of the uptake by DCMU or CCCP in the presence of K(+) that at low P-concentrations the diffusion is limiting the uptake. Only at higher concentrations is an active phosphate uptake measured.Furthermore it is concluded that the observed Na(+)-stimulation of the (32)P-labelling of the TCA-soluble and insoluble compounds inside the cell is indirect and depends only on the action of Na(+) and K(+) ions at the first transport site in the plasmalemma.