An Inwardly Rectifying Cation Current across the Plasma Membrane of the Green Alga Eremosphaera viridis

Ion currents across the plasma membrane of the unicellular green alga Eremosphaera viridis were characterized with electrophysiological methods, especially the two electrode voltage-clamp-technique. Under different conditions, at increased external Cl− concentrations or after perfusion of different anion channel blockers (A9C (anthracen-9-carboxylic acid), NPPB ((5-nitro-2-3-phenylpropylamino) benzoic acid) and ZnCl2), increased instantaneous negative currents were observed. The negative currents were carried by cation fluxes into the cell. The transporter responsible had low selectivity among potassium and sodium. Additionally also divalent cations were transported. The cation influx was not affected by the potassium channel inhibitors TEA (tetraethylammoniumsulfate), Ba2+ or Cs+ at concentrations of 1 mM, but was strongly reduced by 100 μM AlCl3. Our results with E. viridis demonstrate, for the first time for an unicellular alga, the existence of an inwardly rectifying cation current across the plasma membrane. Parallels and differences to inwardly rectifying cation currents and channels described in plasma membranes of other plant cells are discussed.