Lamprothamnium, a Euryhaline Charophyte

Ionic fluxes, membrane potentials and permeabilities of the plasmalemma and tonoplast to K1, Na1 and Cl− have been examined under steady-state conditions in the brackish-water charophyte. Lamprothamnium papulosum. Cells were placed in one of three aerated solutions of artificial seawater (500, 750, 1050 mosmol kg−1). Mean vacuolar potentials were −175, −166 and −157 mV respectively in the three solutions. Compartmental analysis of fluxes indicated that sodium and potassium were moved from the cytoplasm to both the vacuole and the external solution against the electrochemical gradient, whereas inward movements of chloride from the external solution and the cytoplasm were against the gradient. The Na/K ratio in the cytoplasm was greater than one. The low passive permeability of the tonoplast resulted in only a slow loss of ions, particularly K1 from the vacuole. These results are discussed in relation to osmotic regulation under steady-state conditions.