Active Sodium and Chloride Transport in the Single Celled Marine Alga Halicystis Ovalis

Abstract The active transports of sodium and chloride ions, between the vacuole and environmental solutions, were measured in the giant coenocyte of the marine alga Halicystis ovalis. Ion fluxes, determined isotopically, of individual single cells were measured by the short‐circuit technique of U ssing and Z erahn (1951). Concentric pipettes were used to replace the vacuole sap with sep water and to short‐circuit the vacuole potential difference to zero. The mean net efflux of sodium represented 39.2%, S. D. = 5.4, and the mean net influx of chloride 57.6%, S. D. = 5.3, of the current flowing through a short‐circuited cell. Therefore a summation of the current carried by the two net active fluxes can account for the total short‐circuit current. Micro‐electrode penetration of the protoplasm of the cell indicated that the potential difference of the protoplasm was identical with the vacuole potential; that the total potential difference develops at the outer membrane. A theory is presented which evaluates the effect of these transport systems on the total potential difference and on the osmotic stability of the cell relative to its environment.