Cation effects on volume and water permeability in the halophilic algae Dunaliella parva

Rates of change of cell volume were measured in suspensions of the halophilic green algae Dunaliella parva subjected to changes in cation composition and concentration of the outside medium. Measurements were made with a particle size analyzer and results were checked by direct microphotography. For any one salt solution, changes in cell volume with concentration were consistent with the Boyle-Van't Hoff model of an osmometer. Nonosmotic volume comprised 60-80% of total cell volume and was sensitive to the nature of the cation, increasing in the order Cs<K<Na<Ca<Mg. Kinetics of volume change in response to changes in outside salt concentration are best described by two kinetic coefficients differing by one order of magnitude and dependent on the nature of the outside cation (decreasing in order Cs>K>Na>Mg) as well as on direction of water flow.