Measurements of Ion Concentrations and Fluxes inDunaliella parva

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Abstract
Measurements of K+, Na+, and Cl− were made on a halotolerant strain of Dunaliella growing at 500 mM NaCl, 25 °C, and a relatively low light intensity (6000 Lx). Much effort was spent in searching for a means of measuring the extracellular volume of fluid trapped between the cells of centrifuged pellets. All of the sugars tried as markers were rejected because they were found to be digested in the cell suspension. The most suitable marker was found to be [14C]polyethylene glycol2 (mol. wt. 4000); although this substance was apparently adsorbed to the cell exterior, it was found possible to correct for adsorption and then obtain a reasonable figure for the trapped fluid. The final concentrations of cell K+ and Na+ were 128 ± 53 mM and 131 ± 117 mM respectively. Cl− balanced the sum of K+ Na+. Influxes of 22Na+, 42K+, and 36C1− were measured in cells in which the ions were in the steady state. Averages of 610 and 6.6 nmol m−2 s−1 were obtained for Na+ and K+ respectively. Cl− influx was divided into 2 phases with values of 1540 and 178 mmol m−2 s−1. The faster influx was considered to be across the outer cell membrane. The membrane responsible for the slower influx has not been identified. By comparing values of the potential difference calculated from the Nernst and Goldman equations, it was concluded that Na+ and K+ are probably controlled by active mechanisms, whereas cell Cl− is likely to be at thermodynamic equilibrium with the medium.

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