A salt-sensitive Dunaliella mutant I. Growth and osmoregulatory responses to increases in salt concentration

Abstract

Dunaliella is a genus of green unicellular algae distributed in all the oceans and saline bodies of water throughout the world and distinguished by unusual tolerance to salt. Since the cells of this genus do not success a rigid cell-wall, they respond to changes in salt concentration by rapid alterations in cell volume and then return to their original volume as a result of adjustments in the amounts of intracellular ions and glycerol, this latter being the major organic osmoticum. The paper describes the behaviour of a mutant of D. parva 19/9 with reduced capabilities of growth above 0.5 kmol m -3 NaCl. The mutant is unusual in that its abilities to synthesize glycerol and pump out Na + and Cl - do not appear to be impaired; volume changes in the hyperosmotic range also appear to be roughly the same as in D. parva. The average and volume of mutant cells is reduced (206 μm 3 as opposed to 255 μm 3 in D. parva) and their rate of change of cell volume after an increase in salt concentration is lower; it took about 10 min for mutant cells in the light to reach a new cell volume whereas D. parva cells reached their new volume in less than 1 min. Both factors may be dependent on components of the cyto-skeleton. The mutant throws light on adaptations necessary to allow Dunaliella cells to grow at high salt concentrations and demonstrates that halotolerance includes, but is not equivalent to, osmoregulation.