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

Dunaliella is a genus of green unicellular algae distributedin all the oceans and saline bodies of water throughout theworld and distinguished by unusual tolerance to salt. Sincethe cells of this genus do not possess a rigid cell-wall, theyrespond to changes in salt concentration by rapid alterationsin cell volume and then return to their original volume as aresult of adjustments in the amounts of intracellular ions andglycerol, this latter being the major organic osmoticum. Thepaper describes the behaviour of a mutant of D. parva 19/9 withreduced capabilities of growth above 0.5 kmol m^–3 NaCl.The mutant is unusual in that its abilities to synthesize glyceroland pump out Na⁺ and Cl^– do not appear to be impaired;volume changes in the hyperosmotic range also appear to be roughlythe same as in D. parva. The average cell volume of mutant cellsis reduced (206µm³ as opposed to 255 µm³ in D. parva)and their rate of change of cell volume after an increase insalt concentration is lower; it took about 10 min for mutantcells in the light to reach a new cell volume whereas D. parvacells reached their new volume in less than 1 min. Both factorsmay be dependent on components of the cytoskeleton. The mutantthrows light on adaptations necessary to allow Dunaliella cellsto grow at high salt concentrations and demonstrates that halotoleranceincludes, but is not equivalent to, osmoregulation. Key words: Dunaliella, salt tolerance, mutant