A SALT-SENSITIVE DUALIELLA MUTANT .1. GROWTH AND OSMOREGULATORY RESPONSES TO INCREASES IN SALT CONCENTRATION

Dunaliella is a genus of green unicellular algae distributed in all th e oceans and saline bodies of water throughout the world and distingui shed by unusual tolerance to salt. Since the cells of this genus do no t possess a rigid cell-wall, they respond to changes in salt concentra tion by rapid alterations in cell volume and then return to their orig inal 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 reduc ed capabilities of growth above 0.5 kmol m(-3) NaCl. The mutant is unu sual 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 c ell volume of mutant cells is reduced (206 mu m(3) as opposed to 255 m u m(3) in D. parva) and their rate of change of cell volume after an i ncrease in salt concentration is lower; it took about 10 min for mutan t 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 depe ndent on components of the cytoskeleton. The mutant throws light on ad aptations necessary to allow Dunaliella cells to grow at high salt con centrations and demonstrates that halotolerance includes, but is not e quivalent to, osmoregulation.