PLASMA-MEMBRANE STEROLS ARE ESSENTIAL FOR SENSING OSMOTIC CHANGES IN THE HALOTOLERANT ALGA DUNALIELLA

The halotolerant alga Donaliella responds to hyperosmotic stress by sy nthesis of massive amounts of glycerol. The trigger for this osmotic r esponse is the change in cell volume, but the mechanism that senses vo lume changes is not known. Preincubation of Donaliella salina with tri demorph, a specific inhibitor of sterol biosynthesis, inhibits glycero l synthesis and volume recovery. The inhibition is associated with sup pression of [C-14]bicarbonate incorporation into sterols and is correl ated with pronounced depletion of plasma membrane sterols. Incubation of sterol-depleted cells with cholesterol hemisuccinate restores the c apacity for volume regulation in response to hyperosmotic stress. Trid emorph as well as lovastatin also inhibit volume changes that are indu ced by high light in Donaliella bardawil, a species that responds to h igh light intensity by synthesis of large amounts of beta-carotene. Th ese volume changes result from accumulation of glycerol and are associ ated with de novo synthesis of sterols. The major plasma membrane ster ol in D. salina and the high-light-induced sterol in D. bardawil co-mi grate with ergosterol on thin-layer chromatography and on reversed-pha se high-performance liquid chromatography. These results suggest that the osmosensory mechanism in Donaliella resides in the plasma membrane , and that sterols have an important role in sensing osmotic changes.