The Effect of Osmotic and Ionic Stress on the Primary Processes of Photosynthesis in Dunaliella tertiolecta

Cells of the euryhaline unicellular green alga Dunaliella tertiolectawere exposed to a range of osmotic potentials imposed by theaddition of NaCl, KCl, sucrose or ethylene glycol to the algalsuspending medium. The alga synthesized glycerol in responseto increased concentrations of NaCl, KCl or sucrose, but notto increased concentrations of ethylene glycol. The overallprocess of photosynthesis was inhibited by all four treatmentsduring the first 100 min, but especially by NaCl and KCl. Theinduction of chlorophyll fluorescence indicated that photosystemIl was inhibited by such treatment although to a lesser extentthan the overall process. The size and half-time of decay ofthe flash-induced field-indicating absorption change at 519nm were decreased by NaCl and KCl but the effect of sucroseand ethylene glycol on the absorption change was much less.Therefore, the photosynthetic processes in intact Dunaliellacells are more markedly inhibited by the combined osmotic andionic stresses due to NaCl and KCl than by osmotic stress alone.In most cases some recovery of the inhibition of photosynthesiswas seen concomitant with the synthesis of intracellular glycerol,suggesting that the alga has the ability to overcome osmoticand ionic stresses. Key words: Dunaliella, Photosynthesis, Osmotic stress