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

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Abstract
Cells of the euryhaline unicellular green alga Dunaliella tertiolecta were exposed to a range of osmotic potentials imposed by the addition of NaCl, KCl, sucrose or ethylene glycol to the algal suspending medium. The alga synthesized glycerol in response to increased concentrations of NaCl, KCl or sucrose, but not to increased concentrations of ethylene glycol. The overall process of photosynthesis was inhibited by all four treatments during the first 100 min, but especially by NaCl and KCl. The induction of chlorophyll fluorescence indicated that photosystem Il was inhibited by such treatment although to a lesser extent than the overall process. The size and half-time of decay of the flash-induced field-indicating absorption change at 519 nm were decreased by NaCl and KCl but the effect of sucrose and ethylene glycol on the absorption change was much less. Therefore, the photosynthetic processes in intact Dunaliella cells are more markedly inhibited by the combined osmotic and ionic stresses due to NaCl and KCl than by osmotic stress alone. In most cases some recovery of the inhibition of photosynthesis was seen concomitant with the synthesis of intracellular glycerol, suggesting that the alga has the ability to overcome osmotic and ionic stresses.