Salt shock induces state II transition of the photosynthetic apparatus in dark-adapted Dunaliella salina cells

Previous evidence indicates that hypoosmotic shock decreases the photosynthetic rate and induces a state I transition of the photosynthetic apparatus in the dark-adapted halotolerant green alga, Dunaliella salina. The present study investigated the regulation of state transition by salt shock, another stressor D. salina commonly encounters. An upward-shift in the external NaCl concentration decreased the photosynthetic rate, the photosystem II (PSII) maximal fluorescence, and the respiratory rate of D. salina. An ATP synthase inhibitor, an uncoupler, or salt shock decreased intracellular ATP content and induced phosphorylation of the major light harvesting chlorophyll a/b proteins in dark-adapted D. salina cells. Furthermore, salt shock increased cyclic electron flow around photosystem I (PSI), as indicated by the enhancement in the post-illumination transient increase in chlorophyll fluorescence. These results suggest that salt shock induces state II transition in dark-adapted D. salina cells and that ATP content depression is likely involved in the regulation. Together, salt shock and hypoosmotic shock exert opposite effects on state transition and may decrease the photosynthetic rate via different mechanisms.