Recovery of the photosynthetic apparatus from photoinhibition during dark incubation of the green alga <i>Dunaliella salina</i>

The light-independent recovery of the photosynthetic apparatus from photoinhibition was monitored upon a transition of irradiance-stressed Dunaliella salina Teod. to darkness. Upon dark incubation, the chlorophyll (Chl) a /Chl b ratio of the cells decreased promptly with a half-time of 2.5 h from about 12:1 to about 5:1. In contrast, dark incubation of control cells resulted in only a negligible change of the Chl a /Chl b ratio. During dark incubation of irradiance-stressed cells, the level of the Chl a and b light-harvesting proteins of photosystem II (PSII) increased, a change accompanied by alterations in the composition of these light-harvesting proteins. The amount of photodamaged PSII, measured from the relative amount of a 160 kDa protein complex which contains the photodamaged D1 reaction center protein, decreased during dark incubation after an initial lag period. Concomitantly, the amount of functional PSII, measured from the 32 kDa form of D1, increased slightly in the dark. The results show that, in the dark, photodamaged D1 is slowly removed upon degradation from the thylakoid membrane and replaced by a de novo synthesized D1 protein. The amount of reaction center proteins and number of photochemically active PSI centers increased in the dark. These results suggest that thylakoid membranes of irradiance-stressed D. salina exist in a state of dynamic flux. We conclude that several aspects of the D. salina recovery from photoinhibition are light independent.