Photoinhibition of Chlamydomonas reinhardtii in State 1 and State 2

The relationship between state transitions and photoinhibition has been studied in Chlamydomonas reinhardtii cells. In State 2, photosystem II activity was more inhibited by light than in State 1. In State 2, however, the D1 subunit was not degraded, whereas a substantial degradation was observed in State 1. These results suggest that photoinhibition occurs via the generation of an intermediate state in which photosystem II is inactive but the D1 protein is still intact. The accumulation of this state is enhanced in State 2, because in this State only cyclic photosynthetic electron transport is active, whereas there is no electron flow between photosystem II and the cytochrome b6f complex (Finazzi, G., Furia, A., Barbagallo, R. P., and Forti, G. (1999) Biochim. Biophys. Acta 1413, 117‐129). The activity of photosystem I and of cytochrome b6f as well as the coupling of thylakoid membranes was not affected by illumination under the same conditions. This allows repairing the damages to photosystem II thanks to cell capacity to maintain a high rate of ATP synthesis (via photosystem I-driven cyclic electron flow). This capacity might represent an important physiological tool in protecting the photosynthetic apparatus from excess of light as well as from other a-biotic stress conditions. The photochemical utilization of absorbed light is a critical step in the photosynthetic process. Because harvesting of light, photochemistry, and electron transfer occur on widely different scales of time, a correct balance among these different processes is required to optimize the efficiency of CO2 fixation. When light is absorbed in excess of what can actually be utilized by photochemistry, damage to the photosynthetic apparatus may be induced. Impairment of both photosystem I (PSI) 1