The effect of strong irradiance (2000 micromole photons per square meter per second) on PSII heterogeneity in intact cells of Chlamydomonas reinhardtii was investigated. Low light (LL, 15 micromole photons per square meter per second) grown C. reinhardtii are photoinhibited upon exposure to strong irradiance, and the loss of photosynthetic functioning is due to damage to PSII. Under physiological growth conditions, PSII is distributed into two pools. The large antenna size (PSII(alpha)) centers account for about 70% of all PSII in the thylakoid membrane and are responsible for plastoquinone reduction (Q(b)-reducing centers). The smaller antenna (PSII(beta)) account for the remainder of PSII and exist in a state not yet able to photoreduce plastoquinone (Q(b)-nonreducing centers). The exposure of C. reinhardtii cells to 60 minutes of strong irradiance disabled about half of the primary charge separation between P680 and pheophytin. The PSII(beta) content remained the same or slightly increased during strong-irradiance treatment, whereas the photochemical activity of PSII(alpha) decreased by 80%. Analysis of fluorescence induction transients displayed by intact cells indicated that strong irradiance led to a conversion of PSII(beta) from a Q(b)-nonreducing to a Q(b)-reducing state. Parallel measurements of the rate of oxygen evolution revealed that photosynthetic electron transport was maintained at high rates, despite the loss of activity by a majority of PSII(alpha). The results suggest that PSII(beta) in C. reinhardtii may serve as a reserve pool of PSII that augments photosynthetic electron-transport rates during exposure to strong irradiance and partially compensates for the adverse effect of photoinhibition on PSII(alpha).
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