Chiamydomonas reinhardtii Counteracts Photoinhibition'

The effect of strong irradiance (Q000 micromole photons per square meter per second) on PSII heterogeneity in intact cells of Chiamydomonas 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 (PSIIa) centers account for about 70% of all PSII in the thylakoid membrane and are responsible for plastoquinone reduction (OB-reducing centers). The smaller antenna (PSII) account for the remainder of PSII and exist in a state not yet able to photoreduce plastoquinone (0B-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,content remained the same or slightly increased during strong-irradiance treatment, whereas the photochemical activity of PSIIa decreased by 80%. Analysis of fluorescence induction transients displayed by intact cells indicated that strong irradiance led to a conversion of PSlI,6 from a 0.-nonreducing to a QO-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 PSIIa. The results suggest that PSlI, 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,. Light energy drives photosynthesis but excess light is potentially damaging to the photosynthetic apparatus. The latter phenomenon is called photoinhibition and is manifested as a loss in chloroplast electron-transport capacity (for recent reviews see 38, 39). Though plants differ greatly in their sensitivity to photoinhibition, current evidence indicates widespread occurrence of photoinhibition in both terrestrial and aquatic environments (2, 33). In most cases, the loss of PSII function is responsible for decreased electron-transport activity, though damage to the PSI complex has also been reported (39). Recent investigations from several laboratories have sought to establish the single or several sites of light-dependent damage within the PSII complex. One group ofstudies has focused This research was supported by the U.S. Department of Agriculture, competitive research grant number 88-37264-3915 to P. J. N. and National Science Foundation grant DCB-88 15977 to A. M. on the primary electron-transport events within the PSII reaction center complex, i.e. P680* Pheo QA -* P680'Pheo QA P680+Pheo QA(1) When isolated spinach thylakoids were illuminated with strong-irradiance (2500 Amol.m-2 s-') at 0C, both the QA2 (320 nm absorbance change) and Pheo (685 nm absorbance change) signal were lowered in parallel (8, 9, 12), suggesting inhibition of primary photochemistry. The PSII primary charge separation was also inhibited when intact cells of the green alga, Chiamydomonas reinhardtii, were exposed to strong irradiance (12). The rapid loss of the pheophytin photoreduction during photoinhibition has also been detected using EPR difference spectroscopy (46). These results indicated that photoinhibitory damage affected the ability of PSII to form the P680+ Pheocharge separation. An alternative proposal (22, 37) is that photoinhibition results from light-dependent damage to the plastoquinone binding site, which is located on the 32 kD 'Dl' polypeptide of PSII. According to the latter model, only electron transfer from QA to the bound plastoquinone, QB, is disrupted, whereas no damage occurs to the water splitting enzyme or the reaction center. Support for this proposal was found in the rapid in vivo turnover of Dl in C. reinhardtii (22, 37). Subsequently, it has been accepted that the functional components of the PSII reaction center (Mn, Z, P680, Pheo, QA and QB) are all bound to the Dl/D2 heterodimer in analogy with the structure of the crystallized bacterial reaction center (32, 44). In the context of the latter model of PSII, enhanced turnover of Dl would be a likely consequence of processes that repair a damaged PSII reaction center (3, 28). The precise events leading to the loss of PSII primary charge separation remain to be resolved (36, 43). There is considerable experimental support for the concept of heterogeneity among PSII reaction centers both in the functional antenna size and electron-transport properties on the reducing side of QA (5). The concept of PSII heterogeneity was originally introduced to explain the biphasic nature of PSII activity, measured either by fluorescence induction ki2 Abbreviations: QA, primary quinone electron acceptor of PSII; F,, nonvariable fluorescence yield; Fe,, initial plateau of fluorescence yield in the fluorescence induction curve; Fp, peak fluorescence in the fluorescence induction curve; Fm, maximum fluorescence; F,, variable fluorescence = Fm-F1,; P680, reaction center of PSII; Pheo, pheophytin; Z, secondary PSII donor; QBsecondary quinone electron acceptor of PSII; LHC, light-harvesting complex; PS II-QB1-nonreducing, PSII center with impaired QA-QB interaction.