VARIABILITY OF PHOTOSYSTEM-II QUANTUM YIELD AND RELATED PROCESSES IN CHLORELLA-PYRENOIDOSA (CHLOROPHYTA) ACCLIMATED TO AN OSCILLATING LIGHT REGIME SIMULATING A MIXED PHOTIC ZONE

The regulation of photosystem II (PSII)-related parameters was characterized for Chlorella pyrenoidosa Chick grown in continuous nutrient-replete culture and acclimated to a light field simulating fluctuations of a natural environment. The maximum quantum yield for charge separation at PSII showed a Eight-driven decrease during the first half of the photoperiod when incident irradiance (integrated hourly) was increasing to a midday maximum. This change was accompanied by a decrease in the effective cross-section for all PSIIs and the connectivity between PSIIs, while the cross-section for inactive centers increased. Furthermore, a decrease in the number of active PSII centers and the amount Of Q(B) bound to D1 became evident. During the second half of the day, as irradiance declined toward the end of the photoperiod, all the observed changes in PSII photochemistry were reversed. The low quantum yield values at the middle of the photoperiod returned to 85% of the value at the end of the photoperiod by the addition of far-red light, suggesting that a State I-II-I transition took place during the day. Measured rates of oxygen production correlated well with fluorescence-derived estimates of photosynthesis. The overall results suggest that PSII photochemistry is optimized to minimize the susceptibility to photoinhibition and to allow balanced growth under fluctuating irradiation and that the impact of these alterations on primary production can be adequately monitored through bio-optical measurements alone.