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Antenna systems serve to absorb light and to transmit excitation energy tothe reaction center (RC) in photosynthetic organisms. As the emitted(bacterio)chlorophyll fluorescence competes with the photochemical utilizationof the excitation, the measured fluorescence yield is informed by the migrationof the excitation in the antenna. In this work, the fluorescence yieldconcomitant with the oxidized dimer (P+) of the RC were measured during lightexcitation (induction) and relaxation (in the dark) for whole cells ofphotosynthetic bacterium Rhodobacter sphaeroides lacking cytochrome c_2 asnatural electron donor to P+ (mutant cycA). The relationship between thefluorescence yield and P+ (fraction of closed RC) showed deviations from thestandard Joliot-Lavergne-Trissl model: 1) the hyperbola is not symmetric and 2)exhibits hysteresis. These phenomena originate from the difference between thedelays of fluorescence relative to P+ kinetics during induction and relaxation,and in structural terms from the non-random distribution of the closed RCsduring induction. The experimental findings are supported by Monte Carlosimulations and by results from statistical physics based on random walkapproximations of the excitation in the antenna. The applied mathematicaltreatment demonstrates the generalization of the standard theory and sets thestage for a more adequate description of the long-debated kinetics offluorescence and of the delicate control and balance between efficient lightharvest and photoprotection in photosynthetic organisms.

Correlated clusters of closed reaction centers during induction of intact cells of photosynthetic bacteria