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

TL;DR

This study investigates the distribution and dynamics of closed reaction centers in photosynthetic bacteria, revealing deviations from classical models and highlighting the importance of spatial organization in light energy management.

Contribution

It demonstrates non-random clustering of closed reaction centers during induction, supported by experimental data and advanced simulations, extending the standard fluorescence kinetics theory.

Findings

Deviations from symmetric hyperbola in fluorescence-P+ relationship

Hysteresis observed during induction and relaxation phases

Non-random clustering of closed reaction centers during induction

Abstract

Antenna systems serve to absorb light and to transmit excitation energy to the reaction center (RC) in photosynthetic organisms. As the emitted (bacterio)chlorophyll fluorescence competes with the photochemical utilization of the excitation, the measured fluorescence yield is informed by the migration of the excitation in the antenna. In this work, the fluorescence yield concomitant with the oxidized dimer (P+) of the RC were measured during light excitation (induction) and relaxation (in the dark) for whole cells of photosynthetic bacterium Rhodobacter sphaeroides lacking cytochrome c_2 as natural electron donor to P+ (mutant cycA). The relationship between the fluorescence yield and P+ (fraction of closed RC) showed deviations from the standard Joliot-Lavergne-Trissl model: 1) the hyperbola is not symmetric and 2) exhibits hysteresis. These phenomena originate from the difference between the delays of fluorescence relative to P+ kinetics during induction and relaxation, and in structural terms from the non-random distribution of the closed RCs during induction. The experimental findings are supported by Monte Carlo simulations and by results from statistical physics based on random walk approximations of the excitation in the antenna. The applied mathematical treatment demonstrates the generalization of the standard theory and sets the stage for a more adequate description of the long-debated kinetics of fluorescence and of the delicate control and balance between efficient light harvest and photoprotection in photosynthetic organisms.