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

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. Photosynthesis is responsible for the genesis, development and regulation of vast majority forms of life on the Earth by using the ultimate free energy source of the sun. The conversion of (sun)light to chemical energy is initiated by the absorption of the photons in the closely packed network of protein-pigment complexes (antenna) followed by funneling of the excitation energy (exciton) to a specially organized (B)Chl dimer (P) in the reaction centers (RC) 1. Here an electron is stripped from P (P→P +) converting the energy of the exciton into chemical (redox) energy of P/P +. The electron is transferred via the primary quinone acceptor Q A to the secondary quinone acceptor Q B producing a series of transient charge separated states (P + Q-). While Q A can accept one electron only, Q B performs two-electron chemistry: by binding two protons and forming reduced quinone QH 2 , it is exchanged for an oxidized quinone from the quinone pool in the membrane 2-4. To describe the functional cooperation of the antenna pigments in light collection, the loose concept of the photosynthetic unit (PSU) was introduced 5. According to the present knowledge, the structure of the PSU of photosynthetic bacteria can be identified as the core complex including the photochemical RC and the closely attached light-harvesting (core) antenna (LH1, B870 in Rhodobacter (Rba.) sphaeroides) together (if exists) with the peripheral antenna (LH2, B800-850 complex in Rba. sphaeroides) loosely arranged in ...

Roadmap of electrons from donor side to the reaction center dimer of photosynthetic purple bacteria with mutated cytochromes

“…Induction and relaxation of BChl fluorescence. The home-built experimental set-up (BChl fluorometer) and the data processing of fluorescence of intact cells have been described earlier [Kocsis et al 2010;Maróti et al 2020]. The fluorescence (900 nm) emitted in the direction perpendicular to the actinic light beam were detected by a near-infrared-sensitive, large-area (diameter 10 mm), and high-gain Siavalanche photodiode (APD; model 394-70-72-581; Advanced Photonix Inc., USA).…”

“…which is related to the measured yield of BChl fluorescence via the connectivity parameter between the photosynthetic units p [de Rivoyre et al 2010;Kis et al 2020]:…”

Light-induced reversible reorganizations in closed Type II reaction centre complexes: physiological roles and physical mechanisms

“…In a recent study, Maróti et al . [101] recorded simultaneously the induction and the relaxation kinetics of the fluorescence yield and the oxidation level of P—using short laser-diode probing flashes and transient absorption spectroscopy, respectively—in intact purple bacterial cells. The carefully selected experimental conditions—using cycA mutant of Rb.…”

Capacity and kinetics of light-induced cytochrome oxidation in intact cells of photosynthetic bacteria

“…In purple non-sulphur bacteria, electron and proton transfer reactions convert light energy into biochemically amenable energy that covers the energy cost required for these organisms to live. The primary process of photosynthesis is initiated by the absorption of light via light-harvesting pigments followed by the transfer of the electronic excitation energy to the special pair, P, of the reaction center (RC) protein 1 – 3 . The excitation of the special pair (P*) leads to charge separation (P + Q A ─ ) by oxidation of P and passage of the electron to the primary quinone (Q A ) of the acceptor complex via bacteriochlorophyll monomer and bacteriopheophytin pigments.…”