Model quantification of the light-induced thylakoid membrane processes in Synechocystis sp. PCC 6803 in vivo and after exposure to radioactive irradiation.

Measurements of OJIP-SMT patterns of fluorescence induction (FI) in Synechocystis sp. PCC 6803 (Synechocystis) cells on a time scale up to several minutes were mathematically treated within the framework of thylakoid membrane (T-M) model (Belyaeva et al., Photosynth Res 140:1-19, 2019) that was renewed to account for the state transitions effects. Principles of describing electron transfer in reaction centers of photosystems II and I (PSII and PSI) and cytochrome bf complex remained unchanged, whereas parameters for dissipative reactions of non-radiative charge recombination were altered depending on the oxidation state of Q-site (neutral, reduced by one electron, empty, reduced by two electrons). According to our calculations, the initial content of plastoquinol (PQH) in the total quinone pool of Synechocystis cells adapted to darkness for 10 min ranged between 20 and 40%. The results imply that the PQ pool mediates photosynthetic and respiratory charge flows. The redistribution of PBS antenna units responsible for the increase of Chl fluorescence in cyanobacteria (qT) upon state 2 → 1 transition or the fluorescence lowering (qT) due to state 1 → 2 transition were described in the model by exponential functions. Parameters of dynamically changed effective cross section were found by means of simulations of OJIP-SMT patterns observed on Synechocystis cells upon strong (3000 μmol photons ms) and moderate (1000 μmol photons ms) actinic light intensities. The corresponding light constant values kL = 1.2 ms and 0.4 ms define the excitation of total antenna pool dynamically redistributed between PSII and PSI reaction centers. Although the OCP-induced quenching of antenna excitation is not involved in the model, the main features of the induction signals have been satisfactorily explained. In the case of strong illumination, the effective cross section decreases by approximately 33% for irradiated Synechocystis cells as compared to untreated cells. Under moderate light, the irradiated Synechocystis cells showed in simulations the same cross section as the untreated cells. The thylakoid model renewed with state transitions description allowed simulation of fluorescence induction OJIP-SMT curves detected on time scale from microseconds to minutes.