Dynamics of In Vivo Membrane Processes in Algal Thylakoids as Analyzed from Chlorophyll Fluorescence Induction using the Photosystem II and Thylakoid Models

The OJIPSMT pattern of chlorophyll (Chl) a fluorescence induction (FI) was obtained using Scenedesmus obliquus ( Scenedesmus ) microalgal cells exposed, after dark adaptation, to moderate and high light intensities (1200 and 1800 μmol photons m –2 s –1 ) in a time range from the microseconds to 10 minutes. The fast and, particularly, slow OJIPS(M)T stages of FI extending up to 100 s were quantitatively described by fitting a thylakoid membrane (TM) model. The calculations adequately reproduced the dynamics of P700 oxidoreduction transients. Parameters of TM processes were determined, and the transients of the TM model variables are shown in calculations. Dynamic stages of redox reactions in reaction centers (RCs) of photosystem II (PSII), the PQ/PQH 2 quinone pool, the cytochrome b 6 f complex (Cyt bf ), and P700 of photosystem I (PSI) RCs were found to accompany the adaptation of the TM system to light. In silico analysis revealed the role that Cyt bf plays in regulating electron fluxes when b L/H hemes remain in a more reduced state at a high light intensity than at moderate illumination in an interval from 100 ms to 3 s. A dependence of the rate constants k 42–49 on the level of Q_B^(2) - reduction was revealed for non-radiative recombination of separated charges in the RC Phe – P680 + and Q _A^ - P680 + , together with a dependence of k 42–49 on chloroplast illumination. Setting the dynamic rate constants k FNR (t) and k D-qE (t) made it possible to compare the generation ΔpH o-i ( t ) = pH S ( t ) – pH L ( t ), initiation of qE quenching at pH L|trigg = 6.7, activation of FNR reductase in the light, and achievement of stationary charge flows within 1–2 minutes of light induction. Fitting the model of isolated PSII to phytoplankton FI signals showed similar electron transfer parameters of PSII RCs for Scenedesmus and Chlorella monocultures and phytoplankton samples. Markers of the phytoplankton community state were an exception, differing between degrading and ecologically functional water bodies. The differences in markers included a reduced size of the antennas, an increased energy dissipation, and a decrease in pH in the lumen.