The dynamic behavior of phycobilisome movement during light state transitions in cyanobacterium Synechocystis PCC6803

Abstract

Light state transition is a physiological function of oxygenic organisms to balance the excitation of photosystem II (PSII) and photosystem I (PSI), hence a prerequisite of oxygen-evolving photosynthesis. For cyanobacteria, phycobilisome (PBS) movement during light state transition has long been expected, but never observed. Here the dynamic behavior of PBS movement during state transition in cyanobacterium Synechocystis PCC6803 is experimentally detected via time-dependent fluorescence fluctuation. Under continuous excitation of PBSs in the intact cells, time-dependent fluorescence fluctuations resemble "damped oscillation" mode, which indicates dynamic searching of a PBS in an "overcorrection" manner for the "balance" position where PSII and PSI are excited equally. Based on the parallel model, it is suggested that the "damped oscillation" fluorescence fluctuation is originated from a collective movement of all the PBSs to find the "balance" position. Based on the continuous fluorescence fluctuation during light state transition and also variety of solar spectra, it may be deduced that light state transition of oxygen-evolution organisms is a natural behavior that occurs daily rather than an artificial phenomenon at extreme light conditions in laboratory.

New mechanism revealed for light-state transition in cyanobacterium Arthrospira platensis according to 77-K fluorescence kinetics

“…Importantly, this implies that cyanobacteria sense any imbalance of the excitation distribution and respond to it via state transition. Previously, it was proposed that state transition was a natural behavior of cyanobacteria under solar irradiation and not short-term acclimation under an artificial light condition [13], which is further confirmed by this work.…”

“…From the spectra, the time-dependent fluorescence amplitudes for PBS rods (C-phycocyanin; C-PC), PBS core (allophycocyanin; APC) and PS I and PS II components were derived, as shown in Figure 1(b). A remarkable feature is that the PBS fluorescence components do not always keep pace with the fluorescence components of the photosystems, which completely differs from the case for orange-light-induced transition from state 1 to state 2 [13]. Figure 1(b) shows that the transition from state 2 to state 1 took 8 min, at which time the PS I fluorescence and PS II fluorescence became saturated.…”

“…However, quantitatively, PS I and PS II fluorescence components became saturated at 2 min while the PBS components became saturated at 1 min, suggesting that the state transition was accelerated compared with that under 100 µmol m -2 s -1 illumination. It was observed that the frequency of fluorescence oscillation or the state transition time for cyanobacterium Synechocystis PCC6803 cells was a function of orange-light intensity, which is ascribed to the acceleration of PBS movement at higher intensity [13]. The kinetics demonstrated in Figures 1 and 2 suggest that not only PBS movement but also inverse spillover was accelerated at higher light intensity, in turn suggesting that the state transition rate is a function of light intensity irrespective of whether a single mechanism or multiple mechanisms are involved.…”

“…Previously, temporal fluctuation of the 77-K fluorescence for PBS rods, core, and PS I and PS II components was monitored when cyanobacterium Synechocystis PCC6803 cells were continuously illuminated with orange light, and the "damped-oscillation" fluorescence fluctuation revealed the dynamic behavior of PBSs to find a "balanced" position. Furthermore, it demonstrated that PBS and photosystem fluorescence components varied in homogeneous kinetics, indicating that PBSs regulated the excitation distribution independently [13]. While orange light is mainly absorbed by PBSs, it is understandable that a transition from state 1 to state 2 is completely regulated by mobile PBS.…”

Changes in the transthylakoid proton gradient are caused by the movement of phycobilisomes in the cyanobacterium Synechocystis sp. strain PCC 6803

“…It has been shown previously that the speed of PBS movement depends on the intensity of the inducing light [27]. Under 5.2 μE m -2 s -1 FR and 15 μE m -2 s -1 GL, the movement of PBSs occurred rapidly and was complete within 1-2 min (Figures 2 and 3).…”