Fig. 1 Major electron transport pathways around PSINote： Dark adapted cells at final chlorophyll concentration 10 μg·mL-1 were illuminated by either orange light （630 nm， 940 μmol photons m-2·s-1） or far-red light （725 nm， 1 400 μmol photons m-2·s-1） for 20 s and followed by darkness. The absorbance changes at 810 nm were monitored by dark-pulse mode using Jts-10 （Biologics）. The measurements were performed in the absence or presence of 20 μmol·L-1 DCMU， 200 μmol·L-1 MV or 1 mmol·L-1 NaN3.

Fig. 2 Verification of pgr5 and flv1 mutants by PCRA： Segregation of pgr5 in Δpgr5 and M55/Δpgr5； B： Segregations of flv1 in WT， Δflv1， Δflv1/Δpgr5， flv1/M55 and flv1/M55/Δpgr5

Fig. 3 Characterization of various flv1 mutantsA： Transcript accumulation of flv1 gene in the WT， M55， flv1/M55， and flv1/M55/Δpgr5 analyzed by qRT-PCR， and the result was shown as the average of 3 independent replicates ± SD； B： P700 oxidation kinetics of the WT， Δflv1， M55， flv1/M55， and flv1/M55/Δpgr5. Dark adapted cells were illuminated by 940 μmol photons m-2 s-1 actinic light （630 nm）， and the absorbance changes at 810 nm were monitored

Fig. 4 Growth of WT and mutants under different conditionsA： Growth of WT and various mutants under different CO2 conditions， light intensities and additional Na2S2O3； liquid cells in logarithmic growth were centrifuged and resuspended in fresh BG-11 medium， and diluted to OD750= 0.5， 0.05 and 0.005. 4 μl cell suspension were plated on the agar plate； B： Growth of WT， mutants and overexpression strains under fluctuating light； C： Growth of WT and mutant under combination of fluctuating light and darkness conditions

Fig. 5 Hydrogen peroxide treatment and SOD activities of WT and mutantsA： Growth of WT and mutants in the presence of hydrogen. LC grown cells （0.6-0.8 of OD750） were collected and resuspended in BG-11 medium （pH 8.3） to OD750=0.2. The cultures were supplemented with 1.0， 1.5， 2.0 and 3.0 mmol·L-1 H2O2， respectively， and photographed after 16 and 48 h. B： SOD activity of WT and mutant cells grown under LC and HC conditions. The results are the average of the measurements from three independent cultures

Fig. 6 Expression of major photosynthetic proteins on the thylakoid membranes and in the soluble fractionNote： Membrane or soluble proteins were separated by SDS-PAGE， and immunodetected using specific antibodies against D1， PsaB， NdhK， AtpB， FNR， CpcA and RbcL. In each well， 10 μg protein was loaded.

Fig. 7 Kinetic analysis of P700+ reduction in WT and mutantsNote： Kinetics of P700 in WT， Δpgr5， Δflv1， Δflv1/Δpgr5， M55， M55/Δpgr5， flv1/M55， and flv1/M55/Δpgr5. Black square line， 20 μmol·L-1 DCMU； yellow circle line， no addition； blue triangle line， 1 mmol·L-1 NaN3； pink rhombus line， 200 μmol·L-1 MV. The 0.0 and 1.0 on the y axis correspond to fully reduced and oxidized P700， respectively.

Fig. 8 Cyclic electron flow in WT and mutantsA： CET rate determined by P700+ rereduction in WT and mutants under LC and HC； B： CET rate in WT and NDH and PGR5 mutants grown under LC

Fig. 9 Electron flows around P700 and PQ pool in WT and mutants under LC and HCA： Electron flows of P700 oxidation via Flv1/3 mediated Mehler-like reaction and FNR； B： Electrons from metabolites； C： Electrons to O2 via terminal oxidases

Fig. 10 Working scheme of CET， Mehler-like reaction and SOD in cyanobacteriaNote： Arrows represents electron flows and thickness represents amount of electron flows. Accumulation of NdhK representing NDH-1 complexes of WT grown under LL （20 μmol photons·m-2·s-1）， HL （300 μmol photons·m-2·s-1） and FL made by Western blot was shown in right corner.