Evaluation of Cyanobacterial Photosynthetic Activity

For evaluation of CEF activity, P700⁺ re-reduction and the redox kinetics of P700 were determined according to previously described methods^{65 (link),66 (link)} with some modifications. Briefly, Synechococcus cells were cultured in BG11 medium at an initial OD₇₃₀ of 0.05 in column photobioreactors and cultivated in MC1000 under 30 °C and 50 μmol photons/m²/s bubbled with air. Cells in the mid-log phase were harvested and adjusted to approximately 6 × 10⁹ cells in 3 mL and were dark-acclimated for 20 min at room temperature (23 °C). P700 redox was monitored after the termination of AL illumination (2800 μE/m²/s for 35 s) under a background of FR light (Intensity level 20) using Dual-PAM 100 (Heinz Walz) (Dual PAM v1.19 software). The P700 levels were normalized by equating the absorbance minimum after the termination of AL illumination to 0 and equating the absorbance maximum of 3 s before the termination of AL illumination to one. The re-reduction of P700⁺ in darkness was measured by monitoring absorbance changes at 830 nm and using 875 nm as a reference. After dark-acclimated for 20 min, 20 μM DCMU was added to the cultures before the measurement. The P700 was oxidized by FR light (Intensity level 20) for 40 s, and the subsequent re-reduction of P700⁺ in the dark was monitored. The curves were normalized by equating the absorbance maximum of 1 s before termination of FR light to one. The initial rate of P700⁺ re-reduction was acquired from normalized curves by calculating the slope of the initial values after FR was turned off and the R² was more than 0.99.