Effect of redox state on the dynamics Photosystem II during steady-state photosynthesis in eucaryotic algae

Using a pump and probe-flash technique we simultaneously measured changes in fluorescence yields (ΔΦ) and the yield of oxygen produced by the pump flash ( Y ) during steady-state photosynthesis in continuous background light in two species of eucaryotic algae, Chlorella pyrenoidosa , a chlorophyte, and Chaetoceros gracilis , a diatom. By varying the pump-flash intensity, we examined the flash-intensity saturation curves for ΔΦ and Y . When a small fraction of Photosystem (PS) II traps is closed by a blue-green background light the flash-intensity saturation curves for ΔΨ and Y closely followed a cumulative one-hit Poisson function. However, in cells exposed to a far-red background light (more than 720 nm) the flash-intensity saturation curve for ΔΦ (but not Y ) rose faster than predicted by a one-hit Poisson model. These results suggest that energy transfer between PS II reaction centers is moderated by the spectral quality of a continuous photon flux. Using saturating pump flashes we calculated the relative fraction of open/closed PS II reaction centers as perceived from the donor and acceptor sides as the background irradiance was changed. The relative changes in ΔΦ and Y were highly correlated at low and moderate levels of continuous background light, regardless of spectral quality. At high continuous photon fluxes, however, Y declined faster with increasing background irradiance than ΔΦ. Based on kinetic data is appears that the uncoupling between Y and ΔΦ at high photon flux densities is largely due to cyclic electron flow around PS II, which is negligible at subsaturating background irradiance levels. Cyclic electron flow around PS II accounted for 15–28% of the linear electron flow and may reduce damage to PS II reaction centers at supraoptimal irradiance levels.