Evidence for Cyclic Electron Flow around Photosystem II in Chlorella pyrenoidosa

Electron flow around photosystem II was investigated in Chlorella pyrenoidosa. Using a bare platinum O₂ electrode, simultaneous measuremnts were made of steady-state photosynthesis in continuous light, the yield of oxygen (Y_o2) produced by a superimposed saturating xenon flash, and the change in fluorescence yield of a weak flash triggered before and 70 microseconds after the saturating flash. Throughout most of the continuous photosynthesis-irradiance curve, normalized O₂ flash yields (Y_o2/Y_o2max) and normalized variable fluorescence yields (Δ/Δ′) were linearly correlated with a slope of 1.0. As photosynthetic rates reached light saturation, however, the variable fluorescence yields remained relatively constant while O₂ flash yields decreased. These results strongly suggest that there is a cyclic electron flow around photosystem II in unpoisoned intact cells at light saturation and supraoptimal light intensities.