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Evidence for Cyclic Electron Flow around Photosystem II in Chlorella pyrenoidosa

Authors:	Falkowski P G Fujita Y Ley A Mauzerall D

Affiliation:	Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973.

Abstract:	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.

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