THE FUNCTION OF P700 AND CYTOCHROME f IN THE PHOTOSYNTHETIC REACTION CENTER OF SYSTEM 1 IN RED ALGAE

Abstract— Kinetics and quantum yields of light-induced oxidation of P700 and the f -type cytochrome were measured in marine red algae from absorbance changes in the region 420, 435 and 705 nm. The quantum yield for cytochrome oxidation in Iridaea splendens and Schizymenia pacifica was 0.5-0.65 in far-red light, at 20 and at 0C.
Oxidation rates of P700 measured when varying amounts of cytochrome were in the oxidized state indicated that a reaction center of system 1 in Iridaea contains 1 P700 and 4 cytochrome molecules. Oxidized P700 only accumulates when all 4 cytochromes are oxidized. The rate of photochemistry of system 1, measured as the sum of the rates of P700 and cytochrome oxidation, was independent of the oxidation level of cytochrome, but decreased with accumulation of oxidized P700. This decrease was less than proportional to the fraction of P700 that was in the oxidized state, which suggested transfer of excitation energy between reaction centers.
The quantum yield for cytochrome oxidation after dark periods of 1 min or more was only about 0.2. This effect was tentatively ascribed to a dark reduction of the cytochrome coupled to phosphorylation.