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Water oxidation by photosystem (PS) II in oxygenic photosynthetic organisms is a major source of energy on the earth, leading to the production of a stable reductant. Mechanisms generating a high oxidation potential for water oxidation have been a major focus of photosynthesis research. This potential has not been estimated directly but has been measured by the redox potential of the primary electron acceptor, pheophytin (Phe)a . However, the reported values for Phea are still controversial. Here, we measured the redox potential of Phea under physiological conditions (pH 7.0; 25 °C) in two cyanobacteria with different special pair chlorophylls (Chls):Synechocystis sp. PCC 6803, whose special pair for PS II consists of Chla , andAcaryochloris marina MBIC 11017, whose special pair for PS II consists of Chld . We obtained redox potentials of −536 ± 8 mV forSynechocystis sp. PCC 6803 and −478 ± 24 mV forA. marina on PS II complexes in the presence of 1.0 M betaine. The difference in the redox potential of Phea between the two species closely corresponded with the difference in the light energy absorbed by Chla versus Chld . We estimated the potentials of the special pair of PS II to be 1.20 V and 1.18 V forSynechocystis sp. PCC 6803 (P680) andA. marina (P713), respectively. This clearly indicates conservation in the properties of water-oxidation systems in oxygenic photosynthetic organisms, irrespective of the special-pair chlorophylls.

Redox potential of pheophytin a in photosystem II of two cyanobacteria having the different special pair chlorophylls