Exogenous Manganese as an Electron Source for Both Photosystem II and Radical Superoxides

Abstract

Exogenous manganese acts as an electron source for both photosystem I1 (PS 11) and radical superoxides formed after PS I. This conclusion is supported by measurements of external acidification when chloroplasts containing exogenous manganese and methyl viologen are illuminated. The acidification and increased oxygen consumption when manganese was added shows two moles of protons to be produced per mole of oxygen consumed. Reactions for manganese oxidation preceding PS I1 and following PS I are given with the net result:

2H₂O + Mn²⁺ + O₂ ͛4 MnO₂ + H₂O₂ + 2H⁺

This reaction was shown to reverse in the dark and to have a chlorophyll dependency for both the rate of the forward photoreaction and reverse dark reaction. The pH-dependency shows manganese can compete with water as an electron source for PS II only under alkaline conditions and the electrochemical potentials for the competition are discussed in terms of the half reactions involved.