Recent advances in the understanding of superoxide anion radical formation in the photosynthetic electron transport chain

The Mehler reaction is the major source of superoxide anion radicals (O₂^•−) and hydrogen peroxide molecules in the chloroplast. In terms of mechanism, the Mehler reaction starts as a one-electron oxidation of electron transfer components of the photosynthetic apparatus by molecular oxygen producing O₂^•−, which is further reduced to H₂O₂, the stable product of the Mehler reaction. This review covers recent data on the mechanisms of O₂ photoreduction in the photosynthetic electron transport chain, with particular emphasis on photosystem I (PSI), which has long been considered as the major site of O₂^•− production in the chloroplast. Despite the long history of the study, until recently, there was some contradiction as to the precise location(s) of O₂ photoreduction within PSI. Here, I explain some controversies that still exist and discuss evolutionary trends toward optimizing PSI activity in the presence of O₂.