Introduction Every oxygen molecule we breathe is produced from two water molecules in the photosystem 2 protein complex of higher plants, algae and cyanobacteria. This highly endothermic reaction is carried out during photosynthesis using visible light energy under ambient conditions. To perform this task a unique water oxidising catalytic complex, Mn 4 CaO 5/6 , evolved some three billion years ago. This complex oxidises two water molecules to molecular oxygen at a rate approaching 1000 s -1 at ambient temperatures and pressure. Besides being one of the most important reactions in biology, it is also of intense interest from a green energy perspective, where it is recognized to be the main barrier to the development of commercial solar devices for the generation of hydrogen from water. Water oxidation to dioxygen is a challenging due to the high endergonicity (E° = 0.82 V (vs NHE) at pH 7) for the reaction, 2H 2 O → O 2 + 4H + + 4e - and the associated need to remove four protons and four electrons with the formation of an oxygen-oxygen, O-O, bond. Two broad mechanistic proposals, either water nucleophilic attack of metal oxo or direct metal oxo radical coupling, have been proposed for artificial water oxidation catalysts. Somewhat similar proposals have been put forward for the WOC namely water nucleophilic attack or oxyl radical-oxo coupling These require the generation of a reactive oxo species in the final Kok-cycle S 4 state. Artificial catalysts generally use very high strength oxidising agents to generate reactive oxo species, either radical oxygen species or highly charged metal electrophilic species. The WOC on the other hand is limited to the approximately 1 V oxidising power of the nearby tyrosyl radical, Y Z OX . The current mechanisms for the WOC which propose generation of a reactive "hot" oxo species in the S 4 state need to explain how such a species can be generated when the oxidising capability available from the visible light energy available via the S 3 Y Z ox oxidant is around 1V. It is also unclear how triplet O 2 can be produced from peroxo with such a mechanism given that the last oxidising equivalent has been used.