The oxidation of two water molecules to form one molecule of oxygen has been performed uniquely at ambient conditions by the Water Oxidising Complex (WOC) of Photosystem II (PSII) for around three billion years. Visible light energy is used to sequentially extract four electrons from two water molecules via its core catalytic centre a, Mn4O5/6Ca cluster, Figures and. The stepwise S state cycle, Figure , allows the WOC to produce oxidised Mn centres, and deprotonated substrate water molecules facilitating the ultimate formation of an O-O bond at the S3 or S4 state.