The oxidation of water to dioxygen gas is an energy demanding and mechanistically complex chemical reaction. These limitations have restricted the development of practical water-splitting catalysts needed for renewable hydrogen production. By contrast in photosynthesis, water oxidation proceeds efficiently using visible light energy that is absorbed by chlorophyll photopigments and made chemically available through charge separation in the protein complex, photosystem II (PSII). [1] Water is oxidized and O 2 evolved ultimately at an inorganic active site (Mn 4 O x Ca 1 Cl y ) [2,3] known as the water oxidizing complex (WOC), by a mechanism that is debated. [4,5] X-ray structural evidence suggesting that inorganic carbon in the form of (bi)carbonate may serve as another required inorganic cofactor within the WOC has been circumstantial [2] and disputed. [6] However, studies have shown that (bi)carbonate unquestionably participates in the lightdriven assembly of the inorganic core starting from the cofactor-depleted apo-WOC-PSII protein (called photoactivation). This is seen both by its acceleration of the rate of Mn 2+ photooxidation, [7,8] and by EPR spectroscopy of the resulting Mn 3+ assembly intermediate which revealed a strong influence on the strength of both the ligand field and the 55 Mn magnetic hyperfine coupling. [9] Although these observations indicate that (bi)carbonate acts to significantly alter the structural environment of the first Mn 3+ formed during photoassembly of the cluster and its electrochemical potential, they have failed to identify where it actually binds within PSII. Thus, no direct evidence exists for the postulated inner