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Manganese is an essential micronutrient for all organisms. Its requirement by plants is particularly high because of its role in the oxidation of water in photosynthesis1–5. According to thermodynamic considerations, manganese should exist in oxic waters as MnO2(ref. 6) which is insoluble and, therefore, not directly available for plant nutrition. In contrast to thermody-namic predictions, however, most of the manganese in near surface seawater exists as soluble reduced Mn(II) (ref. 7). Although slow oxidation kinetics are at least partially responsible for the presence of Mn(II) in oxic waters8,9, reduced manganese, nevertheless, should be converted to particulate manganese oxides (at rates that depend on several kinetic factors10,11) and be lost from the water column by sinking12. We report here experiments that demonstrate photoreduction of manganese oxides by dissolved organic substances (humic substances) in seawater. Such reactions appear to be important in maintaining manganese in a dissolved reduced form in photic waters, thereby enhancing its supply to phytoplankton.