Electron transfer and Hydrogen ion loss resulting in generation of molecular oxygen.
This process is seen in the biological world in the regeneration of photosystem II of the light reaction of photosynthesis, where water supplies the electrons necessary for photosystem II's reduction back to its un-oxidized state. The result is pretty much all of the world's atmospheric oxygen (O2).
The oxidation of water as seen in chloroplasts and cyanobacteria is likely built upon more ancient systems where electrons necessary to restore photosystems are removed from less electronegative atoms than oxygen, such as sulfur as found in hydrogen sulfide, i.e., H2S (hydrogen sulfide), a molecule that more than a little bit resembles H2O (water). Why the switch? Perhaps it simply was because water can be so remarkably abundant in locations where light also is substantially present.
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