Cyanobacteria hydrogen sulfide oxidation

In the case of oxygenic photosynthesis, catalyzed in the bacterial world by cyanobacteria, the electron donor (X ed) is water, and the oxidized product (Xqx) is O2. This is the same type of photosynthesis that is carried out by green plants, which indeed ultimately derived that ability from the cyanobacteria. Anoxygenic photosynthesis utilizes electron donors other than water (specifically, molecules for which less energy is required to extract electrons—e.g., hydrogen sulfide, ferrous iron), and accordingly, does not generate O2 as a product. 

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