Cyanobacteria anoxygenic photosynthesis

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. 

Cohen Y, Jorgensen BB, Revsbech NP, et al. 1986. Adaptation to hydrogen sulfide of oxygenic and anoxygenic photosynthesis among cyanobacteria. Appl Environ Microbiol 51 398-407. 

Padan E. (1979) Facultative anoxygenic photosynthesis in cyanobacteria. Ann. Rev. Plant Physiol. Plant Molecul. Biol. 30, 27-40. 

Figure 3 A schematic representation of photosystems and associated standard potentials involved in oxygenic photos5mthesis in cyanobacteria (left panel) and anoxygenic photosynthesis (right panel). The flow of electrons in the...

Unlike photosynthesis by other organisms, including cyanobacteria, higher algae, and plants, oxygen is not evolved. Hence, this process is referred to as anoxygenic photosynthesis. Perhaps the best evidence that this process was occurring before 3.0-... 

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