Anoxygenic Photosynthesis

Anoxygenic photosynthesis is a light-driven process in purple and green sulfur bacteria that converts carbon dioxide and alternate electron donors, such as hydrogen sulfide, into organic compounds without producing oxygen.

Anoxygenic phototrophs operate with a single photosystem and rely on bacteriochlorophyll pigments located in the chromatophores, chlorosomes, or cytoplasmic membranes.

In purple bacteria, the P870 reaction center excites electrons, transferring them through bacteriopheophytin to a quinone pool, iron-sulfur cluster proteins, and cytochromes, ultimately returning them to the reaction center.

This electron transfer process generates the proton motive force for ATP synthesis.

Additionally, these bacteria use reverse electron flow, powered by the proton motive force, to reduce NAD⁺ using external electron donors like hydrogen sulfide or elemental sulfur.

Green sulfur bacteria, with P840 as their reaction center, transfer electrons via bacteriochlorophylls, FeS cluster proteins, quinones, and cytochromes.

These bacteria use ferredoxins to directly reduce NAD⁺ to NADH, bypassing the reverse electron flow.