Photosynthesis

The enzymes contained in these species are not configured to use water as their reducing agent, and as a result, do not produce oxygen. Instead they use hydrogen sulfide, which is oxidized to produce granules of elemental sulfur. This in turn may be oxidized to form sulfuric acid.

Anoxygenic photosynthesis is the phototrophic process where light energy is captured and stored as ATP, without the production of oxygen. Anoxygenic phototrophs have photosynthetic pigments called bacteriochlorophylls (similar to chlorophyll found in eukaryotes). Bacteriochlorophyll a and b have maxima wavelength absorption at 775 nm and 790 nm, respectively in ether. Unlike oxygenic phototrophs, anoxygenic photosynthesis only functions using a single photosystem. This restricts them to cyclic electron flow only, and they are therefore unable to produce O2 from the oxidization of H2O.

In purple sulfur bacteria, the electron transport chain begins when the reaction centre bacteriochlorophyll pair, P870, becomes excited from the absorption of light. Excited P870 will then donate an electron to Bacteriopheophytin, which then passes it on to a series of electron carriers down the electron chain. In the process, it will generate a proton motor force which can then be used to synthesize ATP by oxidative phosphorylation. The electron returns to P870 at the end of the chain so it can be used again once light excites the reaction-center.