Anoxygenic photosynthesis

Bacterial anoxygenic photosynthesis is distinguished from the more familiar terrestrial plant oxygenic photosynthesis by the nature of the terminal reductant (e.g. hydrogen sulfide rather than water) and in the byproduct generated (e.g. elemental sulfur instead of molecular oxygen). As its name implies, anoxygenic photosynthesis does not produce oxygen as a byproduct of the reaction. Additionally, all known organisms that carry out anoxygenic photosynthesis are obligate anaerobes. Several groups of bacteria can conduct anoxygenic photosynthesis: green sulfur bacteria (GSB), red and green filamentous phototrophs (FAPs e.g. Chloroflexi), purple bacteria, Acidobacteria, and heliobacteria.

The pigments used to carry out anaerobic photosynthesis are similar to chlorophyll but differ in molecular detail and peak wavelength of light absorbed. Bacteriochlorophylls a through g absorb electromagnetic photons maximally in the near-infrared within their natural membrane milieu. This differs from chlorophyll a, the predominant plant and cyanobacteria pigment, which has peak absorption wavelength approximately 100 nanometers shorter (in the red portion of the visible spectrum).

Some archaea (e.g. Halobacterium) capture light energy for metabolic function and are thus phototrophic but none are known to "fix" carbon (i.e. be photosynthetic). Instead of a chlorophyll-type receptor and electron transport chain, proteins such as halorhodopsin capture light energy with the aid of diterpenes to move ions against the gradient and produce ATP via chemiosmosis in the manner of mitochondria.

There are two main types of anaerobic photosynthetic electron transport chains in bacteria. The type I reaction centers found in GSB, Chloracidobacterium, and Heliobacteria and the type II reaction centers found in FAPs and Purple Bacteria

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