Bacterial anaerobic photosynthesis is distinguished from more familiar terrestrial plant oxygenic photosynthesis by the nature of the terminal reductant (eg hydrogen sulfide rather 'hydrogen oxide' or water) and in the byproduct generated (eg elemental sulfur instead of molecular oxygen). Any photosynthetic process that does not generate oxygen is called anoxygenic. A phototrophic process captures energy; it may or not be photosynthetic (fix carbon). Several groups of bacteria can conduct anoxygenic photosynthesis: green sulfur bacteria, green and red filamentous phototrophs]], purple bacteria, Acidobacteria, and heliobacteria.
The anaerobic photosynthetic pigments can be similar to chlorophyll but differ in molecular detail and peak wavelength of light adsorbed. Bacteriochlorophyll a and b have wavelengths of maximum absorption in the far red at 775 nm and 790 nm in a laboratory solvent but absorb wavelengths maximally further into the near-infrared in their cellular setting. Bacteriochlorophylls c–g again have peak absorbance at shorter wavelengths in solvents and are similarly red-shifted within their natural membrane milieu.
Some archaea are phototrophic (for example Halobacterium( but none are photosynthetic. Instead of a chlorphyll-type receptor and electron transport chain, proteins such as halorhodopsin use captured light energy to move ions against the gradient and produce ATP via chemiosmosis in the manner of mitochondria. Despite the name and other similarities, this class of proteins has no homology to rhodopsins which arose much later within a large receptor class in early animals.
There are two main types of anaerobic photosynthetic electron transport chains in bacteria:
Purple non-sulfur bacteria
The electron transport chain of purple non-sulfur bacteria -- which are quite diverse phylogenetically and include Rhodobacter sphaeroides -- 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 an electro-chemical gradient which can then be used to synthesize ATP by chemiosmosis. P870 has to to be regenerated (reduced) to be available again for a photon reaching the reaction-center to start the process anew. Molecular hydrogen in the bacterial environment is the usual electron donor.
Green sulfur bacteria
The electron transport chain of green sulfur bacteria such as model organism Chlorobium tepidum uses the reaction centre bacteriochlorophyll pair, P840. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential, and so readily donates the electron to bacteriochlorophyll 663 which passes it on down the electron chain. The electron is transferred through a series of electron carriers and complexes until it is used to reduce NAD+. P840 regeneration is accomplished with the oxidation of sulfide ion from hydrogen sulfide (or hydrogen or ferrous iron) by cytochrome c555.
- Donald A. Bryant; Niels-Ulrik Frigaard (November 2006). "Prokaryotic photosynthesis and phototrophy illuminated". Trends in Microbiology 14 (11): 488–496. doi:10.1016/j.tim.2006.09.001. ISSN 0966-842X. Retrieved 11 March 2016.
- Candidatus Chloracidobacterium thermophilum: An Aerobic Phototrophic Acidobacterium Donald A. Bryant, Amaya M. Garcia Costas, Julia A. Maresca, Aline Gomez Maqueo Chew, Christian G. Klatt, Mary M. Bateson, Luke J. Tallon, Jessica Hostetler, William C. Nelson, John F. Heidelberg, and David M. Ward Science 27 July 2007: 317 (5837), 523-526. doi:10.1126/science.1143236