Phototrophy is the process by which organisms trap light energy (photons) and store it as chemical energy in the form of ATP and/or reducing power in NADPH. There are two major types of phototrophy: chlorophyll-based chlorophototrophy and rhodopsin-based retinalophototrophy. Chlorophototrophy can further be divided into oxygenic photosynthesis and anoxygenic phototrophy. Oxygenic and anoxygenic photosynthesizing organisms undergo different reactions either in the presence of light or with no direct contribution of light to the chemical reaction (colloquially called "light reactions" and "dark reactions", respectively).
Anoxygenic photosynthesis is the phototrophic process where light energy is captured and converted to ATP, without the production of oxygen. Water is therefore not used as an electron donor. There are several groups of bacteria that undergo anoxygenic photosynthesis: Green sulfur bacteria, green and red filamentous anoxygenic phototrophs (FAPs), phototrophic purple bacteria, phototrophic Acidobacteria, and phototrophic heliobacteria.
Anoxygenic phototrophs have photosynthetic pigments called bacteriochlorophylls (similar to chlorophyll found in eukaryotes). Bacteriochlorophyll a and b have wavelengths of maximum absorption at 775 nm and 790 nm, respectively in ether. In vivo however, due to shared extended resonance structures, these pigments were found to maximally absorb wavelengths out further into the near-infrared. Bacteriochlorophylls c-g have the corresponding "peak" absorbance at more blue wavelengths when dissolved in an organic solvent, but are similarly red-shifted within their natural environment (with the exception of bacteriochlorophyll f, which has not been naturally observed).
Unlike oxygenic phototrophs, anoxygenic photosynthesis only functions using (by phylum) either one of two possible types of photosystem. This restricts them to cyclic electron flow and are therefore unable to produce O2 from the oxidization of H2O.
Photosynthetic electron transport chain
Purple non-sulfur bacteria
The electron transport chain of purple non-sulfur bacteria 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 (PMF) 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.
Green sulfur bacteria
The electron transport chain of green sulfur bacteria 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 returns to P840 or is used to reduce NAD+. If the electron leaves the chain to reduce NAD+, P840 must be reduced for the ETC to function again.[clarification needed] This is accomplished with the oxidation of hydrogen sulfide (or other inorganic sulfur compound) by cytochrome c555.
- [Donald A. Bryant, Niels-Ulrik Frigaard, Prokaryotic photosynthesis and phototrophy illuminated, Trends in Microbiology, Volume 14, Issue 11, November 2006, Pages 488-496, ISSN 0966-842X, 10.1016/j.tim.2006.09.001. (http://www.sciencedirect.com/science/article/pii/S0966842X06002265)]
- 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