Anoxygenic photosynthesis

Phototrophy is the process by which organisms trap light energy (photons) and store it as chemical energy in the form of ATP. There are three major types of phototrophy: Oxygenic photosynthesis, Anoxygenic photosynthesis, and Rhodopsin-based phototrophy. Oxygenic and anoxygenic photosynthesis undergo different reactions in the presence and absence of light (called Light reactions and Dark reactions, respectively).

Overview

Anoxygenic photosynthesis is the phototrophic process where light energy is captured and stored as ATP, without the production of oxygen. This means water is not used as primary 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, phototrophic heliobacteria.

Anoxygenic phototrophs have photosynthetic pigments called bacteriochlorophylls (similar to chlorophyll found in eukaryotes). Bacteriochlorophyll a and b have wavelengths of maximum wabsorption 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. 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. This is accomplished with the oxidation of hydrogen sulfide (or other inorganic sulfur compound) by cytochrom c₅₅₅.