Anoxygenic photosynthesis

Sulfide is used as a reducing agent during photosynthesis in green and sulfur bacteria.

1. Energy in the form of sunlight
2. The light dependent reactions take place when the light excites a reaction center, which donates an electron to another molecule and starts the electron transport chain to produce ATP and NADPH.
3. Once NADPH has been produced, the Calvin cycleproceeds as in oxygenic photosynthesis, turning CO₂into glucose.

Anoxygenic photosynthesisis a special form ofphotosynthesisused by somebacteriaandarchaea,which differs from the better knownoxygenic photosynthesisinplantsin thereductantused (e.g.hydrogen sulfideinstead ofwater) and thebyproductgenerated (e.g. elementalsulfurinstead ofmolecular oxygen).

Bacteria and archaea

Several groups of bacteria can conduct anoxygenic photosynthesis:green sulfur bacteria(GSB), red and green filamentousphototrophs(FAPs e.g.Chloroflexia),purple bacteria,acidobacteriota,andheliobacteria.

Somearchaea(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 ofditerpenesto move ions against a gradient and produceATPviachemiosmosisin the manner of mitochondria.

Pigments

Thephotopigmentsused to carry out anaerobic photosynthesis are similar tochlorophyllbut differ in molecular detail and peak wavelength of light absorbed.Bacteriochlorophyllsathroughgabsorb electromagnetic radiation maximally in thenear-infraredwithin their natural membrane milieu. This differs from chlorophyll a, the predominant plant andcyanobacteriapigment, which has peak absorption wavelength approximately 100 nanometers shorter (in the red portion of the visible spectrum).

Reaction centers

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

Type I reaction centers

The electron transport chain of green sulfur bacteria—such as is present in the model organismChlorobaculum tepidum—uses thereaction centerbacteriochlorophyll 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 an electron transport chain. The electron is transferred through a series of electron carriers and complexes until it is used to reduce NAD⁺to NADH. P840 regeneration is accomplished with the oxidation of a sulfide ion from hydrogen sulfide (or of hydrogen or ferrous iron) bycytochromec₅₅₅.

Type II reaction centers

Although the type II reaction centers are structurally and sequentially analogous tophotosystem II(PSII) in plant chloroplasts and cyanobacteria, known organisms that exhibit anoxygenic photosynthesis do not have a region analogous to theoxygen-evolving complexof PSII.

The electron transport chain of purple non-sulfur bacteria begins when thereaction centerbacteriochlorophyll pair, P870, becomes excited from the absorption of light. Excited P870 will thendonate an electrontobacteriopheophytin,which then passes it on to a series of electron carriers down theelectron chain.In the process, it will generate anelectrochemical gradientwhich can then be used to synthesize ATP bychemiosmosis.P870 has to be regenerated (reduced) to be available again for a photon reaching the reaction-center to start the process anew. Molecularhydrogenin the bacterial environment is the usual electron donor.