Nonoxygenic photosynthesis

Several photosynthetic bacteria, such as the sulfur bacteria, do not generate oxygen. Nonoxygenic, or aerobic photosynthesis differs from oxygenic photosynthesis in other ways than lack of oxygen production:

1) Pigments called bacteriochlorophylls (bacteriochlorophyll-a and carotenoids in purple bacteria) absorb light of longer wavelengths than the chlorophylls and accessory pigments of oxygenic Cyanobacteria or photosynthetic eukaryotes.
2) Reduced compounds such as hydrogen sulfide or organic molecules provide the necessary electrons for the reduction of carbon dioxide. (The oxygenic phototrophs employ reduced oxygen – water – as electron donor for reduction of CO2.)
3) The purple bacteria utilize only one photosystem, PSI, while oxygenic phototrophs utilize two photosystems (PSI and PSII).
4) Photophosphorylation is cyclic in nonoxygenic photosynthesis.

The photosynthetic machinery of nonoxygenic photosynthetic purple bacteria is often located in intracytoplasmic membranes. It is not yet known whether or not these membranes are similar to the thylakoid membrane of oxygenic phototrophs, or whether these intracytoplasmic membranes of nonoxygenic phototrophs are merely an extension of the plasma membrane.

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