Chromatiaceae
Ectothiorhodospiraceae
Halothiobacillaceae
The purple sulfur bacteria are a group of Proteobacteria capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in sulfur springs or stagnant water. Unlike plants and algae, they do not use water as their reducing agent, and so do not produce oxygen. Instead they use hydrogen sulfide, which is oxidized to produce granules of elemental sulfur. This in turn may be oxidized to form sulfuric acid.
The purple sulfur bacteria are divided into two families, the Chromatiaceae and Ectothiorhodospiraceae, which respectively produce internal and external sulfur granules, and show differences in the structure of their internal membranes. They make up the order Chromatiales, included in the gamma subdivision of the Proteobacteria. The genus Halothiobacillus is also included in the Chromatiales, in its own family, but it is not photosynthetic.
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