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Purple bacteria or purple photosynthetic bacteria are proteobacteria that are phototrophic, i.e. capable of producing energy through photosynthesis.1 They are pigmented with bacteriochlorophyll a or b, together with various carotenoids. These give them colours ranging between purple, red, brown, and orange. Photosynthesis takes place at reaction centers on the cell membrane, which is folded into the cell to form sacs, tubes, or sheets, increasing the available surface area.
Like most other photosynthetic bacteria, purple bacteria do not produce oxygen, because the reducing agent involved in photosynthesis is not water. In some, called purple sulfur bacteria, it is either sulfide or elemental sulfur. The others, called purple non-sulfur bacteria (aka PNSB), typically use hydrogen although some may use other compounds in small amounts. At one point these were considered families, but RNA trees show the purple bacteria make up a variety of separate groups, each closer relatives of non-photosynthetic proteobacteria than one another.
Purple non-sulfur bacteria are found among the alpha and beta subgroups, including:
| Rhodospirillales | |
| Rhodospirillaceae | e.g. Rhodospirillum |
| Acetobacteraceae | e.g. Rhodopila |
| Rhizobiales | |
| Bradyrhizobiaceae | e.g. Rhodopseudomonas |
| Hyphomicrobiaceae | e.g. Rhodomicrobium |
| Rhodobiaceae | e.g. Rhodobium |
| Other families | |
| Rhodobacteraceae | e.g. Rhodobacter |
| Rhodocyclaceae | e.g. Rhodocyclus |
| Comamonadaceae | e.g. Rhodoferax |
Purple sulfur bacteria are included among the gamma subgroup, and make up the order Chromatiales. The similarity between the photosynthetic machinery in these different lines indicates it had a common origin, either from some common ancestor or passed by lateral transfer.
Purple bacteria were the first bacteria that was discovered to photosynthesize without having an oxygen byproduct. Instead, their byproduct is sulfur. This was proved by first establishing the bacteria's reactions to different concentrations of oxygen. What was found was that the bacteria moved quickly away from even the slightest trace of oxygen. Then a dish of the bacteria was taken, and a light was focused on one part of the dish leaving the rest dark. As the bacteria cannot survive without light, all the bacteria moved into the circle of light, becoming very crowded. If the bacteria's byproduct was oxygen, the distances between individuals would become larger and larger as more oxygen was produced. But because of the bacteria's behavior in the focused light, it was concluded that the bacteria's photosynthetic byproduct could not be oxygen.
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