Purple bacteria or purple photosynthetic bacteria are proteobacteria

Proteobacteria

The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....

 that are phototroph

Phototroph

Phototrophs are the organisms that carry out photosynthesis to acquire energy. They use the energy from sunlight to convert carbon dioxide and water into organic material to be utilized in cellular functions such as biosynthesis and respiration.Most phototrophs are autotrophs, also known as...

ic, that is capable of producing energy through photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. They are pigmented with bacteriochlorophyll

Bacteriochlorophyll

Bacteriochlorophylls are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by Von Neil in 1932 . They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacteria. Groups that contain bacteriochlorophyll conduct...

 a or b, together with various carotenoid

Carotenoid

Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...

s, which give them colours ranging between purple, red, brown, and orange.

Metabolism

Photosynthesis takes place at reaction centers on the cell membrane

Cell membrane

The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

, 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

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, because the reducing agent

Reducing agent

A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...

 (electron donor

Electron donor

An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....

) involved in photosynthesis is not water. In some, called purple sulfur bacteria

Purple sulfur bacteria

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 hot springs or stagnant water. Unlike plants, algae, and cyanobacteria, they do not use water as their...

, it is either sulfide

Sulfide

A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...

 or elemental sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

. The others, called purple non-sulfur bacteria (aka PNSB), typically use hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 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.

History

Purple bacteria were the first bacteria 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.

Taxonomy

Purple non-sulfur bacteria are found among the alpha

Alphaproteobacteria

Alphaproteobacteria is a class of Proteobacteria. Like all Proteobacteria, they are Gram-negative.-Characteristics:The Alphaproteobacteria comprise most phototrophic genera, but also several genera metabolising C1-compounds , symbionts of plants and animals, and a group of pathogens, the...

 and beta

Betaproteobacteria

Betaproteobacteria is a class of Proteobacteria. Betaproteobacteria are, like all Proteobacteria, gram-negative.The Betaproteobacteria consist of several groups of aerobic or facultative bacteria which are often highly versatile in their degradation capacities, but also contain chemolithotrophic...

 subgroups, including:

Rhodospirillales Rhodospirillales The Rhodospirillales is an order of Proteobacteria, with two families: the Acetobacteraceae and the Rhodospirillaceae.The Acetobacteraceae comprise the acetic acid bacteria, which are heterotrophic and produce acetic acid during their respiration. The Rhodospirillaceae include mainly purple...
Rhodospirillaceae Rhodospirillaceae The Rhodospirillaceae are a family of Proteobacteria. The majority are purple non-sulfur bacteria, producing energy through photosynthesis; originally all purple non-sulfur bacteria were included here...	e.g. Rhodospirillum
Acetobacteraceae	e.g. Rhodopila
Rhizobiales Rhizobiales Rhizobiales is an order of alpha proteobacteria. They are gram-negative.The rhizobia, which fix nitrogen and are symbiotic with plant roots, appear in several different families here. The Bradyrhizobiaceae, Hyphomicrobiaceae, Phyllobacteriaceae, and Rhizobiaceae are the four families that contain...
Bradyrhizobiaceae Bradyrhizobiaceae The Bradyrhizobiaceae are a family of bacteria, with ten genera. They include plant-associated bacteria such as Bradyrhizobium, a genus of rhizobia associated with some legumes. It also contains animal-associated bacteria such as Afipia felis, formerly thought to cause cat-scratch disease...	e.g. Rhodopseudomonas palustris Rhodopseudomonas palustris Rhodopseudomonas palustris is a gram-negative purple non-sulfur bacteria, notable for its ability to switch between four different modes of metabolism....
Hyphomicrobiaceae Hyphomicrobiaceae The Hyphomicrobiaceae are a family of bacteria. Among others, they include Rhodomicrobium, a genus of purple bacteria....	e.g. Rhodomicrobium
Rhodobiaceae	e.g. Rhodobium Rhodobium (bacterium) Rhodobium is a genus of purple non-sulfur bacteria that reproduces by budding. The cells are rod-shaped and reproduce by budding, as in many other members of the Rhizobiales. RNA trees separate it from the others, however, and it is given its own family. R. orientis, the type species, was...
Other families
Rhodobacteraceae Rhodobacteraceae The Rhodobacteraceae are a family of proteobacteria, given their own order within the alpha subgroup. Like all proteobaceria, they are gram-negative. It contains chemoorganotrophs and photoheterotrophs bacteria. Many occur in aquatic habitats....	e.g. Rhodobacter Rhodobacter In taxonomy, Rhodobacter is a genus of the Rhodobacteraceae.The most famous species of Rhodobacter is Rhodobacter sphaeroides, which is commonly used to express proteins.-External links:...
Rhodocyclaceae Rhodocyclaceae The Rhodocyclaceae are a family of gram-negative bacteria. They are given their own order in the beta subgroup of Proteobacteria, and include many genera previously assigned to the family Pseudomonadaceae. The family contains mainly aerobic or denitrifying rod-shaped bacteria, which exhibit very...	e.g. Rhodocyclus
Comamonadaceae Comamonadaceae The Comamonadaceae are a family of the Beta Proteobacteria. Like all Proteobacteria, they are Gram-negative. They are aerobic and most of the species are motile via flagella and curved rod-shaped.-References:...	e.g. Rhodoferax

Purple sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 bacteria are included among the gamma subgroup

Gammaproteobacteria

Gammaproteobacteria is a class of several medically, ecologically and scientifically important groups of bacteria, such as the Enterobacteriaceae , Vibrionaceae and Pseudomonadaceae. An exceeding number of important pathogens belongs to this class, e.g...

, and make up the order Chromatiales

Purple sulfur bacteria

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 hot springs or stagnant water. Unlike plants, algae, and cyanobacteria, they do not use water as their...

. 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.