The purple sulfur bacteria are a group of 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....

 capable of 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...

, collectively referred to as purple bacteria

Purple bacteria

Purple bacteria or purple photosynthetic bacteria are proteobacteria that are phototrophic, that is capable of producing energy through photosynthesis...

. They are anaerobic

Anaerobic organism

An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...

 or microaerophilic, and are often found in hot spring

Hot spring

A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are geothermal hot springs in many locations all over the crust of the earth.-Definitions:...

s or stagnant water. Unlike plants, algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, and cyanobacteria, they do not use water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 as their 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"...

, and so 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...

. Instead they use hydrogen sulfide

Hydrogen sulfide

Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

, which is oxidized

Redox

Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 to produce granules of 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...

. This in turn may be oxidized to form sulfuric acid

Sulfuric acid

Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

.

The purple sulfur bacteria are divided into two families, the Chromatiaceae

Chromatiaceae

The Chromatiaceae are the main family of purple sulfur bacteria. They are distinguished by producing sulfur globules within their cells. These are an intermediate in the oxidization of sulfide, which is ultimately converted into sulfate, and may serve as a reserve. Members are found in both...

 and Ectothiorhodospiraceae

Ectothiorhodospiraceae

The Ectothiorhodospiraceae are a family of purple sulfur bacteria, distinguished by producing sulfur globules outside of their cells. They are generally marine....

, which respectively produce internal and external sulfur granules, and show differences in the structure of their internal membranes.