Purple bacteria or purple photosynthetic bacteria are Gram-negative proteobacteria that are phototrophic, capable of producing their own food via photosynthesis. They are pigmented with bacteriochlorophyll a or b, together with various carotenoids, which give them colours ranging between purple, red, brown, and orange. They may be divided into two groups – purple sulfur bacteria (Chromatiales, in part) and purple non-sulfur bacteria. Purple bacteria are anoxygenic phototrophs widely spread in nature, but especially in aquatic environments, where there are anoxic conditions that favor the synthesis of their pigments. Taxonomy All purple bacteria belong in the phylum of Pseudomonadota. This phylum was established as Proteobacteria by Carl Woese in 1987 calling it "purple bacteria and their relatives". Purple bacteria are distributed between 3 classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria each characterized by a photosynthetic phenotype. All these classes also contain numerous non-photosynthetic numbers, such as the nitrogen-fixing Rhizobium and the human gut bacterium Escherichia coli. Purple non-sulfur bacteria are found in Alphaproteobacteria and Betaproteobacteria. The families are: Class Alphaproteobacteria (17 purple genera) Order Rhodospirillales Family Rhodospirillaceae, e.g. Rhodospirillum rubrum Family Acetobacteraceae, e.g. Rhodopila globiformis Order Hyphomicrobiales Family Nitrobacteraceae, e.g. Rhodopseudomonas palustris Family Hyphomicrobiaceae, e.g. Rhodomicrobium Family Rhodobiaceae, e.g. Rhodobium (1 purple genus) Order Rhodobacterales, family Rhodobacteraceae (3 purple genera) Class Betaproteobacteria (3 purple genera) Family Rhodocyclaceae, e.g. Rhodocyclus (1 purple genus) Family Comamonadaceae, e.g. Rhodoferax (2 purple genera) Purple sulfur bacteria are named for the ability to produce elemental sulfur. They are included in the class Gammaproteobacteria, in the two families Chromatiaceae and Ectothiorhodospiraceae. While the former family stores the produced sulfur inside the cell, the latter sends the sulfur outside the cell. According to a 1985 phylogeny, Gammaproteobacteria is divided into three sub-lineages, with both families falling into the first along with non-photosynthetic species such as Nitrosococcus oceani. The similarity between the photosynthetic machinery in these different lines indicates that it had a common origin, either from some common ancestor or passed by lateral transfer. Purple sulfur bacteria and purple nonsulfur bacteria were distinguished on the basis of physiological factors of their tolerance and utilization of sulfide: was considered that purple sulfur bacteria tolerate millimolar levels of sulfide and oxidized sulfide to sulfur globules stored intracellulary while purple nonsulfur bacteria species did neither. This kind of classification was not absoluted. It was refuted with classic chemostat experiments by Hansen and Van Gemerden (1972) that demonstrate the growing of many purple nonsulfur bacteria species