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Concept
Purple bacteria
Summary
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.
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.
Official source
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