Genus I Chromatium Perty 1852 174AL emend Imhoff Suling and Petri 1998b 1138

Johannes F. Imhoff' Gr. n. chroma color; M.L. neut. n. Chromatium one which is colored.

Cells are straight or slightly curved rods, more than 3 |im wide, single or in pairs, multiply by binary fission. Motile by a polar tuft of multitrichous flagella that can be seen in the light microscope, Gram negative, belong to the Gammaproteobacteria, and contain internal photosynthetic membranes of vesicular type in which the photosynthetic pigments bacteriochlorophyll a and carotenoids are located.

Strictly anaerobic, obligately phototrophic, and require sul-fide-reduced media. Photolithoautotrophic growth occurs under anoxic conditions in the light with sulfide and S0 as electron donors. Thiosulfate and hydrogen are not used. During oxidation of sulfide, S0 is transiently stored inside the cells in the form of highly refractile globules. Sulfate is the final oxidation product. In the presence of sulfide or sulfur and bicarbonate, a few simple organic substrates are photoassimilated. Assimilatory sulfate reduction is lacking. Ammonia is used as nitrogen source, N2 may be used, nitrate is not reduced. Storage materials: polysaccharides, poly-b-hydroxybutyrate, S0, and polyphosphates. Vitamin B12 required.

Mesophilic freshwater bacteria with optimum growth temperatures from 20 to 35°C and without a distinct requirement for salt.

Habitat: water and sediment surface layers of stagnant freshwater habitats such as ditches, ponds, and lakes, containing hydrogen sulfide and exposed to light.

Type species : Chromatium okenii (Ehrenberg 1838) Perty 1852,174 (Moras okenii Ehrenberg 1838, 15.)

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