Chromatiaceae ML neut n Chromatium type genus of the family aceae ending to denote a family ML fem pl n Chromatiaceae the Chromatium family

Cells are spherical, ovoid, vibrioid, spiral, or rod shaped, multiply by binary fission, may contain gas vesicles, may be motile by flagella or nonmotile. Motile forms have either monotrichous or multitrichous polar flagella. Gram negative, belong to the Gam-maproteobacteria, and contain internal photosynthetic membranes, which are continuous with the cytoplasmic membrane and ofvesicular type in most species (Fig. BXII.y.1), though some species have tubular internal membranes (Fig. BXII.y.2) or lamellae. As photosynthetic pigments, bacteriochlorophyll a or b and carotenoids of the spirilloxanthin, okenone, or rhodopinal groups are present in most species, but other carotenoids such as tetrahydrospirilloxanthin, may occur. In general, the color of cultures of strains with carotenoids of the spirilloxanthin group is orange-brown to brownish red or pink, of those with okenone, purple-red, and of those with carotenoids of the rhodopinal group, purple-violet.

Under anoxic conditions in the light, all species are capable of photolithoautotrophic growth with sulfide or S0 as electron donor. S0 globules accumulate as an intermediate oxidation product inside the cells. Many species are able to use H2 as an electron donor under reducing culture conditions, and some use reduced iron ions. All species are potentially mixotrophic and photo-assimilate a number of simple organic compounds, of which acetate and pyruvate are the most widely used. Some species are able to photoassimilate organic substances in the absence of sulfide or sulfur, or grow photoorganoheterotrophically. Many species are strictly anaerobic and obligately phototrophic, while others are capable of chemolithoautotrophic or chemoorganohet-

erotrophic growth under microoxic to oxic conditions in the dark.

The pathway of autotrophic CO2 assimilation was shown to be the reductive pentose phosphate cycle in all species tested so far. The fixation of dinitrogen has been demonstrated in several species. Storage materials are polysaccharides, poly-b-hydroxy-butyrate, elemental sulfur, and polyphosphate. Vitamin B12 is required by several species.

In nature, members of the Chromatiaceae occur in the illuminated, anoxic, and sulfide-containing parts of a large variety of aquatic environments and sediments from moist and muddy soils, ditches, ponds, lakes, rivers, sulfur springs, estuaries, marine habitats, and even salt and soda lakes.

Type genus: Chromatium Perty 1852, 174 emend. Imhoff, Silling and Petri 1998b, 1138.

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