The phototrophic bacteria, the cyanobacteria, and the prochlorophyta are organisms with a prokaryotic cellular organization and the capacity to transform light energy into chemical energy. The phototrophic bacteria perform an anoxygenic photometabolism under anaerobic conditions and develop their photosynthetic apparatus only at low-oxygen tensions. Although the prototrophic bacteria have developed a great diversity of morphological and physiological types, all members produce ATP principally by the membrane-bound process of electron transport chain phosphorylation. The photosynthetic apparatus of green bacteria, however, is localized in and attached to the cytoplasmic membrane. Phototrophic bacteria are capable of both evolution and consumption of molecular hydrogen. Apparently all of the phototrophic bacteria, except for the obligate halophilic strains, have an outer membrane which in thin sections appears as a unit membrane with a width of 7 to 8 nm. The composition and macromolecular organization of the outer membrane are basically different from that of the cytoplasmic membrane.