"The phototrophic bacteria are a large and diverse category of bacteria that do not represent a taxon but, rather, a group of bacteria that use sunlight as their primary source of energy. This group contains both Proteobacteria and nonproteobacteria. They use solar energy to synthesize ATP through photosynthesis. When they produce oxygen, they perform oxygenic photosynthesis. When they do not produce oxygen, they perform anoxygenic photosynthesis. With the exception of some cyanobacteria, the majority of phototrophic bacteria perform anoxygenic photosynthesis. One large group of phototrophic bacteria includes the purple or green bacteria that perform photosynthesis with the help of bacteriochlorophylls, which are green, purple, or blue pigments similar to chlorophyll in plants. Some of these bacteria have a varying amount of red or orange pigments called carotenoids. Their color varies from orange to red to purple to green, and they are able to absorb light of various wavelengths. Traditionally, these bacteria are classified into sulfur and nonsulfur bacteria; they are further differentiated by color. The sulfur bacteria perform anoxygenic photosynthesis, using sulfites as electron donors as well as releasing free elemental sulfur. Nonsulfur bacteria use organic substrates, such as succinate and malate, as donors of electrons. The purple sulfur bacteria oxidize hydrogen sulfide into elemental sulfur and sulfuric acid. They get their purple color from the pigments bacteriochlorophylls and carotenoids. Bacteria of the genus Chromatium are purple sulfur Gammaproteobacteria. These microorganisms are strict anaerobes and live in water. They use carbon dioxide as their only source of carbon, but their survival and growth are possible only in the presence of sulfites, which they use as electron donors. Chromatium has been used as a model for studies of bacterial photosynthesis since the 1950s. The green sulfur bacteria use sulfide for oxidation and produce large amounts of green bacteriochlorophyll. The genus Chlorobium is a green sulfur bacterium that is implicated in climate change because it produces methane, a greenhouse gas. These bacteria use at least four types of chlorophyll for photosynthesis. The most prevalent of these, bacteriochlorophyll, is stored in special vesicle-like organelles called chlorosomes. Purple nonsulfur bacteria are similar to purple sulfur bacteria, except that they use hydrogen rather than hydrogen sulfide for oxidation. Among the purple nonsulfur bacteria is the genus Rhodospirillum. These microorganisms are facultative anaerobes, which are actually pink rather than purple, and can metabolize (“fix”) nitrogen. They may be valuable in the field of biotechnology because of their potential ability to produce biological plastic and hydrogen fuel. The green nonsulfur bacteria are similar to green sulfur bacteria except for their use of substrates other than sulfides for oxidation. Chloroflexus is an example of a green nonsulfur bacterium. It often has an orange color when it grows in the dark, but it becomes green when it grows in sunlight. It stores bacteriochlorophyll in chlorosomes, similar to Chlorobium, and performs anoxygenic photosynthesis, using organic sulfites at low concentrations or molecular hydrogen as electron donors, so it can survive in the dark if oxygen is available. Chloroflexus does not have flagella but can glide, like Cytophaga. It grows at a wide range of temperatures, from 35 °C to 70 °C, and thus can be thermophilic. Table 2.13 summarizes important phototrophic bacteria: Microcystis aeruginosa is an oxygenic cyanobacterium with no sulfur deposition; Chlorobium is an anoxygenic green sulfur bacterium that deposits sulfur outside the cell; Chloroflexus is an anoxygenic green nonsulfur bacterium with no sulfur deposition; Rhodospirillum and Rhodocyclus are anoxygenic purple nonsulfur bacteria with no sulfur deposition; and Chromatium is an anoxygenic purple sulfur bacterium that deposits sulfur inside the cell."