The Chromatiaceae are one of the two families of purple sulfur bacteria, together with the Ectothiorhodospiraceae. They belong to the order Chromatiales of the class Gammaproteobacteria, which is composed by unicellular Gram-negative organisms. Most of the species are photolithoautotrophs and conduct an anoxygenic photosynthesis, but there are also representatives capable of growing under dark and/or microaerobic conditions as either chemolithoautotrophs or chemoorganoheterotrophs. Both Ectothiorhodospiraceae and Chromatiaceae bacteria produce elemental sulfur globules, the difference is that in the second case they are stored inside the cells rather than outside. Sulfur is an intermediate in the oxidization of sulfide, which is ultimately converted into sulfate, and may serve as a reserve. History of classification Although purple sulfur bacteria have been known for some time, the difficulty in cultivating these microorganisms in the laboratory has made that few scientific depositions are available to date, and even less are those that provide comparative studies between the two families of the order: Chromatiaceae and Ectothiorhodospiraceae. This is evidenced by the fact that most of the publications at disposal of the scientific community are the result of the work of a relatively small number of scientists, first of all Norbert Pfennig, Johannes Imhoff and Jörg Overmann. The taxonomy of these two families was originally drawn up entirely on characteristics easy to observe, such as the storage of elemental sulfur inside or outside the cells, the presence of gas vesicles and the internal membrane systems. This led to a first subdivision based on phenotypic properties without taking into account the phylogenetic relationships between the two families. The first taxonomic system (1907) was based on the Molisch's pigment and the storage of elemental sulfur. According to these traits, Molisch made a first distinction of the purple sulfur bacteria into two groups: the Thiorhodaceae, including all the members that store globules of elemental sulfur inside their cells, and the Athiorhodaceae, without this feature. These two groups appeared as families of the order Pseudomonales in the Bergey's Manual of Determinative Bacteriology, 7th ed. In the next edition, they were split in the Rhodospirillaceae (Pfennig and Triiper, 1971) and the Chromatiaceae (Bavendamm, 1924), respectively. Then, the term Chromatiaceae was used for the first time by Bavendamm in 1924, in particular as referred to all those purple bacteria which follow the pathway of anoxygenic photosynthesis, that is to say without using water as electron donor (and consequently without oxygen production), but rather