oxidizing sulfide and storing the resulting sulfur either inside or outside the cells. As a consequence of this definition, both the bacteria that accumulate sulfur inside (current Chromatiaceae) and outside the cells (current Ectothiorhodospiraceae) were initially grouped together. With the development of chemotaxonomy and DNA sequencing, thanks to analyses on 16S rRNA and pufLM genes as well as on some peculiar phenotypic features (such as lipid and fatty acid composition, quinone structure and amino acid sequence of cytochrome c551), it was possible to demonstrate the phylogenetic distance between the two families. This emerging data led Imhoff (1984) to redefine them: he assigned the genus Ectothiorhodospira (Pelsh, 1936) to a new family called Ectothiorhodospiraceae, and the genus Chromatium (Perty, 1852) to the family Chromatiaceae. Referring to the latest definition, Chromatiaceae family includes only those purple sulfur bacteria that perform anoxygenic photosynthesis and store elemental sulfur inside their cells. This is in line with the first Molisch definition of the Thiorhodaceae group and reflects the distance, but phylogenetic correlation between the two families. Therefore, the current taxonomic system for Chromatiaceae corresponds primarily to the phylogenetic knowledge, but it also takes into account phenotypic characteristics and ecological features, which allow to broadly distinguish the members of this family to the Ectothiorhodospiraceae ones. For example, the pH and the response to salinity play an important role in terms of ecological distribution: the species belonging to the family Ectothiorodospiraceae prefer basic pH and habitats with high salt concentration, while the optimal conditions for the Chromatiaceae species include a pH close to neutral and a higher range of salinity, enabling them to occupy more varied habitats, from fresh to brackish and marine water. The Chromatiaceae bacteria have been divided into four major phylogenetic branches set on the basis of all the aforementioned information: Allochromatium spp., Thermochromatium spp., Thiocystis spp. and Chromatium okenii. They share some cetological properties, such as rod-shaped or spherical cells, flagellar movements and the absence of vesicles. This group includes the most versatile species of the family Chromatiaceae: although they are considered primarily freshwater bacteria as they do not have a specific need for salt, some members can grow under low salinity conditions and thus also in brackish and marine habitats. Thiocapsa spp., Thiolamprovum spp., Thiobaca spp., Lamprocystis spp. and Thiodictyon spp. As well as in the previous case, the members of this group are primarily freshwater bacteria, even if some of them