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 Pseudomonadales in