A group of phototrophic bacteria that contain a green pigment, *bacteriochlorophyll
, similar to the chlorophyll of plants, that enables them to photosynthesize. They obtain hydrogen from hydrogen sulphide or other reduced sulphur compounds (or occasionally from molecular hydrogen or organic acids) and fix carbon dioxide by the Calvin series of reactions. There are three groups of photosynthetic bacteria: namely the green sulphur bacteria (Chlorobiaceae or Chlorobacteriaceae); the purple sulphur bacteria (Chromatiaceae or Thiorhodaceae); and the purple nonsulphur bacteria (Rhodospirillaceae or Athiorhodaceae). Together these form a suborder of the *Pseudomonadales
. In the purple bacteria additional red and brown pigments are also present.