The Rhodocyclaceae are a family of gram-negative bacteria. They are given their own order in the beta subgroup of Proteobacteria, and include many genera previously assigned to the family Pseudomonadaceae. The family contains mainly aerobic or denitrifying rod-shaped bacteria, which exhibit very versatile metabolic capabilities. Most species live in aquatic habitats and prefer oligotrophic conditions. Many occur in waste water and play an important role in biological remediation in waste water treatment (e.g. Zoogloea). The name-giving genus Rhodocyclus is rather untypical for the group, being the only phototroph among them. Rhodocyclus sp. are performing anoxygenic photosynthesis under anoxic conditions in a similar way as other bacterial genera among the alpha-Proteobacteria. The species Rhodocyclus purpureus also deviates from the other known members of the family (including other Rhodocyclus species) in its ring-shaped cell form, which brings the two cell poles close to each other and has inspired the genus name.