Details for Rhodospirillaceae

Purple nonsulfur bacteria have been called "nonsulfur" because it was originally thought that they were unable to use sulfide as an electron donor for the reduction of CO₂ to cell material. However, sufide can be used by most species, although the levels of sulfide utilized well by purple sulfur bacteria are toxic to most purple nonsulfur bacteria. Some purple nonsulfur bacteria can grow anaerobically in the dark using fermentative or anaerobic respiratory metabolism, and most can grow aerobically in darkness by respiration. Under the latter conditions, the electron donor can be an organic compound or in some species even an inorganic compound, such as H₂. However, it is the great ability of this group to practice photoheterotrophy (where light is the energy source and an organic compound is the carbon source), that likely accounts for their competitive success in nature. Purple nonsulfur bacteria are typically nutritionally diverse in this regard, using fatty, organic, or amino acids; sugars; alcohols; and even aromatic compounds like benzoate as carbon sources. Most can also grow photoautotrophically with (CO₂ + H₂) or (CO₂ + low levels of H₂S).

Enrichment and isolation of purple nonsulfur bacteria is easy using a mineral salts medium supplemented with an organic acid as carbon source. Such media, inoculated with a mud, lake water, or sewage sample and incubated anoxically in the light, invariably select for purple nonsulfur bacteria. Enrichment cultures can be made even more selective by omitting fixed nitrogen sources (for example, NH₄⁺) or organic nitrogen sources (for example, yeast extract or peptone) from the medium and supplying a gaseous headspace of N₂; virtually all purple nonsulfur bacteria can fix N₂ and will thrive under such conditions, usually outcompeting other organisms.

The morphological diversity of purple nonsulfur bacteria is typical of that of purple sulfur bacteria and it is clearly a heterogeneous group in this regard.