purple nonsulfur

The purple nonsulfur photosynthetic eubacteria constitute a non-taxonomic group, most of which can grow as photoheterotrophs, photoautotrophs or chemoheterotrophs. Chemotrophic growth for the purple nonsulfur bacteria is achieved by respiration, although some exceptional strains and species can obtain energy by fermentation or anaerobic respiration.

These organisms switch modes depending on environmental conditions, particularly: presence or lack of oxygen, availability of carbon source (CO2 for autotrophic growth, organic compounds for heterotrophic growth), and availability of light. Sulfide can be employed as an electron donor when present at low concentrations. However, higher concentrations of H2S (in which the purple sulfur and green sulfur bacteria can thrive) are toxic to purple nonsulfur bacteria.


There are four phylogenetically distinct groups of purple nonsulfur bacteria:
Order "Rhodocyclales" Family "Rhodocyclaceae" are β-proteobacteria,
Rhodospirillum, Rhodopseudomonas, and Rhodobacter groups are α-proteobacteria.

Purple nonsulfur families: Rhodospirillaceae Rhodospirillum
chlorophylls bchl a or b
electron donor photoautotrophy = H2 probably for all, and for some low levels of S–, S2O3–, S^o
photoheterotrophy? all spp. chemotrophy? probably all spp.