Contain bacteriochlorophyll which enables them to carry out a photosynthetic metabolism. The latter is dependent upon the presence of extraneous oxidizable substances and proceeds without the evolution of molecular oxygen. Though some members can oxidize inorganic substrates, none appears to be strictly autotrophic due to the need for special organic growth factors. Produce accessory pigments causing the cultures, especially when kept in light, to appear in various shades of brownish yellow to deep red.
The type species is Rhodopseudomonas palustris (Molisch) van Niel.
Keys to the species of genus Rhodopseudomonas.
I. Based upon morphological characters.
- A. Cells clearly rod-shaped in all media.
- 1. Cells short, somewhat curved, to long branched rods, size of young and short cells 0.6 to 0.8 by 1.2 to 2 microns; in older cultures up to 10 microns long; do not form slime; liquid cultures, when young, or after shaking, evenly turbid. Color red to dark brown-red.
1. Rhodopseudomonas palustris.
- 2. Cells slender rods, 0.5 by 1.2 microns, usually clumped together in extensive slime masses. Cultures pale brown to peach-colored.
2. Rhodopseudomonas gelatinosa.
- B. Cells more or less spherical in media at pH below 7.
- 1. In media at pH about 7 clearly rod-shaped, 1 by 1 to 2.5 microns. Chains of cells frequent and in characteristic zigzag arrangement.
3. Rhodopseudomonas capsulata.
- 2. In media at pH above 7 cells still predominantly spherical, 0.7 to 4 microns in diameter. Mostly single, little tendency to chain formation.
4. Rhodopsendomonas spheroides.
II. Based chiefly on physiological properties.
- A. Gelatin liquefied.
2. Rhodopseudomonas gelatinosa.
- B. Gelatin not liquefied.
- 1. Does not produce mucus in media at pH above 8. Color the same under aerobic and anaerobic conditions of growth.
1. Rhodopseudomonas palustris.
- 2. Produce mucus in media at pH above 8. Color brown in anaerobic, red in aerobic culture.
- a. Develops readily in media with 0.2 per cent propionate as the chief oxidation substrate. Mucus production marked at pH above 8, but very limited between 7 and 8.
3. Rhodopseudomonas capsulata.
- aa. Does not develop in media with 0.2 per cent propionate as the main oxidation substrate. Slime formation extensive at pH above 7.
4. Rhodopseudomonas spheroides.
III. Based principally upon biochemical characters.
- A. Thiosulfate used as main oxidation substrate.
1. Rhodopseudomonas palustris.
- B. Thiosulfate not used.
- 1. Propionate (0.2 per cent) used.
3. Rhodopseudomonas capsulata.
- 2. Propionate not used.
- a. Mannitol and sorbitol (0.2 per cent) used.
4. Rhodopseudomonas spheroides.
- aa. Mannitol and sorbitol not used.
2. Rhodopseudomonas gelatinosa.
