54 ORDER I. PSEUDOMONADALES 



Contain bacteriochlorophjdl 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 Rhodopsendornonas palnstris (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 j'oung 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 capsulaia. 



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 brow^n 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 be- 

 tween 7 and 8. 



3. Rhodopseudomonas capsulata. 



aa. Does not develop in media with 0.2 per cent propionate as the main oxida- 

 tion 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. 



