36 ORDER I. PSEUDOMONADALES 



Cells spherical, rod-, vibrio- or spiral-shaped. Diameter of individual cells from less than 

 1.0 to over 10 microns. Red, purple, brown or green bacteria which contain bacteriochloro- 

 phj-ll or other chloroph3dl-like green pigments, and which usually also possess one or more 

 carotenoid pigments. Capable of carrying out a photosynthetic metabolism which differs 

 from that of green plants in that it does not proceed with the evolution of oxygen, and de- 

 pends upon the presence of extraneous oxidizable compounds which are dehydrogenated 

 with the simultaneous reduction of carbon dioxide. As oxidizable substrates, a varietj' of 

 simple substances can be used, such as sulfide, or other reduced sulfur compounds, molecu- 

 lar hydrogen, alcohols, fatty acids, hydroxy- and keto-acids, etc. All can be grown in 

 strictly anaerobic cultures when illuminated. Those members which can grow in the pres- 

 ence of air can also be cultured in the dark under aerobic conditions. Color depends mark- 

 edly on environmental conditions; small individuals appear colorless unless observed in 

 masses. May contain sulfur globules. Described species have largely been found in fresh- 

 water habitats. Some species occur in marine habitats. 



Key to the families of suborder Rhodobacteriineae. 



I. Purple bacteria whose pigment system consists of bacteriochlorophyll and various 

 carotenoids capable of carrying out a photosynthetic metabolism. 



A. Contain sulfur globules in the presence of hj'drogen sulfide. The sulfur purple bac- 

 teria. 



Family I. TJiiorlwdaceae, p. 38. 



B. Do not contain sulfur globules even in the presence of hydrogen sulfide. All require 

 organic growth factors. The non-sulfur purple and brown bacteria. 



Family II. Athiorhodaceae, p. 53. 

 II. Green sulfur bacteria containing a pigment system which has the characteristics of a 

 chlorophyllous compound although it differs from the chlorophyll of green plants and 

 from the bacteriochlorophyll of the purple bacteria. 



Family III. Chlorobacteriaceae , p. 61. 



The organisms previously included in the order Thiohacteriales Buchanan do not consti- 

 tute a taxonomic entity; they represent rather a physiological-ecological community. In 

 this sense, however, a special treatment of this group as a unit has decided advantages from 

 a determinative point of view. 



When first proposed as a systematic assemblage, the order Thiobacferia Migula (Sj'st. d. 

 Bakt., 3, 1900, 1039) was intended to include the morphologically conspicuous organisms 

 which, in their natural habitat, contain globules of sulfur as cell inclusions. Since Wino- 

 gradsky (Beitr. z. Morph. u. Physiol, d. Bact., I, Schwefelbacterien, 1888) had elucidated 

 the function of hydrogen sulfide and of sulfur in their metabolism, the characteristic inclu- 

 sions appeared linked with a hitherto unrecognized type of physiology, viz. the oxidation 

 of an inorganic substance instead of the decomposition of organic materials. From this 

 oxidation the sulfur bacteria derive their energy for maintenance and growth. 



Two groups of sulfur bacteria could be distinguished, one consisting of colorless, the 

 other of red or purple organisms. The members of both groups presented an unusual mor- 

 phology apart from the sulfur droplets : in all cases the individual cells were considerably 

 larger than those of the common bacteria, while many species grew as distinctive colonial 

 aggregates. Migula separated these sulfur bacteria into two families, Beggiatoaceae and 

 Rhodobucteriaceae. Even at that time, however, some difficulties e.xisted as to just what 

 organisms should properly be considered as sulfur bacteria. Miyoshi (Jour. Coll. Sci., Imp. 

 Univ., Tokyo, 10, 1897, 143) had discovered a bacterium which forms strands, incrusted with 

 sulfur, in sulfur springs but which does not store sulfur globules in its cells. Although 

 physiologically this organism appeared to comply with Winogradsky's concept of a sulfur 

 bacterium, the absence of the typical cell inclusions made Miyoshi decide it could not be 



