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 bacteriochlorophyll or other chlorophyll-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 depends upon the presence of extraneous oxidizable compounds which are dehydrogenated with the simultaneous reduction of carbon dioxide. As oxidizable substrates, a variety of simple substances can be used, such as sulfide, or other reduced sulfur compounds, molecular 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 presence of air can also be cultured in the dark under aerobic conditions. Color depends markedly on environmental conditions; small individuals appear colorless unless observed in masses. May contain sulfur globules. Described species have largely been found in freshwater 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 hydrogen sulfide. The sulfur purple bacteria.
Family I. Thiorhodaceae, 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 Thiobacteriales Buchanan do not constitute 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 Thiobacteria Migula (Syst. d. Bakt., 2, 1900, 1039) was intended to include the morphologically conspicuous organisms which, in their natural habitat, contain globules of sulfur as cell inclusions. Since Winogradsky (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 inclusions 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 morphology 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 Rhodobacteriaceae. Even at that time, however, some difficulties existed 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
