FAMILY THIORHODACEAE 841 



FMIILY I. THIORHODACEAE MOLISCH. 



(Molisch, Die Purpurbakterien, Jena, 1907, 27; Subfamily Chromaliuideae Bu- 

 chanan, Jour. Bact., 3, 1918, 464; Rhodo-Thiobacteria Bavendamm, Die farblosen 

 und roten Schwefelbacterien, Pflanzenforschung, Heft 2, 1924, 102; Rhodothiobac- 

 teria Bavendamm, Ergeb. Biol., 13, 1936, 49.) 



Unicellular organisms, often developing as cell aggregates or families of variable 

 size and shape. Single cells have the form of spheres, ovoids, short rods, vibrios, 

 spirals, long rods or, occasionally, chains. They occur in nature in environments 

 containing sulfides and require light for their development; infra-red irradiation of 

 a wave-length extending to about 900 millimicrons is effective. They produce a pig- 

 ment system composed of green bacteriochlorophyll, and j-ellow and red carotenoids. 

 As a result they appear as pale purple, brownish to deep red cell masses. Single 

 cells, unless thej' are of considerable size, usually appear to be unpigmented. These 

 are anaerobic or microaerophilic organisms, with a photosynthetic metabolism in 

 which carbon dioxide is reduced with the aid of special hj^drogen donors without the 

 liberation of molecular oxygen. Where these organisms are found in nature, hydro- 

 gen sulfide acts as a hydrogen donor, and sulfur, the first intermediate oxidation 

 product, accumulates as sulfur droplets in the cells. Probabh' all members of the 

 group can utilize a number of organic substances other than hydrogen sulfide as hy- 

 drogen donors for photosj'nthesis. Thus they are potentially mixotrophic. 



Characterization of the genera in this group has since Winogradsky's studies 

 (Beitrage zur IMorphologie und Physiologie der Schwefelbacterien, Leipzig, 1888) 

 been based upon the mode of development of the cell aggregates. Pure culture 

 studies (Bavendamm, Die farblosen und roten Bakterien, I. Schwefelbakterien, 

 Pflanzenforschung, Heft 2, 1924, 74 pp.; van Niel, Arch. f. Mikrobiol., 3, 1931, 1-112; 

 Manten, Antonie van Leeuwenhoek, 8, 1942, 164 pp.) have since shown that not only 

 the sequence of events in the formation of the aggregates but also the appearance 

 and form of the latter even including the size and shape of the component cells are 

 influenced to a considerable extent by environmental conditions. This obviously 

 casts doubt upon the usefulness of the previously used diagnostic criteria for genera 

 and species. On the other hand, the scope of pure culture studies has not yet at- 

 tained sufficient breadth to warrant the use of a different approach. As a provi- 

 sional measure, Winogradsky's genera are therefore maintained. Even the larger 

 taxonomic units must be regarded as being of tentative value only. 



Key to the genera of the family Thiorhodaceae. 



I. Cells usually combined into aggregates. 



A. Cells grouped as regular sarcina packets. 



Genus I. Thiosarcina, p. 842. 



B. Cells not in sarcina packets. 



1. Aggregates in the form of a flat sheet. 



a. Cells in regular arrangement, with tetrads as the common structural 

 unit. 



Genus II. Thiopedia, p. 843. 

 aa. Cells in irregular aggregates. 



Genus III. Thiocapsa, p. 844. 



