distinguishing characteristics are not always very clear. For the benefit of those who are familiar with previous methods of classification, it will be indicated where deviations have been adopted.
The non-sulfur purple bacteria (Athiorhodaceae Molisch; Rhodobacterioideae Buchanan) have been subjected to a comparative morphological and physiological study comprising more than 150 strains, among which all previously proposed genera and species are represented (van Niel, Bact. Rev., 8, 1944, 1-118). It has been found that the characteristics upon which Molisch based the seven genera of this group are inadequate, and a new classification with only two distinguishable genera has been proposed. This system will be followed here.
Nadson (Bull. Jard. Imper. Bot., St. Petersburg, 12, 1912, 64) described a new type of small, green bacteria not containing sulfur globules in the presence of hydrogen sulfide but excreting elemental sulfur. They are photosynthetic and are capable of growing in anaerobic culture when illuminated. The green pigment differs from the green plant chlorophylls and from the bacteriochlorophyll of the purple bacteria but has the characteristics of a chlorophyllous compound. These are grouped in the family Chlorobacteriaceae.
FAMILY I. THIORHODACEAE MOLISCH, 1907.
(Die Purpurbakterien, Jena, 1907, 27.)
Thi.o.rho.da′ce.ae. Gr. noun thium sulfur; Gr. noun rhodum the rose; -aceae ending to denote a family; M.L. fem. pl. n. Thiorhodaceae (probably intended to mean) the family of sulfur red bacteria.
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 pigment system composed of green bacteriochlorophyll and yellow and red carotenoids. As a result they appear as bluish violet, pale purple, brownish to deep red cell masses. Single cells, unless they 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 hydrogen donors without the liberation of molecular oxygen. Where these organisms are found in nature, hydrogen sulfide acts as a hydrogen donor, and sulfur, the first intermediate oxidation product, accumulates as sulfur droplets in the cells. Probably all members of the group can utilize a number of organic substances in place of hydrogen sulfide as hydrogen donors for photosynthesis. Thus they are potentially mixotrophic.
Characterization of the genera in this group has, since Winogradsky's studies (Beiträge zur Morphologie 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 shown, however, 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 attained sufficient breadth to warrant the use of a different approach. As a provisional measure, Winogradsky's genera are therefore maintained. Even the larger taxonomic units must be regarded as being of tentative value only.
