Norbert Pfennig

Rhodocyclus purpureus gen. nov. and sp. nov., a Ring-Shaped, Vitamin B12-Requiring Member of the Family Rhodospirillaceae

The dominant phototrophic bacterium in a water sample from a purplish red waste-water lagoon was isolated in pure culture. The individual cells of this organism were half-ring-shaped and ring-shaped before cell division; the half-rings were 0.6 to 0.7 μm wide and 2.5 to 3.0 μm long. Open or compact coils of variable length were also formed. Acetate, pyruvate, and cyclohexane carboxylate were the carbon sources best utilized by this organism; vitamin B12, p-aminobenzoic acid, and biotin were required as growth factors. The photosynthetic pigments produced by the organism are bacteriochlorophyll αp and carotenoids of the rhodopinal series. The new bacterium was facultatively aerobic and was unable to photooxidize sulfide or thiosulfate to sulfur or sulfate. It belongs to the family Rhodospirillaceae and is described herein as a member of a new genus, Rhodocyclus. The name proposed for this new species is Rhodocyclus purpureus. The type strain of R. purpureus is “Ames” 6770 (= DSM 168).

The Family Chromatiaceae

The Family Chromatiaceae (purple sulfur bacteria) comprises physiologically and genetically closely related species and genera (Fowler et al., 1984) that carry out anoxygenic photosynthesis. The most important and selective environmental factors in their aquatic habitats are anoxic conditions, the presence of hydrogen sulfide, and illumination. The only other groups of phototrophic bacteria that thrive under similar environmental conditions are the Ectothiorhodospiraceae (see Chapter 171) and the Chlorobiaceae (green sulfur bacteria; see Chapter 195). Because they live in the same types of habitats, some discussion of the Chlorobiaceae must be included in this chapter. However, since the Chlorobiaceae are not phylogenetically related to the other anoxygenic phototrophic bacteria (Stackebrandt et al., 1984), they are treated in a separate chapter, Chapter 195.

Isolation of Members of the Families Chromatiaceae and Chlorobiaceae

The green and purple sulfur bacteria (Chlorobiaceae and Chromatiaceae) are two physiological-ecological groups of anaerobic phototrophic bacteria with anoxygenic photosynthesis. The two groups display a competitive advantage over other microorganisms in similar aquatic habitats. The most important environmental factors are anaerobic conditions, the presence of hydrogen sulfide, and illumination. Both families are treated in one chapter because they occur under similar environmental conditions and because the strains of both families are isolated with similar methods and media. Only the genus Ecto-thiorhodospira is treated in a seChapaute chapter (this Handbook, Chapter 15) because of methodological differences.

Characterization and Identification of the Anoxygenic Phototrophic Bacteria

The anoxygenic phototrophic bacteria (Anoxyphoto-bacteria Gibbons and Murray 1978) perform photosynthesis with a bacteriochlorophyll (bchl) under anaerobic conditions, using reduced sulfur compounds, molecular hydrogen, or simple organic carbon compounds as electron donors. These bacteria do not use water as a photosynthetic electron donor and do not produce molecular oxygen during photosynthesis. The ability to fix molecular nitrogen occurs in nearly all species tested so far (Siefert, 1976).

The Family Chlorobiaceae

The species and genera included in the physiological and ecological group of the green sulfur bacteria (Chlorobium and related genera; the Chlorobiaceae) share a number of particular characteristics which suggest that these genera may be genetically related. However, until now only a few strains have been studied with respect to the similarities of the oligonucleotide sequences of their 16S rRNA (Woese, 1987) and their 5S rRNA (van den Eynde et al., 1990). Thus, the Chlorobiaceae cannot yet be considered as a family of genetically related genera comparable to the Chromatiaceae (Fowler et al., 1984) (see also Chapter 170). The closest relatives of the Chlorobiaceae analyzed so far form a cluster containing Bacteroides fragilis and other Bacteroides species; with respect to all other groups of anoxygenic phototrophic bacteria, the Chlorobiaceae appear to be a well-isolated group (SAB value < 0.25).

Higher Taxa of the Phototrophic Bacteria

For the families, suborders, and the order of the phototrophic bacteria, legitimate names based on the names of type genera are proposed to replace the illegitimate names used to date. The family names are: Rhodospirillaceae nom. nov., Chromatiaceae Bavendamm 1924, and Chlorobiaceae Copeland 1956; suborder names are: Rhodospirillineae nom. nov. and Chlorobiineae nom. nov.; the order name is Rhodospirillales nom. nov.

Type and Neotype Strains of the Species of Phototrophic Bacteria Maintained in Pure Culture

The type or neotype strains of 41 species of phototrophic bacteria are given. Thirteen types, of which four are designated as types by the authors, are cited. Twenty-eight neotype strains, twenty-four of which are designated as neotypes, are also given. Fifteen of the newly designated neotype strains are described and references are given to previously published descriptions of the other nine neotypes.

Family of Phototrophic Green Sulfur Bacteria: Chlorobiaceae Copeland, the Correct Family Name; Rejection of Chlorobacterium Lauterborn; and the Taxonomic Situation of the Consortium-Forming Species: Request for an Opinion

The family of phototrophic green sulfur bacteria contains five genera: Chlorobium Nadson 1906, Pelodictyon Lauterborn 1913, Clathrochloris Geitler 1925, Chloropseudomonas Czurda and Maresch 1937, and Prosthecochloris Gorlenko 1970. Of these, Chlorobium is the type genus, and the name of the family is Chlorobiaceae Copeland 1956. The familial name Chlorobacteriaceae Lauterborn 1913 is illegitimate. Furthermore, the applications of the names Chlorobacterium Lauterborn 1916 and C. symbioticum Lauterborn 1916 are uncertain, and the Judicial Commission is requested to place these names in the list of rejected names. The taxonomic situation of the consortium-forming “symbiotic species” of the phototrophic green sulfur bacteria (Chlorochromatium aggregatium, Chlorochromatium glebulum, Pelochromatium roseum, and Cylindrogloea bacterifera) is discussed.

New Nomenclatural Combinations in the Phototrophic Sulfur Bacteria

The following new combinations for names of species of phototrophic sulfur bacteria are proposed: (i) Thiocystis gelatinosa (Winogradsky) comb. nov., (ii) Thiodictyon bacillosum (Winogradsky) comb. nov., (iii) Amoebobacter pendens (Molisch) comb. nov., (iv) Pelodictyon luteolum (Schmidle) comb. nov. Each of the two species of Chlorobium Nadson 1906 has an infrasubspecific forma specialis designated: Chlorobium limicola forma specialis thiosulfatophilum and Chlorobium vibrioforme forma specialis thiosulfatophilum. A revised description of each newly named organism is given. Neotype strains are designated.

Introduction to the Phototrophic Prokaryotes

Four taxonomic groups of classical prokaryotes and one group of archaebacteria (Woese, Magrum, and Fox, 1978) can convert light energy into chemical-bond energy. They belong to three functional categories, in terms of the photochemical mechanisms involved (Table 1).