Further descriptive information

The available 16S rDNA sequence information for Ectothiorhodospiraceae (Imhoff and Suling, 1996) and Chromatiaceae species (DeWeerd et al., 1990; Wahlund et al., 1991; Caumette et al., 1997; Guyoneaud et al., 1997, 1998b; Imhoff et al., 1998b) supports the classification of these bacteria in two different families (Imhoff, 1984b). At the same time, it indicates the existence of different genera within the family Ectothiorhodospiraceae (Imhoff and Suling, 1996), and the inconsistency of the previous taxo-nomic classification of the family Chromatiaceae (see Pfennig and Traper, 1989) with its phylogeny (Imhoff et al., 1998b). In addition, the phylogenetic relatedness of both families and their distance to other bacteria of the Gammaproteobacteria support their treatment within one order, for which the name Chromatiales is proposed. The order Chromatiales includes the phototrophic purple sulfur bacteria of the Ectothiorhodospiraceae and Chromatiaceae

(Imhoff, 1984b, 1989; Pfennig and Trfiper, 1989) and their purely chemotrophic relatives (see Figs. BXII.y.3 and BXII.y.4 of the chapter describing the family Chromatiaceae.). The purely chem-otrophic bacteria known so far are specifically related to Ectothi-orhodospiraceae. The 16S rDNA sequence of Arhodomonas aquaeolei (Adkins et al., 1993; Imhoff and Suling, 1996) is similar to those of Halorhodospira species, but clearly distinct at the genus level. In addition, sequences of Nitrococcus mobilis (Teske et al., 1994) and Nitrosococcus oceani (Head et al., 1993) place these bacteria in phylogenetic proximity to the Ectothiorhodospiraceae, though on a more distant level than the genus Arhodomonas.

Differentiation of Chromatiaceae and Ectothiorhodospiraceae

Traditionally, the most important and easily recognized distinguishing property within the phototrophic members of the order Chromatiales is the deposition of S0 during growth on sulfide. Sulfur globules appear exclusively inside the cells ( Chromatiaceae) or outside the cells (Ectothiorhodospiraceae). A clear distinction between the two families is also possible based on differences in quinone, lipid, and fatty acid composition (Imhoff and Bias-Im-hoff, 1995). Several glucolipids are present in Chromatiaceae species, but absent from members of Ectothiorhodospiraceae (Imhoff et al., 1982a). In addition, the lipopolysaccharides show significant differences between members of the two families (Weckesser et al., 1979, 1995). The lipid A of all investigated Chromatiaceae species (Allochromatium vinosum, Thermochromatium tepidum, Thi-ocystis violacea, Thiocapsa roseopersicina, Thiococcus pfennigii) is characterized by a phosphate-free backbone with D-glucosamine as the only amino sugar, having terminally attached D-mannose and amide-bound C14:0 3OH. In the lipid A of all tested Ectothiorhodo-spiraceae species (Ectothiorhodospira vacuolata, E. shaposhnikovii, E. haloalkaliphila, and Halorhodospira halophila), phosphate is present, 2,3-diamino-2,3-dideoxy-D-glucose is the major amino sugar (D-glucosamine is also present), D-mannose is lacking (D-galact-uronic acid, and D-glucuronic acid are present instead) and, quite remarkably, C10:0 3OH is present as an amide-bound fatty acid (Zahr et al., 1992; Weckesser et al., 1995; Imhoff and Suling, 1996). These distinctive properties of the lipid A appear to be characteristic features of the two families. In addition, sequences of 16S rDNA are significantly different in representatives of the two families, as demonstrated by a number of characteristic signatures (Table BXII.y.3) and by their overall dissimilarity (Figs. BXII.y.3 and BXII.y.4 of the chapter describing the family Chromatiaceae).

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