Taxonomic Comments

The taxonomic position of bacteria now classified as Ectothiorho-dospiraceae has been disputed since their discovery by Pelsh (1936). He distinguished these bacteria, which he called "Ecto-thiorhodaceae", from those phototrophic purple bacteria with elemental sulfur inside their cells, which he called "Endothiorho-daceae"(Pelsh, 1937). Pelsh's isolates were poorly characterized and were lost soon after their isolation. Traper (1968) reisolated Ectothiorhodospira mobilis, and Pfennig and Traper (1971a) included Ectothiorhodospira as an exceptional genus into the Chro-matiaceae because of its ability to perform an oxidative dissimi-latory sulfur metabolism similar to that of other phototrophic purple sulfur bacteria. Because bacteria known at that time as Ectothiorhodospira species were significantly different from all other genera of the Chromatiaceae, and because representatives of the purple nonsulfur bacteria had been found to oxidize sulfide to extracellular elemental sulfur, Pfennig (1977) suggested the removal of Ectothiorhodospira from the Chromatiaceae and its placement in the purple nonsulfur bacteria. However, on the basis of significant differences in their polar lipid composition, three major groups of phototrophic purple bacteria were distinguished by Imhoff et al. (1982a): Chromatiaceae, the bacteria known at that time as Ectothiorhodospira, and the purple nonsulfur bacteria (see also Imhoff and Bias-Imhoff, 1995). A clear differentiation of Ectothiorhodospira species from Chromatiaceae was later demonstrated by the oligonucleotide patterns of 16S rRNA molecules (Stackebrandt et al., 1984). With the support of this information, it was proposed that the Ectothiorhodospiraceae be separated as a family distinct from the Chromatiaceae (Imhoff, 1984b). The separation of two groups of species within the family had been recognized not only based on physiological properties and lipid composition (Tindall, 1980; Imhoff et al., 1982a; Imhoff, 1984a), but also by similarities of rRNA oligonucleotide patterns (Stackebrandt et al., 1984). However, a formal separation into genera was not proposed until later. The complete sequence analysis of the 16S rDNA from a large number of strains, including all available type strains of Ectothiorhodospiraceae, not only supported their separation from the Chromatiaceae, but also the divergence of the two groups within this family (Imhoff and Suling, 1996). Because of the large phylogenetic distance between these two groups, which is also reflected in a number of distinctive phenotypic properties, the extremely halophilic species were transferred to the new genus Halorhodospira (Imhoff and Suling, 1996). With this taxonomic reevaluation of the Ec-tothiorhodospiraceae, their taxonomy now follows their phyloge-netic relationships, based on 16S rDNA sequences.

Two new isolates have expanded our knowledge of the Ecto-thiorhodospiraceae. A new phototrophic sulfur bacterium isolated from a Siberian soda lake, Thiorhodospira sibirica, has been shown to be genetically related to the genus Ectothiorhodospira, and many of its properties resemble those of Ectothiorhodospira species (Bry-antseva et al., 1999b). This impressively large bacterium is unique in that it forms S0 globules outside the cells as well as inside. Sulfur globules are located in the cells only in the very peripheral part, which is represented by an apparently large periplasmic space; they are not located in the cell interior. Therefore, the mechanism of S0 formation and the location of the involved enzymes appear to be the same as in other Ectothiorhodospiraceae. However, particular care is needed to distinguish between internal and external deposition of S0.

Another new halophilic and strictly chemotrophic bacterium has been found to be genetically related to the Ectothiorhodospi-raceae, in particular to Halorhodospira (Adkins et al., 1993; Imhoff and Suling, 1996). This bacterium, Arhodomonas aquaeolei, is a close, purely chemotrophic relative to the phototrophic Ectothi-orhodospiraceae. It uses a restricted spectrum of carbon sources for growth. It is considered as a member of the Ectothiorhodospi-raceae primarily because of its phylogenetic relationship by 16S rDNA sequence.

Differentiation of the genera of Ectothiorhodospiraceae Characteristics for differentiation of the genera Ectothiorhodospira, Ha-lorhodospira, Thiorhodospira, and Arhodomonas are summarized in Table BXII.y.7. Characteristic fatty acid compositions of Halorhodospira and Ectothiorhodospira species are shown in Table BXII.y.8. The genetic relationships of Ectothiorhodospiraceae based on 16S rDNA sequences are presented in Fig. BXII.y.19.

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