Differentiation of the genus Halorhodospira from other genera

Halorhodospira species represent the most halophilic species known in the phototrophic purple sulfur bacteria, and they are well adapted to form massive blooms in hypersaline environments and in alkaline soda lakes. Halorhodospira species are extremely halophilic bacteria and do not grow at total salt concentrations below 10%, whereas Ectothiorhodospira species have growth optima at salt concentrations below 10%.

Species of the genus Halorhodospira are easily differentiated from other purple sulfur bacteria of the Chromatiaceae by the deposition of S0 outside their cells, when grown with sulfide as the photosynthetic electron donor. Differentiation of Ectothiorho-dospira and Halorhodospira species is possible based on both molecular and physiological properties (Table BXII.y.7 of the chapter describing the family Ectothiorhodospiraceae). Absorption spectra of H. halophila have a clear maximum at approximately 890 nm, but do not show absorbance at 830 nm. The long wavelength absorption maximum of the species having bacteriochlorophyll b is between 1010 and 1020 nm. Whereas several species of the genus Ectothiorhodospira are able to grow under chemotrophic conditions in the dark, all Halorhodospira species are obligately phototrophic. In contrast to Ectothiorhodospira species, Halorho-dospira species do not contain significant proportions of quinone homologs with 7 isoprenoid units, but instead have MK-8, Q-8, and a short chain MK component as major components (Imhoff, 1984a; Ventura et al., 1993). Both genera also form separate groups according to their fatty acid compositions (Thiemann and Imhoff, 1996; Table BXII.y.8 of the chapter describing the family Ectothiorhodospiraceae). Their 16S rDNA sequences are significantly different; this is evident from a number of signature sequences as well as sequence similarities of 87.2-89.9% (Table BXII.y.7 of the chapter describing the family Ectothiorhodospiraceae).

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