Differentiation of the genus Ectothiorhodospira from

other genera

Species of the genus Ectothiorhodospira are differentiated from other purple sulfur bacteria of the Chromatiaceae family by the deposition of elemental sulfur outside their cells, when grown with sulfide as the photosynthetic electron donor. Differentiation of Ectothiorhodospira and Halorhodospira species is possible based on both molecular and physiological properties (Table BXII.y.7). Ectothiorhodospira species contain MK-7 and either Q-7 or Q-8 as major components, whereas Halorhodospira species lack significant proportions of homologs with 7 isoprenoid units, butinstead have MK-8, Q-8, and a short chain MK component as major components (Imhoff, 1984a; Ventura et al., 1993). Both genera form separate groups according to their fatty acid compositions (Thiemann and Imhoff, 1996; Table BXII.y.8). They differ significantly according to their 16S rDNA sequences, as can be seen by a number of signature sequences and sequence similarities of 87.2-89.9% (Table BXII.y.7). In contrast to Ectothiorhodospira species that form sulfur globules outside their cells, Thiorhodospira also forms sulfur globules enclosed by the outer membrane (Bry-antseva et al., 1999b).

Metabolic properties such as utilization of carbon, sulfur, and nitrogen sources are very similar among the species of both genera. Whereas several species of the genus Ectothiorhodospira are able to grow under chemotrophic conditions in the dark, Ha-lorhodospira species are obligately phototrophic. Ectothiorhodospira species have growth optima under conditions of 1-8% total salts, whereas Halorhodospira species do not grow below 10% total salts.

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