Further descriptive information

Cells of Halorhodospira species are spirals, but under severe osmotic stress, i.e., at low salt concentrations, they tend to become vibrioid to rod-shaped and increase in volume. The photosyn-thetic membranes of Halorhodospira halochloris have been intensively studied. In negatively stained preparations under the electron microscope, a regularly granulated structure is observed (Imhoff and Traper, 1977). More detailed work, including image analysis, has demonstrated that the photosynthetic complexes are arranged in a regular hexagonal array in Halorhodospira halochloris and Halorhodospira abdelmalekii (as well as in other bacteria containing bacteriochlorophyll b) (Engelhardt et al., 1983). Bac-teriochlorophyll b of H. halochloris and H. abdelmalekii is esterified with D-2,10-phytadienol, not with phytol as is bacteriochlorophyll a of H. halophila (Steiner et al., 1981; R. Steiner, personal communication). Carotenoids of the normal spirilloxanthin series are present in H. halophila with spirilloxanthin as predominant component and negligible amounts of rhodopin (Schmidt and TrUper, 1971). The content of carotenoids in H. halochloris and H. abdelmalekii is low. Major components were methoxy-hydrox-ylycopene glucoside (methoxy-rhodopin glucoside), hydroxyly-copene glucoside (rhodopin glucoside), and dihydroxylycopene diglucoside (Takaichi et al., 2001). Substantial amonuts of di-hydroxylycopene diglucoside diester were also found.

Halorhodospira species are among the most halophilic eubac-teria known (Imhoff, 1988a, b). With regard to their salt responses and the salt concentrations required for optimum growth, Halorhodospira species are extremely halophilic (see Genus Ectothiorhodospira in this volume and Imhoff, 1993). In alkaline and highly hypersaline salt and soda lakes, they commonly develop massive blooms that cause red or green coloration of the water or sediment horizons (Jannasch, 1957; Imhoff et al., 1979). To adapt to high concentrations of salts and to cope with the high external osmotic pressure, bacteria and other unicellular microorganisms have to accumulate osmotically active molecules that are compatible with the molecular cell structures and metabolic processes; these are called compatible solutes or osmotica (see Imhoff, 1986, 1993). In contrast to halophilic algae and archaebacteria that selectively accumulate glycerol and potassium ions, respectively, Halorhodospira species accumulate glycine betaine, ectoine, and trehalose. In H. halochloris, glycine betaine was initially reported as the main osmotic active cyto-plasmic component (Galinski and Triiper, 1982). More recently, a novel component that is active as an osmoprotectant and accumulates in high concentrations in cells of H. halochloris and other Halorhodospira species has been identified as 1,4,5,6-tetra-

hydro-2-methyl-4-pyrimidinecarboxylic acid (Galinski et al., 1985). The trivial name ectoine has been given to this component because of its first discovery in an Ectothiorhodospiraceae (at that time Ectothiorhodospira) species. This compound was later found to be widely distributed among the halophilic eubacteria (Severin et al., 1992). Unlike the Halorhodospira species, ectoine and tre-halose have not been found as osmotica in Ectothiorhodospira species (E. haloalkaliphila and E. marismortui) that characteristically accumulate glycine betaine, sucrose, and an unidentified component (Oren et al., 1991; Imhoff and Riedel, unpublished. results).

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