Further descriptive information

Cell morphology of the Ectothiorhodospira species varies with the growth conditions, in particular with pH and salinity. Cells of Ectothiorhodospira mobilis are spirals under optimum growth conditions, but tend to form vibrioid cells or bent rods. Ectothiorho-dospira shaposhnikovii and Ectothiorhodospira vacuolata form straight or slightly bent rods under suitable growth conditions. Carote-noids are of the normal spirilloxanthin series with spirilloxanthin as the predominant component (Schmidt and Traper, 1971). The in vivo absorption spectra of Ectothiorhodospira species are characterized by long wavelength absorption bands at 798, 822827, 851-855, and 892-895 nm. The band at -850 nm is the absolute maximum; the band at -890 nm is mostly recognized as a shoulder, sometimes as a maximum of similar strength to that at 850 nm; and the one at —820-830 nm can be seen only in second derivative spectra (Imhoff, unpublished results).

All species grow well under anoxic conditions in the light with reduced sulfur compounds as photosynthetic electron donors and in the presence of organic carbon sources and inorganic carbonate. Ectothiorhodospira mobilis, Ectothiorhodospira shaposhni-kovii, and Ectothiorhodospira haloalkaliphila also grow microaerob-ically in the dark if sulfide is present. During phototrophic growth with sulfide as electron donor, the oxidation of sulfide and S0 strictly follow each other, as has been shown for E. mobilis (see Traper, 1978). Under the alkaline growth conditions, which are optimal for Ectothiorhodospira species, polysulfides are stable intermediates in sulfide oxidation. As a result, polysulfides are the first measurable oxidation products, and the medium becomes translucent yellow at this stage. After sulfide depletion, S0 droplets are formed rather rapidly, and the medium becomes opaque and whitish. Upon further growth, cultures become pinkish and finally red, if S0 disappears. The knowledge of enzymes involved in the oxidation of reduced sulfur compounds by Ectothiorhodo-spira species has been summarized by Traper and Fischer (1982).

The fixation of carbon dioxide via the Calvin cycle is apparently the major route of carbon assimilation in Ectothiorhodospira species under autotrophic growth conditions. High activities of ribulosebisphosphate carboxylase have been found in E. shaposh-nikovii (Firsov et al., 1974) and E. mobilis (Sahl and Traper, 1977). However, the assimilation of several organic carbon sources, such as acetate and propionate, depends on the presence of carbon dioxide and proceeds via several carboxylation reactions (Firsov and Ivanovskii, 1974, 1975). Under these conditions, a considerable proportion of the cellular carbon is therefore derived from carbon dioxide, which is not assimilated via the ribulose-bisphosphate pathway. Phosphoenolpyruvate carboxylase, ferre-doxin-dependent pyruvate synthase and oxoglutarate synthase have been found in E. shaposhnikovii (Firsov et al., 1974); phos-phoenolpyruvate carboxylase, phosphoenolpyruvate carboxyki-nase, and pyruvate carboxylase have been found in E. mobilis (Sahl and Traper, 1977). All enzymes of the glycolytic pathway and the tricarboxylic acid cycle, with the exception of oxoglu-tarate dehydrogenase, are present in E. shaposhnikovii (Krasil'nikova, 1975). Cells grown on acetate demonstrate increased activities of isocitrate lyase, which indicates activity of the glyoxylic acid pathway. The major reserve material formed from acetate and butyrate in the absence of carbon dioxide is poly-b-hydroxybutyric acid. In the presence of carbon dioxide and from other organic carbon sources, carbohydrates are formed instead (Novikova, 1971).

Ammonia and glutamine are suitable nitrogen sources for all species. Dinitrogen fixation has been demonstrated for some species and is probably a property inherent to the genus as a whole. Glutamate dehydrogenase (NADH-dependent) and glu-tamine synthetase/glutamate synthase (NADH-dependent) have been found in E. mobilis (Bast, 1977). Nitrate can be used as a nitrogen source by E. shaposhnikovii, and its reduction is catalyzed by a ferredoxin-dependent nitrate reductase, which is associated with or bound to the membranes and induced during growth with nitrate (Malofeeva et al., 1975).

In regard to their salt responses and the salt concentrations required for optimum growth, Ectothiorhodospira species are slightly halophilic or marine. Based on the salt concentration at which optimum growth occurs, species are grouped as nonhal-ophilic bacteria (<0.1 M NaCl), brackish water bacteria (—0.10.35 M NaCl), marine or slightly halophilic bacteria (—0.35-1.2 M NaCl), moderately halophilic bacteria (—1.2-2.5 MNaCl), and extremely halophilic bacteria (>2.5 M NaCl) (see Imhoff, 1993, 2001d). Bacteria of each of these groups may show up with different degrees of salt tolerance. Even nonhalophilic bacteria may exhibit extreme salt tolerances and grow at very high concentration, though at a suboptimal level. In this respect, Ectothiorho-dospira species have salt optima at the upper border of marine bacteria, and some, in particular E. haloalkaliphila, are moderately halotolerant. Because of its extended salt tolerance, this species may successfully compete with Halorhodospira species in moderately saline soda lakes. To cope with the salt and osmotic stress, Ectothiorhodospira species (E. haloalkaliphila, E. marismortui) accumulate glycine betaine, sucrose, and a third component (a-N-carbamoyl-L-glutamine amide) as osmotica (Oren et al., 1991; Severin et al., 1992; Imhoff, 1993; Imhoff and Riedel, unpublished results), but not ectoine and trehalose, which along with glycine betaine have been found as major components in Ha-lorhodospira species (Galinski et al., 1985; Severin et al., 1992).

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