Enrichment and Isolation Procedures

Ectothiorhodospira species can be selectively enriched under pho-toautotrophic conditions with sulfide as the electron donor in saline and alkaline media (3% NaCl and pH 7.6-8.5), even when high proportions of Chromatiaceae species occur in the natural sample. Using Pfennig' s medium for purple sulfur bacteria with the addition of appropriate concentrations of NaCl and adjusted to an alkaline pH (Traper, 1970) and alternatively with a medium based on the mineral composition of the soda lakes of the Wadi Natrun (Jannasch, 1957; Imhoff and Traper, 1977; Imhoff et al., 1979) or modifications thereof, Ectothiorhodospira strains have been isolated from marine sources, hypersaline ponds, salt swamps, and salt lakes from many parts of the world. From such sources, isolation can be achieved in agar dilution series with natural samples without prior enrichment. A recipe of the medium specifically designed for species from alkaline lakes containing 5% (and 20%) salts is given below (see Imhoff, 1988a).1

1. The medium is prepared as a basal solution with (final content per liter medium): KH2PO4, 0.8 g; trace element solution SLA (see below), 1 ml; 1 M sodium carbonate buffer pH 9.0, 200 ml; and NaCl, 30 g. This basal solution is sterilized in an autoclave. Solutions of 20% NH4Cl (4 ml/l), 1% CaCl2-2H2O (5ml/l), 2% MgCl2-6H2O (5 ml/l), and 5% Na2S-9H2O (5-10 ml/l) are sterilized separately and added after cooling to room temperature. Sodium thiosulfate (1.0 g/l), if used as electron donor, and sodium acetate (1-2 g/l) may be added to the basal solution before sterilization. The final salinity is adjusted by changing the concentrations of sodium chloride, keeping under consideration the constant level of carbonate buffer, which is equivalent to —2% total salts. The amounts yield a medium with 5% total salts. Part of the sodium chloride (10% of the total final salt content by weight) may be replaced by sodium sulfate. In order to adjust the salinity to a total

The recipe of the standard medium according to Pfennig is given in the description of the genus Chromatium.

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