The current opinion is that facultatively chemolithotrophic thiobacilli, sometimes referred to as “versatile” thiobacilli [ 11, often may have a significant ecological advantage over their more specialized counterparts, the obligately chernolithotrophic thiobacilli. It has been suggested that the presence of both inorganic and organic substrates in the natural environment would particularly favour growth of facultatively chemolithotrophic thiobacilli [ 2 3 ] . There exists, however, still too little direct experimental evidence to substantiate this point of view. Moreover only a very limited number of facultatively chemolithotrophic thiobacilli has been isolated in the past. This might be explained either by the fact that their occurrence in nature is very limited or by the lack of selective enrichment techniques, or by a combination of both. In a study on the ecological niche of facultatively chemolithotropic thiobacilli, we investigated Thiobacillus A2 during growth in pure and in mixed cultures under dual substrate limitation in the chemostat [4,5]. Under these conditions Thiobacillus A2 was found to be able to utilize inorganic and organic compounds simultaneously. The outcome of competition between Thiobacillus A2 and both a specialized Thiobacillus sp. and a specialized heterotroph was shown to depend on the concentration-ratio of the organic and inorganic substrate (acetate and thiosulfate, respectively) in the medium-reservoir of the chemostat [4]. Thiobacillus A2 dominated the mixed population over a large range of different mixtures of thiosulfate and acetate. At relatively high thiosulfateacetate ratios coexistence of Thiobacillus A2 and the specialized thiobacillus occurred, whereas at relatively low thiosulfate-acetate ratios Thiobacillus A2 coexisted with the specialized heterotroph. However, when the inflowing medium contained exclusively thiosulfate or acetate the specialized autotroph or heterotroph respectively, became dominant. These results clearly pointed to a possible ecological niche for facultatively chemolithotrophic thiobacilli, since in many environments growthlimiting concentrations of inorganic and organic energy sources may be available simultaneously [6,7]. The outcome of the competition experiments also implied that it should be possible to enrich for facultatively chemolithotrophic thiobacilli in continuous cultures in which growth is limited simultaneously by organic and reduced sulfur containing inorganic substrates. This paper reports the successful application of this method to isolate selectively these organisms from different fresh-water samples.
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