Summary: The outcome of competition in thiosulphate-limited chemostat culture between the obligate chemolithotroph and the versatile facultative autotroph A2 was in part a function of pH. In pure culture grew faster than A2 at pH values up to pH 7.6, but in competition A2 dominated at pH 7.35 and 7.6. At pH 7.1, dominated, although a significant steady state population of A2 persisted, apparently growing on organic nutrients excreted by . Coexistence of both organisms occurred under all chemolithotrophic growth conditions tested with the dominant organism comprising 85 to 99% of the population, indicating that competition was not the sole interaction between the species. At pH 7.1, the inclusion of glucose in the thiosulphate medium resulted in rapid domination of the culture by A2, with the virtual elimination of . It is concluded that the capacity for mixotrophy is a selective advantage to a facultative thiobacillus in competition with an obligately chemolithotrophic species.


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