Competition in the Chemostat between an Obligately and a Facultatively Chemolithotrophic Thiobacillus

Abstract

The outcome of competition in thiosulphate-limited chemostat culture between the obligate chemolithotroph Thiobacillus neapolitanus and the versatile facultative autotroph Thiobacillus A2 was in part a function of pH. In pure culture T. neapolitanus grew faster than Thiobacillus A2 at pH values up to pH 7·6, but in competition Thiobacillus A2 dominated at pH 7·35 and 7·6. At pH 7·1, T. neapolitanus dominated, although a significant steady state population of Thiobacillus A2 persisted, apparently growing on organic nutrients excreted by T. neapolitanus. 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 Thiobacillus A2, with the virtual elimination of T. neapolitanus. It is concluded that the capacity for mixotrophy is a selective advantage to a facultative thiobacillus in competition with an obligately chemolithotrophic species.