Growth of Thiobacillus A2 under Alternating Growth Conditions in the Chemostat

Abstract

A chemostat culture of Thiobacillus A2 was grown under alternate limitation of acetate and thiosulphate at a dilution rate of 0·05 h−1. With a substrate alternation of 4 h acetate/4 h thiosulphate or 8 h acetate/16 h thiosulphate, uninterrupted growth of the culture was obtained. However, with 16 h acetate/8 h thiosulphate, Thiobacillus A2 required several hours before it attained autotrophic growth at a rate of 0·05 h−11 following the transition from acetate to thiosulphate supply. In a two-membered mixed culture with the heterotrophic spirillum G7, Thiobacillus A2 outcompeted the heterotroph when grown under alternating limitation of acetate (4 h) and thiosulphate (4 h). Under the same growth conditions Thiobacillus A2 coexisted in equal numbers with Thiobacillus neapolitanus, from which it was concluded that Thiobacillus A2 grew only heterotrophically on acetate in this case. Analogous competition experiments with three-membered cultures, grown under the same conditions, resulted in complete elimination of Thiobacillus A2 and coexistence of T. neapolitanus and spirillum G7 in equal numbers. In an enrichment culture, again grown under the same regime, a facultatively chemolithotrophic spirillum-shaped Thiobacillus or Thiomicrospira became dominant; this organism was subsequently isolated in pure culture.