Metabolic flexibility of Thiobacillus A 2 during substrate transitions in the chemostat

During autotrophic growth, cells of Thiobacillus A 2 retained a considerable capacity to oxidize various organic energy sources. Heterotrophically grown cultures, on the other hand, were completely devoid of the capacity to fix CO₂ via the Calvin cycle and to generate energy from thiosulfate. During transitions from organic media to inorganic thiosulfate-containing media in the chemostat, a long lag-phase was observed before energy generation, CO₂ fixation and, consequenctly, measurable growth occurred. This lag-phase was practically abolished if substrates were presentm at very low concentrations in the thiosulfate mineral medium which could be used as an energy source. The same result was obtained when the cells contained reserve material at the moment of the transition. During transitions from thiosulfate-limited growth to starvation, the -thiosulfate and the capacity to fix CO₂ decreased very slowly, after an initial short (± 4 h) increase of both enzyme systems. In contrast, these two metabolic functions were inactivated relatively rapidly in the presence of an oxidizable organic carbon and energy source. This process of inactivation was instantaneously stopped and reversed into rapid enzyme synthesis upon replacement of the organic substrate by thiosulfate.