Journal of General Microbiology (198 l), 125, 55-62. Printed in Great Britain 55 Mixotrophic Growth of Thiobacillus A2 in Chemostat Culture on

In aerobic chemostat culture Thiobacillus A2-GFI grew autotrophically on formate and heterotrophically on glucose with maximum specific growth rates (,urn,,) of 0.21 and 0.33 h-l, respectively. At dilution rates of 0.1 and 0.18 h-l, it grew mixotrophically on formate + glucose mixtures, completely consuming both substrates. Ribulose- 1,5-bisphosphate carboxy-lase and formate dehydrogenase were present at high specific activity in autotrophic and mixotrophic cultures, but were repressed in cultures on glucose alone. A greater proportion of added glucose was assimilated in mixotrophic culture than in heterotrophic culture. Raising the dilution rate of a mixotrophic culture from 0.1 or 0.18 to 0.3 h- ' resulted in washout (with an apparent p,,, for mixotrophic growth of 0.25 h-l) and the establishment of a culture dependent on glucose for growth. Growth yields on formate and glucose were, respectively, 3- 3 and 100 g dry wt (mol substrate consumed)-'. Steady state biomass production in mixotrophic culture indicated additive growth yields. The biomass produced in cultures on formate + glucose at a dilution rate of 0-3 h-l suggested that growth only occurred on glucose, but organisms still contained high activities of ribulose- 1,5-bisphosphate carboxylase and formate dehydrogenase. At a formate:glucose ratio (mM) of 100: 1, some formate was oxidized and CO, was fixed, but formate was not used when this ratio was 50:5. Formate-glucose mixotrophy benefits Thiobacillus A2-GFI when substrates are limited at low growth rates (<,urnax for formate), but is characterized by a pmax below that possible on glucose. Physiological behaviour at high growth rates was influenced by the formate:glucose ratio, resulting under some conditions, at least, in loss of mixotrophy and the establishment of heterotrophic growth