Mixotrophic Growth of Thiobacillus A2 in Chemostat Culture on Formate and Glucose

In aerobic chemostat culture Thiobacillus A2-GFI grew autotrophically on formate and heterotrophically on glucose with maximum specific growth rates (μmax) of 0·21 and 0·33 h−1, respectively. At dilution rates of 0·1 and 0·18 h−1, it grew mixotrophically on formate + glucose mixtures, completely consuming both substrates. Ribulose-1,5-bisphosphate carboxylase 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−1 resulted in washout (with an apparent μ max for mixotrophic growth of 0·25 h−1) 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)−1. 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−1 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 CO2 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 (<μ max for formate), but is characterized by a μ max 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.