In aerobic chemostat culture A2-GFI grew autotrophically on formate and heterotrophically on glucose with maximum specific growth rates (μ) of 021 and 033 h, respectively. At dilution rates of 01 and 018 h, 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 01 or 018 to 03 h resulted in washout (with an apparent μ for mixotrophic growth of 025 h) and the establishment of a culture dependent on glucose for growth. Growth yields on formate and glucose were, respectively, 33 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 03 h 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 (m) 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 A2-GFI when substrates are limited at low growth rates (< μ for formate), but is characterized by a μ 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.


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