초록
Carbon dioxide serves as a co-substrate in succinic acid (SA) production by Actinobacillus succinogenes making it an important consideration in fermentation optimisation. In the current study, the availability of CO<SUB>2</SUB> to the cell, as the dissolved CO<SUB>2</SUB> concentration in the fermentation broth (C<SUB>CO'2</SUB>), is shown to define three distinct steady-state regimes. At C<SUB>CO'2</SUB>values between 8.4mM (+/-36.8% saturation) and saturation (22.8mM), there is no evidence of CO<SUB>2</SUB> limiting SA productivity and flux to SA is constant. As C<SUB>CO'2</SUB> is decreased, an upper C<SUB>CO'2</SUB> threshold (+/-36.8% saturation; 8.4mM) is reached where metabolic flux distributions remain constant but SA productivity and substrate uptake start to decline with decreasing C<SUB>CO'2</SUB> levels. A further decrease in C<SUB>CO'2</SUB> leads to a lower C<SUB>CO'2</SUB> threshold (+/-17.1% saturation, 3.9mM) where SA productivity continues to decrease with a concomitant shift in carbon flux away from SA towards C<SUB>3</SUB> fermentative pathways including ethanol. Since SA production is not limited at relatively low C<SUB>CO'2</SUB> values (+/-36.8% saturation), adequate CO<SUB>2</SUB> supply to the fermenter can be achieved without requiring major CO<SUB>2</SUB> sparging schemes which is favourable from an industrial processing perspective.