Abstract:
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 CO2 to the cell, as the dissolved CO2 concentration in the fermentation broth (CCO2 ), is shown to define
three distinct steady-state regimes. At CCO2 values between 8.4 mM (±36.8% saturation) and saturation
(22.8 mM), there is no evidence of CO2 limiting SA productivity and flux to SA is constant. As CCO2 is
decreased, an upper CCO2 threshold (±36.8% saturation; 8.4 mM) is reached where metabolic flux distributions
remain constant but SA productivity and substrate uptake start to decline with decreasing CCO2
levels. A further decrease in CCO2 leads to a lower CCO2 threshold (±17.1% saturation, 3.9 mM) where SA
productivity continues to decrease with a concomitant shift in carbon flux away from SA towards C3
fermentative pathways including ethanol. Since SA production is not limited at relatively low CCO2 values
(±36.8% saturation), adequate CO2 supply to the fermenter can be achieved without requiring major CO2
sparging schemes which is favourable from an industrial processing perspective.
