Abstract:
Carbon dioxide serves as co-substrate in the production of succinic acid by Actinobacillus succinogenes. The transient concentration of dissolved CO2 in the broth (CCO2) controls the uptake of CO2 in the cell. Based on CCO2 , three distinct regimes could be identified in which the behaviour of the organism differed with CCO2 availability. When CCO2 was higher than 8.4 mM (44.4% saturated at an atmospheric pressure of 86 kPa), there was no evidence of CO2 limiting succinic acid productivity and flux to succinic acid remained constant. When CCO2 decreased below 8.4 mM a decrease in the succinic acid production and glucose consumption rates was observed to 28.01% and 19.89% of their original value respectively, at the lowest CCO2 value investigated. Below a CCO2 of 4 mM (21.16% saturated at an atmospheric pressure of 86 kPa), the productivity continued to decrease along with a shift in the total carbon flux from the succinic acid-producing pathway (C4-pathway) to the by-product-producing pathway (C3-pathway). The fraction of total carbon flux directed to the C4-pathway decreased from 0.48 to 0.33 at the lowest CCO2 value investigated. Although the by-product acetic acid concentration decreased to 88% of the original value, formic acid remained relatively stable and the ethanol concentration increased from an average of 0.26 g.L-1 to 1 g.L-1. The organism starts producing ethanol in order to satisfy the redox balance when the C4-pathway becomes less active. It was calculated that the flux shift to the C3-pathway does not favour ATP production. The organism is, however, still viable at the very low ATP production rates found at very low values of CCO2.
Since succinic acid production is not limited at relatively low values of CCO2 (44.4% saturation), adequate CO2 supply to the fermenter can be achieved without major CO2 sparging which is beneficial from an industrial processing perspective.
