Succinic acid-producing biofilms of Actinobacillus succinogenes: reproducibility, stability and productivity

Continuous anaerobic fermentations were performed in a biofilm reactor packed with Poraver® beads. Dilution rates (D) varied between 0.054 and 0.72 h⁻¹, and D-glucose and CO₂ gas were used as carbon substrates. Steady-state conditions were shown to be repeatable and independent of the operational history. Production stability was achieved over periods exceeding 80 h at values of D below 0.32 h⁻¹. In these situations, steady-state variation (expressed as fluctuations in NaOH neutralisation flow rates) exhibited a standard deviation of less than 5 % while no indication of biofilm deactivation was detected. The total biomass amount was found to be independent of the dilution rate with an average dry concentration of 23.8 ± 2.9 g L⁻¹ obtained for all runs. This suggests that the attachment area controls the extent of biofilm accumulation. Specific succinic acid (SA) productivities, based on the total biomass amount, exhibited a substantial decrease with decreasing D. An SA volumetric productivity of 10.8 g L⁻¹ h⁻¹ was obtained at D = 0.7 h⁻¹—the highest value reported to date in Actinobacillus succinogenes fermentations. SA yields on glucose increased with decreasing D, with a yield of 0.90 ± 0.01 g g⁻¹ obtained at a D of 0.054 h⁻¹. Production of formic acid approached zero with decreasing D, while the succinic to acetic acid ratio increased with decreasing D, resulting in an increasing SA yield on glucose.