FIGURE 3.

Impact of nitrate as an alternative N-source on continuous gas fermentation of C. ljungdahlii using CO₂ and H₂ at different periods with a medium feed rate of 0.10 mL min^–1 (experiment 1). Single values for pH and OD₆₀₀ (A), and for acetate and ethanol production rates in mmol-C L^–1 d^–1 (B). The bioreactors with nitrate feed were grown in ammonium-free PETC medium supplemented with 18.7 mM Na-nitrate. The horizontal dotted lines indicate the continuous process in which medium of different pH was fed to each bioreactor. The red arrows indicate the crash in OD₆₀₀ of each bioreactor with nitrate feed at different time points. The star symbol describes the time point were the pH was lowered manually by adding HCl to the system until a pH of 4.5 was reached. Period: I, pH = 6.0; II, pH = 5.5, III, pH = 5.0; and IV, pH = 4.5.