Efficiency of hydrogen utilization during unitrophic and mixotrophic growth of Acetobacterium woodii on hydrogen and lactate in the chemostat

<it>Acetobacterium woodii</it> is able to grow chemolithoautotrophically on H<inf>2</inf> plus CO<inf>2</inf> or heterotrophically on lactate by forming acetate as sole product (homoacetogenesis). In order to investigate the effect of a second substrate on the utilization of H<inf>2</inf>, the bacteria were grown under substrate limitation in chemostat culture using H<inf>2</inf>/CO<inf>2</inf> or lactate for unitrophic and H<inf>2</inf>/CO<inf>2</inf>+lactate for mixotrophic growth. The chemostat was run at different dilution rates (0.007–0.035 h−1) until steady state was reached. Substrate consumption was balanced by production of acetate and biomass (96–115% recovery). Growth yields increased with increasing dilution rates giving maximum values of 7.7, 9.6, and 9.6 g-dw bacteria per mol acetate produced for growth on H<inf>2</inf>/CO<inf>2</inf>, lactate, and H<inf>2</inf>/CO<inf>2</inf>+lactate, respectively. The maintenance coefficients (expressed as acetate production) were 0.4, 0.08 and 0.17 mmol g-dw−1 h−1, respectively. Residual concentrations of lactate were usually below the detection limit (5 μM). However, H<inf>2</inf> partial pressures could always be analyzed and generally increased with increasing dilution rate. It is noteworthy that steady state H<inf>2</inf> concentrations (11–20 Pa) were also detected in lactate-grown chemostats demonstrating that H<inf>2</inf> was produced. During growth on H<inf>2</inf>/CO<inf>2</inf> residual H<inf>2</inf> partial pressures were much higher (50–2450 Pa, depending on dilution rate) than on lactate. Mixotrophic growth, on the other hand, resulted in intermediate H<inf>2</inf> partial pressures (25–160 Pa, depending on dilution rate). A similar pattern of H<inf>2</inf> partial pressures was obtained when the bacteria were grown at 25°C instead 30°C. Growth yields and H<inf>2</inf> partial pressures were not affected by the concentration of lactate (0.1–1.0 mM) under both unitrophic and mixotrophic conditions. The H<inf>2</inf> partial pressures at the half maximum growth rate on lactate, lactate+H<inf>2</inf>/CO<inf>2</inf>, and H<inf>2</inf>/CO<inf>2</inf> were 16, 42, and 94 Pa, respectively. These results demonstrate that <it>A. woodii</it> is able to utilize H<inf>2</inf> down to lower partial pressures when a second heterotrophic substrate is available. However, the residual H<inf>2</inf> partial pressures were still too high to allow successful competition with H<inf>2</inf>-utilizing methanogens.