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

Abstract

Acetobacterium woodii is able to grow chemolithoautotrophically on H 2 plus CO 2 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 2, the bacteria were grown under substrate limitation in chemostat culture using H 2/CO 2 or lactate for unitrophic and H 2/CO 2+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 2/CO 2, lactate, and H 2/CO 2+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 2 partial pressures could always be analyzed and generally increased with increasing dilution rate. It is noteworthy that steady state H 2 concentrations (11–20 Pa) were also detected in lactate-grown chemostats demonstrating that H 2 was produced. During growth on H 2/CO 2 residual H 2 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 2 partial pressures (25–160 Pa, depending on dilution rate). A similar pattern of H 2 partial pressures was obtained when the bacteria were grown at 25°C instead 30°C. Growth yields and H 2 partial pressures were not affected by the concentration of lactate (0.1–1.0 mM) under both unitrophic and mixotrophic conditions. The H 2 partial pressures at the half maximum growth rate on lactate, lactate+H 2/CO 2, and H 2/CO 2 were 16, 42, and 94 Pa, respectively. These results demonstrate that A. woodii is able to utilize H 2 down to lower partial pressures when a second heterotrophic substrate is available. However, the residual H 2 partial pressures were still too high to allow successful competition with H 2-utilizing methanogens.