EFFICIENCY OF HYDROGEN UTILIZATION DURING UNITROPHIC AND MIXOTROPHIC GROWTH OF ACETOBACTERIUM-WOODII ON HYDROGEN AND LACTATE IN THE CHEMOSTAT

Acetobacterium woodii is able to grow chemolithoautotrophically on H-2 plus CO2 or heterotrophically on lactate by forming acetate as sole p roduct (homoacetogenesis). In order to investigate the effect of a sec ond substrate on the utilization of H-2, the bacteria were grown under substrate limitation in chemostat culture using H-2/CO2 or lactate fo r unitrophic and H-2/CO2+lactate CO2+lactate for mixotrophic growth. T he chemostat was run at different dilution rates (0.007-0.035 h(-1)) u ntil steady slate was reached. Substrate consumption was balanced by p roduction of acetate and biomass (96-115% recovery). Growth yields inc reased 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/CO2+lactate, respectively. The maintenance coeffic ients (expressed as acetate production) were 0.4, 0.08 and 0.17 mmol g -dw(-1) h(-1). respectively. Residual concentrations of lactate were u sually below the detection limit (5 mu M). However, H-2 partial pressu res could always be analyzed and generally increased with increasing d ilution rate. It is noteworthy that steady state H-2 concentrations (1 1-20 Pa) were also detected in lactate-grown chemostats demonstrating that H-2 was produced. During growth on H-2/CO2 residual H-2 partial p ressures were much higher (50-2450 Pa, depending on dilution rate) tha n on lactate. Mixotrophic growth, on the other hand, resulted in inter mediate H-2 partial pressures (25-160 Pa, depending on dilution rate). A similar pattern of H-2 partial pressures was obtained when the bact eria were grown at 25 degrees C instead 30 degrees C. Growth yields an d H-2 partial pressures were not affected by the concentration of lact ate (0.1-1.0 mM) under both unitrophic and mixotrophic conditions. The H-2 partial pressures at the half maximum growth rate on lactate, lac tate+H-2/CO2, and H-2/CO2 were 16, 42, and 94 Pa, respectively. These results demonstrate that A. woodii is able to utilize H-2 down to lowe r partial pressures when a second heterotrophic substrate is available . However, the residual H-2 partial pressures were still too high to a llow successful competition with H-2-utilizing methanogens. (C) 1998 F ederation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.