QUANTIFICATION OF SMALL ENTHALPIC DIFFERENCES IN ANAEROBIC MICROBIAL-METABOLISM - A CALORIMETRY-SUPPORTED APPROACH

A calorimetry-supported approach to the measurement of small enthalpic differences in microbial metabolism is presented. Anaerobic fermentat ions with the bacterium Zymomonas mobilis and the yeast Saccharomyces cerevisiae were carried out in a fermenter-calorimeter at a constant d ilution rate (D=0.15 h(-1)). The glucose concentration of the reed was increased stepwise from 10 to 80 gl(-1). Salts, vitamins and trace el ements were present in excess. Different steady state conditions were analysed. For Z. mobilis, specific production rates for ethanol, carbo n dioxide and heat were three times higher than those measured for S. cerevisiae. If the glucose feed concentration exceeded 20 gl(-1) durin g the S. cerevisiae fermentations, glucose could be measured in the fe rmenter. Further increasing glucose concentrations of the feed caused higher specific ethanol, carbon dioxide and heat production rates, whe reas the biomass yield Y-X/S decreased remarkably. This result can be explained as the Crabtree effect under anaerobic conditions and interp reted as metabolic uncoupling of anabolic biomass formation and catabo lic energy substrate consumption. A futile cycle contributing to the h eat production is considered to provide S. cerevisiae with a regulator y mechanism in the control of the flux of metabolites through the glyc olytic pathway.