GROWTH AND METABOLISM OF SACCHAROMYCES-CEREVISIAE IN CHEMOSTAT CULTURES UNDER CARBON-LIMITING, NITROGEN-LIMITING, OR CARBON-LIMITING AND NITROGEN-LIMITING CONDITIONS

Aerobic chemostat cultures of Saccharomyces cerevisiae were performed under carbon-, nitrogen-, and dual carbon- and nitrogen-limiting conditions. The glucose concentration was kept constant, whereas the ammonium concentration was varied among different experiments and different dilution rates. It was found that both glucose and ammonium were consumed at the maximal possible rate, i.e., the feed rate, over a range of medium C/N ratios and dilution rates. To a small extent, this was due to a changing biomass composition, but much more important was the ability of uncoupling between anabolic biomass formation and catabolic energy substrate consumption. When ammonium started to limit the amount of biomass formed and hence the anabolic flow of glucose, this was totally or at least partly compensated for by an increased catabolic glucose consumption. The primary response when glucose was present in excess of the minimum requirements for biomass production was an increased rate of respiration. The calculated specific oxygen consumption rate, at D = 0.07 h-1, was more than doubled when an additional nitrogen limitation was imposed on the cells compared with that during single glucose limitation. However, the maximum respiratory capacity decreased with decreasing nitrogen concentration. The saturation level of the specific oxygen consumption rate decreased from 5.5 to 6.0 mmol/g/h under single glucose limitation to about 4.0 mmol/g/h at the lowest nitrogen concentration tested. The combined result of this was that the critical dilution rate, i.e., onset of fermentation, was as low as 0.10 h-1 during growth in a medium with a low nitrogen concentration compared with 0.20 h-1 obtained under single glucose limitation.