Aerobic chemostat cultures of Saccharomyces cerevisiae were performed under carbon-, nitrogen-, and dual carbon- and nitrogen-limiting conditions. The glucose concn. was kept const., whereas the ammonium concn. was varied among different expts. and different diln. rates. 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 diln. rates. To a small extent, this was due to a changing biomass compn., but much more important was the ability of uncoupling between anabolic biomass formation and catabolic energy substrate consumption. When ammonium started to limit the amt. 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 min. requirements for biomass prodn. was an increased rate of respiration. The calcd. specific oxygen consumption rate, at D = 0.07 h-1, was more than doubled when an addnl. nitrogen limitation was imposed on the cells compared with that during single glucose limitation. However, the max. respiratory capacity decreased with decreasing nitrogen concn. The satn. 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 concn. tested. The combined result of this was that the crit. diln. rate, i.e., onset of fermn., was as low as 0.10 h-1 during growth in a medium with a low nitrogen concn. compared with 0.20 h-1 obtained under single glucose limitation. [on SciFinder (R)]