CALORIMETRIC AND STOICHIOMETRIC ANALYSIS OF GROWTH OF KLUYVEROMICES FRAGILIS IN CONTINUOUS-CULTURE - NITROGEN LIMITATION IMPOSED UPON CARBON-LIMITED GROWTH

The calorimetric response of the yeast Kluyveromices fragilis was inve stigated for growth in continuous culture where nitrogen limitation wa s imposed on a carbon-limited culture. Calorimetric measurements were combined with off gas analysis, measurements of biomass, substrate and product concentrations, elemental biomass composition, and heat produ ction to study the physiological response of K. fragilis. Regions wher e both carbon and nitrogen limited growth, were found over a broad ran ge of dilution rates and feed carbon-to-nitrogen ratios. The principle mechanism by which K. fragilis accommodated regions of dual carbon an d nitrogen limitation was by partial decoupling of the anabolic and ca tabolic pathways. When the culture was only nitrogen-limited, increase d decoupling of the two pathways was observed. The principal effect of the decoupling was an increased catabolic consumption of glucose, gen erating an increased heat yield. The preferred way to process the exce ss glucose was through respiration but the cells were also capable of fermenting a small percentage of the excess glucose in specific cases where the dissolved oxygen partial pressure approached zero. In additi on, these results were qualitatively compared to similar studies on Sa ccharomices cerevisiae. The two yeasts were similar in their ability t o accommodate dual limitation by uncoupling anabolic biomass formation from substrate consumption. The two yeasts were dissimilar in how the catabolic substrate was processed. For S. cerevisiae the presence of a bottleneck in the respiration pathway dictated that the majority of the catabolic glucose consumption was by fermentation. For K. fragilis , the lack of a bottleneck in the respiration pathway dictated that th e majority of catabolic glucose substrate consumption was by respirati on.