Utilization of N, P, and organic C by heterotrophic bacteria. I. Outline of a chemostat theory with a consistent concept of 'maintenance' metabolism

The effect of variable N : C and P : C ratios in the reservoir medium on chemostat steadystate situations is explored using idealized mathematical models. A simple 'Monod' type model, with fixed biomass composition and constant yield, is used to gmde the general understanding of the outcome to be expected. With this model, 3 uniquely defined areas of reservoir medium N : C and P : C ratios are discerned, corresponding to either carbon, nitrogen, or phosphorus limitation of bacterial growth. A more elaborate 'Droop' type model, allowing for variable biomass composition and an additional respiration term proportional to the 'surplus' amount of carbon per cell, is shown to introduce additional intermediate areas where the medium is exhausted of combinations of 2, or of all 3 nutrients. At maximum growth rate, the predictions of the 'Droop' model reduce to those of a 'Monod' type model. The requirement of organic C for energy production is explored at different growth rates and at different types of growth limitation. With previous models there was an unnoted inconsistency between the simultaneous assumptions of mineral nutrient limitation and a non-zero 'maintenance' metabolism as growth rate approached zero. In the model suggested here, the increase in the areas of mixed N, C and P, C limitations at low growth rates eliminates this inconsistency, w h ~l e still qualitatively explaining existing observational data. From comparison of the model with exlstlng data, 2 further refinements in the descnption of respiration are suggested. One is the assumption of a difference between the level of cell 'surplus' carbon at which growth stops, and the level at which respiration is halted. The other assumption is a dependence of respiration on phosphate starvation. The model is used to explore the role of bacteria as remineralizers and/or consumers of mineral nutrients. Use of the terms 'maintenance', 'waste', and 'growth independent' metabolism is discussed in light of the concept introduced in the model.