Models of metabolic trade-offs as mechanisms underlying diversity

Background/Question/Methods The competitive exclusion principle which states that a simple unstructured environment containing only a single resource can support only one competitor is often used as a starting point for discussion of the evolution and maintenance of microbial diversity. A model for such a simple environment is the chemostat and models of competition predict that diversity cannot be maintained in the chemostat unless the population is subject to product inhibition, cross-feeding or the dilution rate of the chemostat varies periodically. Each of these assumptions creates a form of environmental heterogeneity that allows the coexistence of multiple types. However, recently diversity has been detected in a number of chemostat experiments using both culturing techniques and molecular population genetics. In these experiments there no product inhibition nor cross-feeding took place and the dilution rate was kept constant. Using mathematical models we show that internal metabolic complexities are sufficient to allow creation and maintenance of microbial diversity in simple habitats. In particular we consider two examples. Results/Conclusions The first example demonstrates that the rate vs. yield trade-off and the maximal growth rate vs. affinity trade-offs could lead to creation of a population with diverse maximal uptake rates. In the second example we demonstrate that stress protection and nutritional competence (SPANC) balance is sufficient to cause polymorphism in a core stress response gene rpoS.