Figure 4: Respiring cells can invade a chemostat with an optimal, fermenter resident, and vice versa.

The resident is optimal for the starting conditions of the chemostat (D = 0.3 hr⁻¹ and glu_feed = 111 mM). This means that no other respiring strain can invade the chemostat in (A) and similarly no other fermenter in (B). (A,B) At t = 0, a ‘mutant’ of opposite metabolic strategy appears that settles in the chemostat, but does not take over completely. This indicates that (under these conditions) there is not a single optimal strategy in this chemostat. (C,D) The potential of opposing strategies to invade is clear from the fitness landscapes, which show the difference in growth rate, relative to the dilution rate, as a function of the relative flux through respiration. When a respirer (fermenter) is the resident, more fermenting (respiring) strategies have a positive selection coefficient, indicating that they have the potential to invade.