Evolutionary pressures on microbial metabolic strategies in the chemostat
Late lunch talk by Meike Wortel, VU University, Amsterdam, The Netherlands
Protein expression is shaped by evolutionary pressures. Due to limitations in biosynthetic capacity, the costs and benefits of enzyme production are important determinants of fitness. While these processes are well understood in batch conditions, in chemostats, which are extensively used to study microbial evolution in a laboratory setting, a feedback of the microbial physiology on the conditions in the chemostat hinders an intuitive understanding. Here, we aim to provide a solid theoretical framework of the selective pressures and optimal evolutionary strategies in the chemostat. We show that the optimal enzyme levels can be described with control theory and that optimal strategies are implemented by well-defined metabolic subsystems, known as elementary flux modes, similar to batch conditions. However, as we illustrate with a realistic coarse-grained model of the physiology and growth of the yeast Saccharomyces cerevisiae, the evolutionary dynamics and final outcome of evolution in a chemostat can be very different. Simulated evolution of respiro-fermentative yeast cells in a chemostat at an intermediate dilution rate shows an evolutionary stable coexistence of a strictly respiring and a strictly fermenting strain. Our results connect a kinetic, mechanistic view of metabolism with cellular physiology and evolutionary dynamics. We provide a theoretical framework for interpreting and reasoning about selection and evolution experiments in the chemostat.