Evolution of microbial growth dynamics

The relationship between nutrient availability and growth rate is key to predicting the behavior of microbial ecosystems. Like all biological traits, the nutrient-growth relationship is subject to the fundamental evolutionary processes of mutation, selection, and genetic drift. Using a combination of empirical data and mathematical models, I will show how these evolutionary processes address two longstanding questions about the nutrient-growth relationship. 1) Should an organism's growth affinity for a nutrient be commensurate with that nutrient's environmental concentration? Since environmental concentrations (especially from the past) are often difficult to measure, a proportionality between these quantities has been assumed by many previous studies to infer the environment from growth traits. I will show that this is false under some modes of population dynamics, due to different environmental dependences of selection and genetic drift. This means that populations evolving in nutrient-rich environments can still have fast growth in nutrient-poor environments. 2) Are microbes limited by only one nutrient at a time, or can they be colimited by multiple nutrients? Colimitation has long been suspected in aquatic environments, where multiple nutrients are simultaneously rare. I will show that selection should indeed drive populations toward colimitation in general. When some of the nutrients are cross-fed between species, I will show how the evolved degree of colimitation corresponds to a balance between competition and cooperation between the species. Altogether these results demonstrate the importance of evolutionary processes in shaping fundamental aspects of microbial ecology.