Transient dynamics of competitive exclusion in microbial communities

Microbial metabolism drives our planet's biogeochemistry and plays a central role in industrial processes. Molecular profiling in bioreactors has revealed that microbial community composition can be highly variable while maintaining constant functional performance. Furthermore, following perturbation bioreactor performance typically recovers rapidly, while community composition slowly returns to its original state. Despite its practical relevance, we still lack an understanding of the mechanisms causing the discrepancy between functional and compositional stability of microbial communities. Using a mathematical model for microbial competition, as well as simulations of a model for a nitrifying bioreactor, we explain these observations on grounds of slow non-equilibrium dynamics eventually leading to competitive exclusion. In the presence of several competing strains, metabolic niches are rapidly occupied by opportunistic populations, while subsequent species turnover and the eventual dominance of top competitors proceeds at a much slower rate. Hence, functional redundancy causes a separation of the time scales characterizing the functional and compositional stabilization of microbial communities. This effect becomes stronger with increasing richness because greater similarities between top competitors lead to longer transient population dynamics.