A network optimisation condition uncovers the role offunctional groups in the feasibility and dynamical stability of microbial model ecosystems.

Abstract

Investigating mechanisms favouring stable coexistence of complex microbial communities is central to understand their formation and maintenance in the natural environment and, eventually, controlling and designing synthetic communities. In this work, we studied microbial model ecosystems in which consumption and secretion of resources was explicit, determining ecological interactions (e.g. competition for resources, facilitation, mutualism). We found a sufficient condition for the dynamical stability of microbial model ecosystems. This condition allowed us to derive an objective function whose optimization selected specific configurations of consumption and secretion of resources. Optimized configurations stood out as a compromise between having a large parameter space in which species coexistence was feasible (dominated by systems in which microbes do not secrete any resources), and having a high rate of return to equilibrium after a perturbation (dominated by systems in which every microbe secrete all resources). We explained the behaviour observed for optimized configurations by noting that they host sets of species with differentiated niches -as defined by their consumption and secretion strategies-, termed functional groups. We speculated that, since increasing the number of functional groups increased the number of niches, competition between species should be reduced, favouring coexistence. Therefore, our results suggest that the formation of functional groups have an important role in microbial coexistence, which has ramifications for the design of complex microbial communities.