The diverse effects of stress in cross-feeding systems

Predicting how metabolically interdependent microbial communities will respond to stress is an important but difficult challenge. Synthetic microbial communities provide an important tool for identifying mechanisms that impact stress response in communities. In this review, we discuss the weakest-link hypothesis as a null model that predicts that all species in an interdependent community will be constrained by the species that is most sensitive to a given stress. The hypothesis is contingent on the assumptions that 1) stress does not alter metabolite exchange in a way that changes growth constraints, 2) cross-feeding does not alter the ability of cells to withstand stress, and 3) the amount of stress experienced by a species is not altered by partners through mechanisms unrelated to cross-feeding. We then present cases that violate these assumptions to highlight the complexity of ecological factors involved in community stress response. Finally, we discuss the evolutionary dynamics of cross-feeding microbial systems when exposed to stress and some important challenges for connecting work from synthetic communities to more complex systems.