Syntrophic interactions, where species consume metabolites excreted by others, are ubiquitous in microbial communities, and have important uses in synethetic biology. Syntrophy is likely to arise when trade-offs favor an organism that specializes on consuming particular metabolites, rather than all possible metabolites. Several trade-offs have been suggested for how this might occur; however, few models consider different trade-offs to test which are most consistent with observed patterns. We developed a differential equation model to study competition between a syntrophic processing chain, where each microbe can perform one step in metabolizing an initial resource to a final state, and a metabolic generalist that can perform all metabolic functions. We also examined whether competition will maximize the production of a final metabolic compound. Our model considers both a constant environment, and one in which resource inputs are variable.
Results/Conclusions
We found that competitive outcomes can be predicted by a generalization of the R* rule and relative nonlinearity. In a constant environment, the species that can persist at the lowest resource level is the competitive dominant, and a metabolic generalist cannot coexist with all members of the syntrophic community. In a variable environment, species can coexist if there is a trade-off between growth at low resources vs. growth at high resources. We derived a simple rule for predicting production rates of the final metabolite, and show that competition may not maximize final metabolite production. We showed that processing chains are inherently less efficient, because resources are lost during each step of the process. Our results also suggest that most proposed trade-offs cannot fully explain why some environments appear to favor syntrophy.