Syntrophy, where one species lives off metabolites produced by others, is a mechanism that can promote species coexistence in microbial communities. As a specific form of syntrophy, cross-feeding, where each of two species produces resource the other species needs, has been experimentally shown to have positive effect on stabilizing the abundance of the cooperative species. Although extensive theoretical models have been formulated to study the coexistence in cross-feeding systems, few of them has included cheaters, who benefit from the resources produced by their cooperative partners with below-average contributions or even without any contributions to the communities. To further reveal the role of cross-feeding in the coexistence of species and to examine the competitive outcomes between the syntrophs and the cheater, we formulate a cross-feeding model and analyze it using resource competition theory. In particular, we examine the effects of the synsthesis rates of the syntrophs.
Results/Conclusions
The results show that when the cheater is absent, multiple stable states can be observed if the synthesis rates are high because the synthesis of each syntroph can promote the persistence of the other, which in turn benefits itself, depending on its initial density. Consequently, higher synthesis rates can increase the possibility that they coexist. However, when the cheater is present, multiple stable states disappear because at least one syntroph is outcompeted by the cheater. Moreover, high synthesis rates increase the possibility that the cheater invades the system, and the cheater will be dominant in the community as long as it can invade. Our results suggest that cross-feeding promotes species coexistence and has the potential to favor more complex syntrophic networks.