Recipient-Biased Competition For A Cross-Fed Nutrient Is Required For Coexistence Of Microbial Mutualists

Many mutualistic microbial relationships are based on nutrient cross-feeding. Traditionally, cross-feeding is viewed as being unidirectional, from the producer to the recipient. This is likely true when a producer’s waste, such as a fermentation product, has value only for a recipient. However, in some cases the cross-fed nutrient holds value for both the producer and the recipient. In such cases, there is potential for nutrient reacquisition by producer cells in a population, leading to competition against recipients. Here, we investigated the consequences of interpartner competition for cross-fed nutrients on mutualism dynamics by using an anaerobic coculture pairing fermentative Escherichia coli and phototrophic Rhodopseudomonas palustris. In this coculture, E. coli excretes waste organic acids that provide a carbon source for R. palustris. In return, R. palustris cross-feeds E. coli ammonium (NH$_4$$^+$), a compound that both species value. To explore the potential for interpartner competition, we first used a kinetic model to simulate cocultures with varied affinities for NH$_4$$^+$ in each species. The model predicted that interpartner competition for NH$_4$$^+$ could profoundly impact population dynamics. We then experimentally tested the predictions by culturing mutants lacking NH$_4$$^+$ transporters in both NH$_4$$^+$ competition assays and mutualistic cocultures. Both theoretical and experimental results indicated that the recipient must have a competitive advantage in acquiring cross-fed NH$_4$$^+$ to sustain the mutualism. This recipient-biased competitive advantage is predicted to be crucial, particularly when the communally valuable nutrient is generated intracellularly. Thus, the very metabolites that form the basis for mutualistic cross-feeding can also be subject to competition between mutualistic partners