Abstract
Although mutualisms are often studied as simple pairwise interactions, they typically involve complex networks of interacting species. How multiple mutualistic partners that provide the same service and compete for resources are maintained in mutualistic networks is an open question. We use a model bacterial community in which multiple ‘partner strains’ of Escherichia coli compete for a carbon source and exchange resources with a ‘shared mutualist’ strain of Salmonella enterica. In laboratory experiments, competing E. coli strains readily coexist in the presence of S. enterica, despite differences in their competitive abilities. We use ecological modeling to demonstrate that a shared mutualist can create temporary resource niche partitioning by limiting growth rates, even if yield is set by a resource external to a mutualism. This mechanism can extend to maintain multiple competing partner species. Our results improve our understanding of complex mutualistic communities and aid efforts to design stable microbial communities.
Highlights
Mutualisms are often studied as simple pairwise interactions, they typically involve complex networks of interacting species
The partner guild consists of one E. coli strain that is a methionine auxotroph (“Em”) and another E. coli strain that is an arginine auxotroph (“Er”)—each strain lacks a gene in the biosynthetic pathway for its respective amino acid, so in order for a strain to grow, its required amino acid must be available in the environment
The two E. coli strains compete for lactose, which we provide in the growth media, and excrete acetate as a byproduct of lactose metabolism
Summary
Mutualisms are often studied as simple pairwise interactions, they typically involve complex networks of interacting species. We use ecological modeling to demonstrate that a shared mutualist can create temporary resource niche partitioning by limiting growth rates, even if yield is set by a resource external to a mutualism. This mechanism can extend to maintain multiple competing partner species. Our system consists of a partner guild of Escherichia coli strains that compete with one another for a carbon source and engage in mutualism with a strain of Salmonella enterica, the shared mutualist. When the two E. coli strains obtain their amino acids from the shared mutualist, the two E. coli strains coexist, maintaining the diversity of the multiple mutualist communities. This work helps us understand how diversity is maintained in multiple mutualist communities and can inform efforts to design stable microbial communities
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