Cellular Economics of Exchanged Metabolites Alter Ratios of Microbial Trading Partners in a Predictable Manner

Most microorganisms
exist in interacting consortia, yet the principles
behind consortia assembly, including trading partner structure, still
require deciphering, despite being central to understanding, building,
and controlling consortia. This study tests a cellular-economy-based
hypothesis that proposes consortia engaged in obligate metabolite
exchange will assemble at ratios where the metabolic burden associated
with the exchanges is predictably shared. The metabolic burden was
quantified using ATP equivalents, the quantity of high-energy ATP
phosphoanhydride bonds that could be produced if the exchanged metabolites
were instead retained and catabolized for cellular energy. The hypothesis
was tested using Escherichia coli cocultures
engineered for obligatory exchange of pyruvate and l-arginine.
The burden of metabolite exchange was manipulated by changing the
availability of O2, which altered the bioavailability
of substrate energy, and by deactivating the ATP synthase enzyme,
which prevented oxidative phosphorylation. Three synthetic cocultures
assembled at predicted strain ratios as a function of perturbations.
The strain ratios represented an equivalent metabolic burden between
the trading partners even though the exchanged metabolites varied
substantially in flux magnitude (300+ fold), molecular weight (174
vs 88 g mol–1), enzyme requirements (8 specialized
enzymes vs none), and biological energy density (27.5 vs 9 ATP molecule–1). The strain ratios could be rationally altered up
to 20-fold using O2 availability and cellular phenotype.
The metabolic burden theory was applied to seven additional published
cocultures and was found to accurately predict trading partner ratios.
Quantifying exchanged metabolites on an ATP-equivalent basis provides
a theory for interpreting natural consortia and a toolbox for controlling
bioprocess consortia.