Stoichiometric analysis of microbial communities links function, structure, and biomass carrying capacity.

Microbial communities carry out important ecological functions. Their activities emerge from interactions between species, often potentiated by metabolic traits. We lack a quantitative understanding of how these traits shape community properties. Here, we present theory for microbial communities, leveraging concepts from quantitative microbial physiology. We focus on how steady-state metabolic exchanges between species determine their fractional abundances, given their biomass and byproduct yields on nutrients. We start by deriving formal conditions for the steady states of communities of microbes that grow, die, and cross-feed metabolites. We describe the metabolic stoichiometry of nutrient uptake and the formation of biomass and byproducts for each species in terms of charge- and chemical-element balanced reactions (macrochemical reactions). Byproducts function as nutrients for other species. Next, we express the relative abundances of species (living and dead), the net metabolic conversion of a community, and the biomass carrying capacity in terms of the metabolic stoichiometry, growth rates and death rates of the species. We show how niche creation can emerge from stoichiometric imbalances in cross-feeding communities. Finally, we discuss how relative species abundances depend on the Adenosine Triphosphate (ATP) stoichiometries of intracellular metabolism.