Facilitation–competition tradeoffs structure microbial niches and nitrogen cycling

Abstract The marine nitrogen cycle is regulated by diverse microbial interactions, especially in oxygen minimum zones where denitrifiers and anammox bacteria drive fixed nitrogen loss. While competition for limiting resources is well studied, the combined effects of competition and facilitation remain poorly understood when microbes require multiple, non-substitutable resources such as carbon and nitrogen. Here we develop a trait-based consumer–resource framework to test how recipient populations—whose growth relies on the metabolic products of other groups— reshape the ecological niches of their feeders and competitors. Invasion analysis shows that recipients expand either their feeder’s or the feeder’s competitor’s niche, depending on whether they are superior competitors for resources limiting the feeder’s rivals or the feeder itself. These shifts alter biogeochemical outcomes: high organic matter to nitrate supply ratios favor nitrous oxide cycling over dinitrogen gas production. Moreover, anammox bacteria occupy a wider range of organic matter and nitrate supply regimes than nitrite-reducing denitrifiers, consistent with their more frequent detectability in diverse marine environments. Together, our results link microbial interaction networks to global biogeochemical fluxes, advancing trait-based ecological theory for multi-resource systems.