Dependency-competition tradeoffs structure microbial niches and nitrogen cycling.

The marine nitrogen cycle is regulated by ecological interactions among diverse microbial populations. In anoxic zones, populations carrying out anaerobic metabolisms, mainly multi-step denitrification and anammox, drive the loss of bioavailable nitrogen, some of which is emitted as the potent greenhouse gas nitrous oxide ([Formula: see text]). While competition for limiting resources is well studied, the combined effects of competition and dependencies, where a "feeder" population supplies a required resource to a "recipient," remain poorly understood. Here, we develop a trait-based consumer-resource framework to test how recipient populations reshape the ecological niches of their feeders and competitors. Our analysis demonstrates how recipients may expand either their feeder's or their feeder's competitor's niche, depending on relative competitive abilities on limiting resources. We analyze and identify equilibrium co-existence regions, threshold regimes, and the dominant pathways of nitrogen loss as a function of varying both organic matter (OM) and nitrate supply, rather than just their ratio. Examining this 2D supply space identifies a distinct zone where OM and nitrate co-limitation results in [Formula: see text] production but not consumption, and thus an ecological niche for [Formula: see text] accumulation. Additionally, the model suggests that anammox bacteria occupy a wider range of OM and nitrate supply regimes than denitrifying populations, consistent with their more frequent detection across diverse marine environments. The results link microbial interaction networks to biogeochemical fluxes relevant at global scales and extend ecological theory to multi-resource systems with nested competitive and dependent interactions.