Facilitation-competition tradeoffs structure microbial niches and nitrogen cycling

The marine nitrogen cycle is regulated by ecological interactions among diverse microbial populations, especially in oxygen minimum zones where populations carrying out anaerobic metabolisms, mainly denitrification and anammox, drive fixed nitrogen loss. While competition for limiting resources is well studied, the combined effects of competition and facilitation, 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. Invasion analysis shows that recipients expand either their feeders or the feeders competitors niche, depending on the populations relative competitive abilities on limiting resources. In terms of biogeochemistry, ecological outcomes result in diverse N-loss pathways; specifically, when growth is limited by both organic matter and nitrate, we observe increased nitrous oxide production. Additionally, the model suggests that anammox bacteria occupy a wider range of organic matter 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 facilitative interactions.