Substrate regulation leads to differential responses of microbial ammonia-oxidizing communities to ocean warming.

In the context of continuously increasing anthropogenic nitrogen inputs, knowledge of how ammonia oxidation (AO) in the ocean responds to warming is crucial to predicting future changes in marine nitrogen biogeochemistry. Here, we show divergent thermal response patterns for marine AO across a wide onshore/offshore trophic gradient. We find ammonia oxidizer community and ambient substrate co-regulate optimum temperatures (Topt), generating distinct thermal response patterns with Topt varying from ≤14 °C to ≥34 °C. Substrate addition elevates Topt when ambient substrate is unsaturated. The thermal sensitivity of kinetic parameters allows us to predict responses of both AO rate and Topt at varying substrate and temperature below the critical temperature. A warming ocean promotes nearshore AO, while suppressing offshore AO. Our findings reconcile field inconsistencies of temperature effects on AO, suggesting that predictive biogeochemical models need to include such differential warming mechanisms on this key nitrogen cycle process.