The Xiamen Symposium on Marine Environmental Sciences (XMAS 2027)

Nitrification, comprising ammonia and nitrite oxidation (AO and NO), links marine reduced and oxidized nitrogen pools, thereby influencing productivity and phytoplankton communities. How these two steps jointly respond to ocean warming remains unclear, largely due to limited understanding of NO. Here we combine temperature-manipulation experiments with a prediction framework to compare warming responses of AO and NO across broad environmental gradients. Despite similar temperature-dependent rate patterns, AO and NO exhibit distinct kinetic thermal sensitivities. The thermal sensitivities of substrate affinity and oxidation capacity in NO depend strongly on substrate availability, whereas AO shows weaker coupling to resource conditions. The framework indicates greater thermal tolerance in NO than in AO, especially in coastal regions relative to offshore waters. Model-based extrapolations show that AO is more strongly suppressed in low latitudes and less enhanced at high latitudes than NO. These asymmetric warming responses are predicted to favor imbalanced nitrification, shifting upper-ocean nitrogen partitioning with implications for nutrient availability and ecosystem function under climate change.