Contrasting Oxygen Sensitivities of Ammonia Oxidation and Nitrous Oxide Production in Estuarine Waters

The ocean is losing its oxygen, with hypoxia frequently observed in estuarine and coastal waters. Oxygen concentration changes affect both marine animals and microbe-mediated biogeochemical cycles, such as nitrogen cycling. Understanding the oxygen sensitivity of nitrogen cycling processes is critical to evaluating changes in nitrogen speciation and availability, which regulate marine primary production. Ammonia oxidation transforms ammonia into nitrite, supplying a substrate for nitrogen removal processes, including denitrification and anammox. Ammonia oxidation is also the major source of nitrous oxide (N₂O), a potent greenhouse gas and ozone-depleting substance, in oxygenated parts of the ocean, but observations of its oxygen sensitivity are limited, particularly for estuarine and coastal environments. Here, we showed a strong dependence of ammonia oxidation on oxygen, with an average half-saturation constant of 6.88 ± 4.22 μM O₂ in the seasonally hypoxic waters of Chesapeake Bay, one of the largest estuaries in the world. N₂O production from ammonia oxidation peaked at an average oxygen concentration of 2.68 ± 2.33 μM O₂. Compilation and meta-analysis of previous studies identified spatial differences and constrained the oxygen sensitivity of ammonia oxidation and N₂O production. The overlap in the oxygen affinities of ammonia oxidation and denitrification suggests that the supply of nitrite by ammonia oxidation facilitates nitrogen loss, and both processes contribute to N₂O hotspots under low oxygen conditions.