Low yield and abiotic origin of N2O formed by the complete nitrifier Nitrospira inopinata

Nitrous oxide (N₂O) and nitric oxide (NO) are atmospheric trace gases that contribute to climate change and affect stratospheric and ground-level ozone concentrations. Ammonia oxidizing bacteria (AOB) and archaea (AOA) are key players in the nitrogen cycle and major producers of N₂O and NO globally. However, nothing is known about N₂O and NO production by the recently discovered and widely distributed complete ammonia oxidizers (comammox). Here, we show that the comammox bacterium Nitrospira inopinata is sensitive to inhibition by an NO scavenger, cannot denitrify to N₂O, and emits N₂O at levels that are comparable to AOA but much lower than AOB. Furthermore, we demonstrate that N₂O formed by N. inopinata formed under varying oxygen regimes originates from abiotic conversion of hydroxylamine. Our findings indicate that comammox microbes may produce less N₂O during nitrification than AOB.