Low yield and abiotic origin of N 2 O formed by the complete nitrifier Nitrospira inopinata

Nitrous oxide (N O) 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 N O and NO globally. However, nothing is known about N O 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 N O, and emits N O at levels that are comparable to AOA but much lower than AOB. Furthermore, we demonstrate that N O formed by N. inopinata formed under varying oxygen regimes originates from abiotic conversion of hydroxylamine. Our findings indicate that comammox microbes may produce less N O during nitrification than AOB.