Oxygen production via NO dismutation in different ammonia oxidizers

Ammonia oxidizing archaea (AOA) are widespread and highly abundant in nature. Despite their typical aerobic metabolism, they can be abundant in ecosystems where oxygen is scarce. Recent observations revealed that the AOA isolate Nitrosopumilus maritimus produces oxygen and dinitrogen at nanomolar concentrations, upon oxygen depletion through nitric oxide (NO) dismutation. Here, we explore NO dismutation capability in other ammonia oxidizers with different phylogenetic affinities and from different environmental settings. The organisms explored include three marine AOA, one soil AOA and two soil ammonia-oxidizing bacteria (AOB). Upon oxygen depletion all isolates accumulated oxygen. In incubations with 15 N tracers with ongoing oxygen accumulation, the AOA strains Nitrosopumilus adriaticus and Nitrosopumilus viennensis produced 46 N 2 O from nitrite. Transient 46 N 2 O accumulation followed by 30 N 2 production was detected in the AOA strains Nitrosopumilus piranensis and Nitrosopumilus sp. CCS1, supporting the earlier observation that NO-dismutation is a common metabolism in AOA, albeit with physiological variations between different strains. An important physiological variable is the capability to reduce N 2 O to N 2 . The finding of oxygen production in several AOA, as well as AOB, indicates that this process is widely distributed among the tree of life and adds an explanation for their abundance in oxygen-depleted environments.