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 15N tracers with ongoing oxygen accumulation, the AOA strains Nitrosopumilus adriaticus and Nitrosopumilus viennensis produced 46N2O from nitrite. Transient 46N2O accumulation followed by 30N2 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 N2O to N2. 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.