Oxygen production via nitric oxide dismutation in diverse ammonia oxidizers

Ammonia-oxidizing archaea (AOA) are widespread and highly abundant in nature. Despite their typical aerobic metabolism, they thrive in ecosystems where oxygen is scarce. Recently, the AOA Nitrosopumilus maritimus SCM1 was shown to produce oxygen and N₂O from nitrite upon oxygen depletion, with N₂O subsequently converted to dinitrogen (N₂) supporting nitric oxide (NO) dismutation as the proposed metabolic pathway. Here, we explore the ability of other ammonia oxidizers, with diverse phylogenetic affiliations and from different environmental settings to produce oxygen via NO-dismutation. We studied three marine AOA, one soil AOA, and two soil ammonia-oxidizing bacteria (AOB). Upon oxygen depletion, all strains accumulated oxygen. In incubations of the AOA strains with ¹⁵N tracers, two of them, Nitrosopumilus piranensis D3C and Nitrosopumilus adriaticus CCS1, showed transient ⁴⁶N₂O accumulation followed by linear ³⁰N₂ production, similar to SCM1, while Nitrosopumilus adriaticus NF5 and Nitrososphaera viennensis EN76 mainly produced ⁴⁶N₂O from nitrite without further N₂ accumulation. These findings indicate that oxygen production through NO-dismutation is a common metabolism in cultured representatives of AOA, albeit with physiological variations between different strains such as the reduction of N₂O to N₂. Comparative genomics did not reveal a distinct putative N₂O reductase in the AOA strains with linear N₂ production, suggesting that the observed physiological differences may not directly stem from gene inventory. The finding of oxygen production in several AOA, as well as AOB, suggests that dark oxygen production is a common trait among archaeal and bacterial nitrifiers and adds a potential explanation for their abundance in oxygen-depleted environments.