Nitrous oxide production and consumption by marine ammonia-oxidizing archaea under oxygen depletion.

Ammonia-oxidizing archaea (AOA) are key players in the nitrogen cycle and among the most abundant microorganisms in the ocean, thriving even in oxygen-depleted ecosystems. AOA produce the greenhouse gas nitrous oxide (NO) as a byproduct of ammonia oxidation. Additionally, the recent discovery of a nitric oxide dismutation pathway in the AOA isolate points toward other NO production and consumption pathways in AOA. AOA that perform NO dismutation when exposed to oxygen depletion, produce oxygen and dinitrogen as final products. Based on the transient accumulation of NO coupled with oxygen accumulation, NO has been proposed as an intermediate in this novel archaeal pathway. In this study, we spiked NO to oxygen-depleted incubations with pure cultures of two marine AOA isolates that were performing NO dismutation. By using combinations of N compounds with different isotopic signatures (NO pool +NO spike and NO pool +NO spike), we evaluated the NO spike effects on the production of oxygen and the isotopic signature of N and NO. The experiments confirmed that NO is an intermediate in NO dismutation by AOA, distinguishing it from similar pathways in other microbial clades. Furthermore, we showed that AOA rapidly reduce high concentrations of spiked NO to N. These findings advance our understanding of microbial NO production and consumption in oxygen-depleted settings and highlight AOA as potentially important key players in NO turnover.