Lisa Y. Stein, Martin G. Klotz

Nitrous oxide, a potent greenhouse gas and ozone-depleting molecule, continues to accumulate in the atmosphere as a product of anthropogenic activities and land-use change. Nitrogen oxides are intermediates of nitrification and denitrification and are released as terminal products under conditions such as high nitrogen load and low oxygen tension among other factors. The rapid completion and public availability of microbial genome sequences has revealed a high level of enzymatic redundancy in pathways terminating in nitrogen oxide metabolites, with few enzymes involved in returning nitrogen oxides to dinitrogen. The aerobic methanotrophic bacteria are particularly useful for discovering and analysing diverse mechanisms for nitrogen oxide production, as these microbes both nitrify (oxidize ammonia to nitrite) and denitrify (reduce nitrate/nitrite to nitrous oxide via nitric oxide), and yet do not rely on these pathways for growth. The fact that methanotrophs have a rich inventory for nitrogen oxide metabolism is, in part, a consequence of their evolutionary relatedness to ammonia-oxidizing bacteria. Furthermore, the ability of individual methanotrophic taxa to resist toxic intermediates of nitrogen metabolism affects the relative abundance of nitrogen oxides released into the environment, the composition of their community, and the balance between nitrogen and methane cycling.