As a side effect of their metabolism, microorganisms living on methane can also convert ammonia. In the process, they produce nitric oxide (NO), a central molecule in the global nitrogen cycle. Scientists from the Max Planck Institute for Marine Microbiology, Bremen (DE), and Radboud University, Nijmegen (NL), have now discovered the enzyme that produces NO, closing an important gap in our understanding of how methanotrophs deal with rising environmental ammonia concentrations.