In aquatic ecosystems, the production of nitrous oxide (N2O), an important greenhouse gas, was until recently mostly ascribed to the activity of nitrifiers and denitrifiers. Recent research has revealed that a range of other organisms are also capable of producing N2O, but quantifying their contribution has remained challenging. Plouviez et al. present a novel approach based on the stable nitrogen and oxygen isotope composition of N2O, combined with the intramolecular distribution of 15N ("site preference"). They found that various microalgae grown in darkness present distinct stable isotope patterns, which paves the way for improving our ability to distinguish the contribution of different microbial groups to N2O production in aquatic ecosystems.