Nitrification, the conversion of ammonia via nitrite to nitrate, is carried out by specialized microorganisms called nitrifiers. This process is extremely important for the global biogeochemical nitrogen cycle in virtually all ecosystems, but it plays an ambivalent role in global change. On one hand, nitrification contributes to the emission of the potent greenhouse gas and ozone-depleting substance nitrous oxide and leads to massive fertilizer losses in agriculture, resulting in the eutrophication of water bodies.  On the other hand, nitrification is indispensable as a biological purification step for nutrient removal in wastewater treatment plants, thus protecting water bodies from excessive nitrogen input from wastewater. The discovery of comammox (complete ammonia oxidizers) microbes in 2015 (Daims et al. 2015, van Kessel et al. 2015 both in Nature), has revolutionized our understanding of nitrification, a crucial process in the global nitrogen cycle. Unlike traditional nitrifiers that either oxidize ammonia to nitrite or nitrite to nitrate, comammox bacteria such as Nitrospira inopinata can perform both steps of nitrification in a single cell. Due to their high substrate affinity and growth yield comammox microbes are well adapted to oligotrophic environments and to growth in biofilms (Kits et al. 2017 Nature). Interestingly, comammox microbes produce nitrous oxide (N₂O) at lower levels compared to ammonia-oxidizing bacteria and can thus be considered as green nitrifiers (Kits et al. 2019 Nature Communications). Our recent finding that comammox microbes can utilize guanidine as a sole source of energy, reductant, and nitrogen, offers new opportunities for targeted cultivation of these recalcitrant microbes as well as exciting options to reduce agricultural nitrous oxide emissions (Palatinszky et al. 2024 Nature).