The study investigates the role of Archaea, specifically Crenarchaeota, in marine nitrification. Crenarchaeota are the most abundant prokaryotes in the ocean, but their physiological roles in biogeochemical cycles are poorly understood. The authors enriched a Crenarchaeote from North Sea water and found that its abundance correlated with ammonium oxidation to nitrite, suggesting its ability to nitrify. A time series study in the North Sea revealed that the abundance of the archaeal ammonia mono-oxygenase alfa subunit (amoA) gene correlated with declining ammonium concentrations and increased Crenarchaeota abundance. The archaeal amoA abundance was significantly higher than that of bacterial nitrifiers, which are typically responsible for ammonium oxidation in marine environments. Similar findings were observed in the Atlantic Ocean, where crenarchaeotal amoA copy numbers were 1–3 orders of magnitude higher than bacterial amoA. These results indicate that Crenarchaeota play a major role in oceanic nitrification, potentially contributing more to this process than previously recognized bacterial nitrifiers.The study investigates the role of Archaea, specifically Crenarchaeota, in marine nitrification. Crenarchaeota are the most abundant prokaryotes in the ocean, but their physiological roles in biogeochemical cycles are poorly understood. The authors enriched a Crenarchaeote from North Sea water and found that its abundance correlated with ammonium oxidation to nitrite, suggesting its ability to nitrify. A time series study in the North Sea revealed that the abundance of the archaeal ammonia mono-oxygenase alfa subunit (amoA) gene correlated with declining ammonium concentrations and increased Crenarchaeota abundance. The archaeal amoA abundance was significantly higher than that of bacterial nitrifiers, which are typically responsible for ammonium oxidation in marine environments. Similar findings were observed in the Atlantic Ocean, where crenarchaeotal amoA copy numbers were 1–3 orders of magnitude higher than bacterial amoA. These results indicate that Crenarchaeota play a major role in oceanic nitrification, potentially contributing more to this process than previously recognized bacterial nitrifiers.