Archaeal nitrification in the ocean has been demonstrated through experimental evidence showing that marine Crenarchaeota can oxidize ammonium to nitrite, a key step in the nitrogen cycle. This study isolated and enriched a crenarchaeote from North Sea water, revealing its ability to perform nitrification. The abundance of the archaeal ammonia monooxygenase (amoA) gene was found to correlate with ammonium oxidation and the presence of Crenarchaeota, which were 1–2 orders of magnitude more abundant than bacterial nitrifiers. Analysis of Atlantic waters showed similar results, with archaeal amoA copy numbers significantly higher than bacterial ones. These findings suggest that Crenarchaeota play a major role in oceanic nitrification. Marine Crenarchaeota are abundant in the ocean, particularly in deep neritic and mesopelagic zones, and are thought to account for about 20% of all prokaryotic cells. Their metabolism is debated, but recent isolation of a crenarchaeote, Candidatus "Nitrosopumilus maritimus," showed it to be autotrophic and capable of nitrification. This organism, along with other crenarchaeota, may be involved in the marine nitrogen cycle, possibly as nitrifiers. The study also found that Crenarchaeota in the North Sea were responsible for significant ammonium oxidation, with their amoA abundance correlating with changes in nutrient concentrations. The importance of archaeal nitrification in the open ocean is highlighted by the distribution of Crenarchaeota across a wide depth range, with high cell numbers in the photic zone and dominance in deeper waters. Nitrate profiles in the ocean suggest that nitrification occurs through various processes, including ammonium oxidation by Crenarchaeota. The study's data, combined with literature, suggest that archaeal nitrification may be a significant component of the global nitrogen cycle, though the extent of this role remains uncertain. The study also found that Crenarchaeota in the mesopelagic layer of the open ocean may play a more important role in nitrification than bacterial nitrifiers. These findings, along with the importance of Planctomycetes in denitrification and unicellular cyanobacteria in dinitrogen fixation, highlight the significant role of previously unrecognized prokaryotes in oceanic nitrogen cycling. The study provides experimental evidence that Crenarchaeota are capable of nitrification, contributing to the marine nitrogen cycle.Archaeal nitrification in the ocean has been demonstrated through experimental evidence showing that marine Crenarchaeota can oxidize ammonium to nitrite, a key step in the nitrogen cycle. This study isolated and enriched a crenarchaeote from North Sea water, revealing its ability to perform nitrification. The abundance of the archaeal ammonia monooxygenase (amoA) gene was found to correlate with ammonium oxidation and the presence of Crenarchaeota, which were 1–2 orders of magnitude more abundant than bacterial nitrifiers. Analysis of Atlantic waters showed similar results, with archaeal amoA copy numbers significantly higher than bacterial ones. These findings suggest that Crenarchaeota play a major role in oceanic nitrification. Marine Crenarchaeota are abundant in the ocean, particularly in deep neritic and mesopelagic zones, and are thought to account for about 20% of all prokaryotic cells. Their metabolism is debated, but recent isolation of a crenarchaeote, Candidatus "Nitrosopumilus maritimus," showed it to be autotrophic and capable of nitrification. This organism, along with other crenarchaeota, may be involved in the marine nitrogen cycle, possibly as nitrifiers. The study also found that Crenarchaeota in the North Sea were responsible for significant ammonium oxidation, with their amoA abundance correlating with changes in nutrient concentrations. The importance of archaeal nitrification in the open ocean is highlighted by the distribution of Crenarchaeota across a wide depth range, with high cell numbers in the photic zone and dominance in deeper waters. Nitrate profiles in the ocean suggest that nitrification occurs through various processes, including ammonium oxidation by Crenarchaeota. The study's data, combined with literature, suggest that archaeal nitrification may be a significant component of the global nitrogen cycle, though the extent of this role remains uncertain. The study also found that Crenarchaeota in the mesopelagic layer of the open ocean may play a more important role in nitrification than bacterial nitrifiers. These findings, along with the importance of Planctomycetes in denitrification and unicellular cyanobacteria in dinitrogen fixation, highlight the significant role of previously unrecognized prokaryotes in oceanic nitrogen cycling. The study provides experimental evidence that Crenarchaeota are capable of nitrification, contributing to the marine nitrogen cycle.