Ammonia is oxidised to nitrite via hydroxylamine and nitric oxide intermediates, but there are major gaps in our understanding of the enzymology of the ammonia oxidation pathway. In ammonia oxidising bacteria, hydroxylamine is oxidised to nitric oxide via hydroxylamine dehydrogenase (also known as hydroxylamine oxidoreductase) but this enzyme is not present in ammonia oxidising archaea. This suggests that archaea must have evolved novel enzymes for oxidation of hydroxylamine and nitric oxide.

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Candidate genes encoding for these novel archaeal enzymes have been previously postulated, but not yet confirmed. Multiple different ammonia oxidation pathways have also been proposed in archaea, but likewise, the order and the reactions of the pathway remain unknown.

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We want address the questions:

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• Which enzyme is oxidising hydroxylamine in archaea?

• What is the archaeal ammonia oxidation pathway?