National Academy of Sciences, Proceedings of the National Academy of Sciences, 3(110), p. 1006-1011, 2012
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The ammonia-oxidizing archaea have recently been recognized as a significant component of many microbial communities in the biosphere. Although the overall stoichiometry of archaeal chemoautotrophic growth via ammonia (NH 3 ) oxidation to nitrite (NO 2 − ) is superficially similar to the ammonia-oxidizing bacteria, genome sequence analyses point to a completely unique biochemistry. The only genomic signature linking the bacterial and archaeal biochemistries of NH 3 oxidation is a highly divergent homolog of the ammonia monooxygenase (AMO). Although the presumptive product of the putative AMO is hydroxylamine (NH 2 OH), the absence of genes encoding a recognizable ammonia-oxidizing bacteria-like hydroxylamine oxidoreductase complex necessitates either a novel enzyme for the oxidation of NH 2 OH or an initial oxidation product other than NH 2 OH. We now show through combined physiological and stable isotope tracer analyses that NH 2 OH is both produced and consumed during the oxidation of NH 3 to NO 2 − by Nitrosopumilus maritimus , that consumption is coupled to energy conversion, and that NH 2 OH is the most probable product of the archaeal AMO homolog. Thus, despite their deep phylogenetic divergence, initial oxidation of NH 3 by bacteria and archaea appears mechanistically similar. They however diverge biochemically at the point of oxidation of NH 2 OH, the archaea possibly catalyzing NH 2 OH oxidation using a novel enzyme complex.