Hydroxylamine as an intermediate in ammonia oxidation by globally abundant marine archaea

摘要

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 chemo-autotrophic 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_2OH), the absence of genes encoding a recognizable ammonia-oxidizing bacteria-like hydroxylamine oxidoreductase complex necessitates either a novel enzyme for the oxidation of NH_2OH or an initial oxidation product other than NH_2OH. We now show through combined physiological and stable isotope tracer analyses that NH_2OH 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_2OH is the most probable product of the archaeal AMO homolog. Thus, despite their deep phy-logenetic divergence, initial oxidation of NH_3 by bacteria and archaea appears mechanistically similar. They however diverge biochemically at the point of oxidation of NH_2OH, the archaea possibly catalyzing NH_2OH oxidation using a novel enzyme complex.