Abstract
Bioconversion of ammonium to dinitrogen (N2) is an essential process in the nitrogen cycle, primarily driven by O2-dependent nitrification and followed by O2-limited denitrification, involving multiple redox states of nitrogen (NH4+ → NH2OH → NO2- → NO3- → NO2- → NO→N2O → N2). Here, we discovered a new process termed acetone-mediated ammonium oxidation in Zobellella taiwanensis bacteria under both oxic and anoxic conditions, directly oxidizing ammonium to N2 (NH4+ + acetone → acetoxime → N2 + acetone). Acetone, produced from organic sources, couples with ammonium to form acetoxime in the presence of O2, NO2-, NO3-, or Fe(III). Subsequently, acetoxime is oxidized to N2, thereby releasing recyclable acetone. We purified two new enzymes (acetoxime synthase; acetoxime dehydrogenase) catalyzing this process and identified their corresponding genes. The widespread distribution of homologous amino acid sequences across thousands of prokaryotic and eukaryotic microorganisms suggests the potential ubiquity of this process in nature and its possible substantial contributions to the nitrogen cycle.
Keywords:
AMAO process; N2 production; Zobellella taiwanensis; ammonium oxidation.