The nitrate‐ammonifying and nosZ ‐carrying bacterium B acillus vireti is a potent source and sink for nitric and nitrous oxide under high nitrate conditions

Summary Several G ram‐positive bacteria carry genes for anaerobic reduction of NO 3 − via NO 2 − to NH 4 + or gaseous nitrogen compounds, but the processes are understudied for these organisms. Here, we present results from a whole‐genome analysis of the soil bacterium B acillus vireti and a phenotypic characterization of intermediate and end‐products, formed under anoxic conditions in the presence of NO 3 − . B acillus vireti has a versatile metabolism. It produces acetate, formate, succinate and lactate from fermentation and performs dissimilatory nitrate reduction via NO 2 − to ammonium ( DNRA ) using NrfA , while NirB may detoxify NO 2 − in the cytoplasm. Moreover, it produces NO from an unknown source and reduces it via N 2 O to N 2 using two enzymes connected to denitrification: an unusual NO reductase, qCu A Nor encoded by cbaA , and a z‐type N 2 O reductase, encoded by nosZ . In batch cultures, B . vireti reduced all NO 3 − to NO 2 − before the NO 2 − was reduced further. The quantities of all products varied with the initial NO 3 − concentration. With 5 m M NO 3 − , 90% was reduced to NH 4 + while with ≥ 20 m M NO 3 − , 50% was reduced to NO , N 2 O and N 2 . This organism is thus an aggressive NO 2 − accumulator and may act as a net source and sink of NO and N 2 O .