Exploring the Functions of Efficient Canonical Denitrifying Bacteria as N 2 O Sinks: Implications from 15 N Tracer and Transcriptome Analyses.

In denitrifying reactors, canonical complete denitrifying bacteria reduce nitrate (NO 3 - ) to nitrogen via N 2 O. However, they can also produce N 2 O under certain conditions. We used a 15 N tracer method, in which 15 N-labeled NO 3 - /nitrite (NO 2 - ) and nonlabeled N 2 O were simultaneously supplied with organic electron donors to five canonical complete denitrifying bacteria affiliated with either Clade I or Clade II nosZ . We calculated their NO 3 - , NO 2 - , and N 2 O consumption rates. The Clade II nosZ bacterium Azospira sp. strain I13 had the highest N 2 O consumption rate (3.47 ± 0.07 fmol/cell/h) and the second lowest NO 3 - consumption rate (0.20 ± 0.03 fmol/cell/h); hence, it is a N 2 O sink. A change from peptone- to acetate/citrate-based organic electron donors increased the NO 3 - consumption rate by 4.8 fold but barely affected the N 2 O consumption rate. Electron flow was directed to N 2 O rather than NO 3 - in Azospira sp. strain I13 and Az. oryzae strain PS only exerting a N 2 O sink but to NO 3 - in the Clade I nosZ N 2 O-reducing bacteria Pseudomonas stutzeri strain JCM 5965 and Alicycliphilus denitrificans strain I51. Transcriptome analyses revealed that the genotype could not fully describe the phenotype. The results show that N 2 O production and consumption differ among canonical denitrifying bacteria and will be useful for developing N 2 O mitigation strategies.