Nitrite Control over Dissimilatory Nitrate/Nitrite Reduction Pathways in Shewanella loihica Strain PV-4

Shewanella loihica strain PV-4 harbors both a functional denitrification (NO3--> N-2) and a respiratory ammonification (NO3--> NH4+) pathway. Batch and chemostat experiments revealed that NO2- affects pathway selection and the formation of reduced products. Strain PV-4 cells grown with NO2- as the sole electron acceptor produced exclusively NH4+. With NO3- as the electron acceptor, denitrification predominated and N2O accounted for similar to 90% of reduced products in the presence of acetylene. Chemostat experiments demonstrated that the NO2-:NO3- ratio affected the distribution of reduced products, and respiratory ammonification dominated at high NO2-:NO3- ratios, whereas low NO2-:NO3- ratios favored denitrification. The NO2-:NO3- ratios affected nirK transcript abundance, a measure of denitrification activity, in the chemostat experiments, and cells grown at a NO2-:NO3- ratio of 3 had similar to 37-fold fewer nirK transcripts per cell than cells grown with NO3- as the sole electron acceptor. In contrast, the transcription of nrfA, implicated in NO2--to-NH4- reduction, remained statistically unchanged under continuous cultivation conditions at NO2-: NO3- ratios below 3. At NO2-:NO3- ratios above 3, both nirK and nrfA transcript numbers decreased and the chemostat culture washed out, presumably due to NO2- toxicity. These findings implicate NO2- as a relevant modulator of NO3- fate in S. loihica strain PV-4, and, by extension, suggest that NO2- is a relevant determinant for N retention (i.e., ammonification) versus N loss and greenhouse gas emission (i.e., denitrification).