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

摘要

Shewanella loihica strain PV-4 harbors both a functional denitrification (NO3⁻→N2) 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 ∼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 ∼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).