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