pH Control Enables Simultaneous Enhancement of Nitrogen Retention and NO Reduction in Strain PV-4.

pH has been recognized as one of the key environmental parameters with significant impacts on the nitrogen cycle in the environment. In this study, the effects of pH on NO/NO fate and NO emission were examined with strain PV-4, an organism with complete denitrification and respiratory ammonification pathways. Strain PV-4 was incubated at varying pH with lactate as the electron donor and NO/NO and NO as the electron acceptors. When incubated with NO and NO at pH 6.0, transient accumulation of NO was observed and no significant NH production was observed. At pH 7.0 and 8.0, strain PV-4 served as a NO sink, as NO concentration decreased consistently without accumulation. Respiratory ammonification was upregulated in the experiments performed at these higher pH values. When NO was used in place of NO, neither growth nor NO reduction was observed at pH 6.0. NH was the exclusive product from NO reduction at both pH 7.0 and 8.0 and neither production nor consumption of NO was observed, suggesting that NO regulation superseded pH effects on the nitrogen-oxide dissimilation reactions. When NO was the electron acceptor, transcription was significantly upregulated upon cultivation at pH 6.0, while transcription was significantly upregulated at pH 8.0. The highest level of transcription was observed at pH 6.0 and the lowest at pH 8.0. With NO as the electron acceptor, transcription profiles of , and were statistically indistinguishable between pH 7.0 and 8.0. The transcriptions of and were severely downregulated regardless of pH. These observations suggested that the kinetic imbalance between NO production and consumption, but neither decrease in expression nor activity of NosZ, was the major cause of NO accumulation at pH 6.0. The findings also suggest that simultaneous enhancement of nitrogen retention and NO emission reduction may be feasible through pH modulation, but only in environments where C:N or NO:NO ratio does not exhibit overarching control over the NO/NO reduction pathways.