pH control enables simultaneous reduction of nitrogen loss and $N_2O$ emission in cultures of Shewanella loihica strain PV-4

Abstract

Denitrification causes loss of nitrogen from agricultural soils and is one of the most prominent sources of greenhouse gas ($N_2O$). pH control is key to these problems, as it is known as one of the most influential environmental parameters that determine both the fate of $NO_3^{-}$ in anoxic environments and the extent of $N_2O$ emission. In this study, Shewanella loihica strain PV-4, a proteobacterial strain capable of both denitrification and respiratory ammonification, was examined to demonstrate that simultaneous reduction of N-loss and $N_2O$ emission may be possible through pH control. Strain PV-4 was incubated at varying pH with lactate as the electron donor and $NO_3^{-}$ and $N_2O$ as the $e^-$ acceptors. At pH 6.0, transient accumulation of $N_2O$ was observed while no $NH_4^{+}$ was produced. At pH 7.0 and 8.0, strain PV-4 served as a $N_2O$ sink, as $N_2O$ concentration decreased consistently without accumulation. Respiratory ammonification activity was up-regulated and 9.73 and 83.51 $\mu$moles $NH_4^{+}$ were recovered from reduction of $NO_3^{-}$ at pH 7.0 and pH 8.0, respectively. The results from transcription analyses using reverse transcription polymerase chain reaction targeting nrfA and nirK supported that pH regulated denitrification and respiratory ammonification at transcription level. As nosZ transcript level was higher at pH 6.0 than at pH 7.0 and 8.0, $N_2O$ accumulation at the low pH condition was not attributed to transcriptional regulation. These results with strain PV-4 suggest that $N_2O$ emission reduction and N-retention may be able to be simultaneously achieved by controlling pH of the environment.