pH-Dependent Hydrogenotrophic Denitratation Based on Self-Alkalization

Abstract

Producing stable nitrite is a necessity for anaerobic ammonium oxidation (anammox) but remains a huge challenge. Here, we describe the design and operation of a hydrogenotrophic denitratation system that stably reduced >90% nitrate to nitrite under self-alkaline conditions of pH up to 10.80. Manually lowering the pH to a range of 9.00-10.00 dramatically decreased the nitrate-to-nitrite transformation ratio to <20%, showing a significant role of high pH in denitratation. Metagenomics combined with metatranscriptomics indicated that six microorganisms, including a <i>Thauera</i> member, dominated the community and encoded the various genes responsible for hydrogen oxidation and the complete denitrification process. During denitratation at high pH, transcription of periplasmic genes <i>napA</i>, <i>nirS</i>, and <i>nirK</i>, whose products perform nitrate and nitrite reduction, decreased sharply compared to that under neutral conditions, while <i>narG</i>, encoding a membrane-associated nitrate reductase, remained transcriptionally active, as were genes involved in intracellular proton homeostasis. Together with no reduction in only nitrite-amended samples, these results disproved the electron competition between reductions of nitrate and nitrite but highlighted a lack of protons outside cells constraining biological nitrite reduction. Overall, our study presents a stably efficient strategy for nitrite production and provides a major advance in the understanding of denitratation.