The pH dependency of N‐converting enzymatic processes, pathways and microbes: effect on net N2O production

• Published in: Environmental microbiology Vol. 20; no. 5; pp. 1623 - 1640
• Main Authors: Blum, Jan‐Michael, Su, Qingxian, Ma, Yunjie, Valverde‐Pérez, Borja, Domingo‐Félez, Carlos, Jensen, Marlene Mark, Smets, Barth F.
• Format: Journal Article
• Language: English
• Published: Oxford Wiley Subscription Services, Inc 01-05-2018
• Subjects: Active sites; Amino acids; Ammonia; Bacteria; Catalysis; Cofactors; Consumption; Denitrification; Electron transfer; Enzymes; Feasibility studies; Ligands; Microorganisms; Nitrogen; Nitrous oxide; Oxidation; Oxidoreductions; pH effects; Protonation; Structural members; Water engineering; Water treatment
• ISSN: 1462-2912; 1462-2920
• DOI: 10.1111/1462-2920.14063

Summary
Nitrous oxide (N2O) is emitted during microbiological nitrogen (N) conversion processes, when N2O production exceeds N2O consumption. The magnitude of N2O production vs. consumption varies with pH and controlling net N2O production might be feasible by choice of system pH. This article reviews how pH affects enzymes, pathways and microorganisms that are involved in N‐conversions in water engineering applications. At a molecular level, pH affects activity of cofactors and structural elements of relevant enzymes by protonation or deprotonation of amino acid residues or solvent ligands, thus causing steric changes in catalytic sites or proton/electron transfer routes that alter the enzymes' overall activity. Augmenting molecular information with, e.g., nitritation or denitrification rates yields explanations of changes in net N2O production with pH. Ammonia oxidizing bacteria are of highest relevance for N2O production, while heterotrophic denitrifiers are relevant for N2O consumption at pH > 7.5. Net N2O production in N‐cycling water engineering systems is predicted to display a ‘bell‐shaped’ curve in the range of pH 6.0–9.0 with a maximum at pH 7.0–7.5. Net N2O production at acidic pH is dominated by N2O production, whereas N2O consumption can outweigh production at alkaline pH. Thus, pH 8.0 may be a favourable pH set‐point for water treatment applications regarding net N2O production.