Biological nitrite oxidation in the presence of MnO2 under anoxic conditions

Biological nitrogen transformation driven by manganese redox reactions in anaerobic environments is receiving increasing attention. In particular, microbially mediated Mn(IV) reduction coupled with ammonium oxidation (Mnammox) and nitrate-dependent manganese oxidation (NDMO) have been widely reported. In general nitrification, nitrite is a key intermediate between ammonia and nitrate; however, the fate of nitrite within Mn redox processes remains unclear. In this study, we attempted to enrich anoxic nitrite-oxidizing bacteria using an up-flow reactor and investigated whether nitrite can be biologically oxidized to nitrate using γ-MnO 2 as an electron acceptor. As the anoxic nitrite-oxidizing bacteria became successfully enriched, nitrite was consistently oxidized to nitrate while γ-MnO 2 was reduced to lower-valence Mn species. Interestingly, nitrite was also reduced rather than solely oxidized. Moreover, part of the reduced Mn species was re-oxidized into a new crystalline structure (β-MnO 2 ), instead of reverting to γ-MnO 2 . During this process, nitrite oxidation and reduction occurred simultaneously, and both Mn reduction and Mn oxidation proceeded within the reactor. Across two independently operated runs conducted at different times, two unclassified bacterial taxa, Blastocatellaceae OLB17 and Acidithiobacillaceae KCM-B-112, were consistently enriched. Although their metabolic pathways have not yet been identified, these microorganisms may play distinct roles in mediating nitrite oxidation and reduction under Mn redox cycling. • Anoxic nitrite was biologically oxidized to nitrate using γ-MnO 2 as the electron acceptor. • Nitrite oxidation and reduction occurred simultaneously under manganese redox cycling. • γ-MnO 2 was biologically transformed into β-MnO 2 through manganese redox cycling. • Blastocatellaceae OLB17 and Acidithiobacillaceae KCM-B-112 may play distinct roles in nitrite oxidation and/or reduction.