Nitrogen removal mechanisms of marine anammox consortia induced by zero-valent iron in high-salinity and low-temperature wastewater treatment: Nitrate or nitrite makes differences.

Nitrogen removal mechanisms of marine anammox bacteria (MAB) induced by zero-valent iron (ZVI) were investigated to reveal the differences resulting from nitrate or nitrite under high-salinity (3.5 %) and low-temperature (15 ± 1 °C) stress. The one-time addition of ZVI (20 g) altered the microbial energy metabolism and electron transfer pathways. When nitrate served as the electron acceptor, ZVI-based autotrophic denitrification (ZVI-AD) served as the "engine", providing essential nitrite for MAB through partial nitrate reduction, achieving a total nitrogen removal efficiency (TNRE) of 31.5 %. Conversely, when nitrite was the electron acceptor, marine anammox dominated nitrogen removal, while ZVI-AD acted as a recycler of the anammox by-product (nitrate), establishing a self-coupled nitrogen cycle with a superior TNRE of 94.0 %. Sulfurimonas and Candidatus Scalindua were the core functional microbes, exhibiting different preferences for nitrate and nitrite. Compared to the nitrite-added reactor, Sulfurimonas was enriched more in the nitrate-added reactor. Nevertheless, ZVI significantly promoted the enrichment of Candidatus Scalindua in the nitrite-added reactor, with its relative abundance increasing to 31.3 %. Furthermore, Feammox may have contributed to ammonium removal during the later stage. Overall, the selection of electron acceptors regulated the nitrogen removal mechanisms of ZVI-induced MAB consortia, providing new insights into intensifying nitrogen removal in high-salinity and low-temperature wastewater treatment.