Fe(III)-mediated anaerobic ammonium oxidation: A novel microbial nitrogen cycle pathway and potential applications

Ammonium (NH4+) oxidation is crucial for nitrogen (N) removal, contributing to regional and global N cycles, but is regarded as limited to a few biological pathways. A novel pathway for NH4+ oxidation and the N cycle is provided in the microbial anaerobic NH4+ oxidation coupled with Fe(III) (ferric iron) reduction, called ferric ammonium oxidation (Feammox). Over the past few years, Feammox, which results in significant loss of N in natural environments, has been detected widely in both terrestrial and aquatic ecosystems. Researchers have revealed various Feammox pathways, end products of nitrate (NO3-), nitrite (NO2-), and gaseous nitrogen (N2), and the interactions within the nitrogen and iron-cycle-related microbial communities, which might offer some novel alternative processes for wastewater treatment. However, there are substantial variations among different studies in terms of the key functional microorganisms. The underlying mechanisms of Feammox, as well as the effect of environmental factors, remain poorly understood. In this review of the emerging process, we detail the end-products and microbes involved in the Feammox process and discuss possible mechanisms and the main influential factors. In particular, we assess the potential applications in wastewater treatment based on previous experimental studies and highlight knowledge gaps and future research opportunities.