Anaerobic ammonium oxidation (anammox) bacteria contribute significantly to the global nitrogen cycle and play a major role in sustainable wastewater treatment. Anammox bacteria convert ammonium (NH₄⁺) to dinitrogen gas (N₂) using intracellular electron acceptors such as nitrite (NO₂⁻) or nitric oxide (NO). However, it is still unknown whether anammox bacteria have extracellular electron transfer (EET) capability with transfer of electrons to insoluble extracellular electron acceptors. Here we show that freshwater and marine anammox bacteria couple the oxidation of NH₄⁺ with transfer of electrons to insoluble extracellular electron acceptors such as graphene oxide or electrodes in microbial electrolysis cells. ¹⁵N-labeling experiments revealed that NH₄⁺ was oxidized to N₂ via hydroxylamine (NH₂OH) as intermediate, and comparative transcriptomics analysis revealed an alternative pathway for NH₄⁺ oxidation with electrode as electron acceptor. Complete NH₄⁺ oxidation to N₂ without accumulation of NO₂⁻ and NO₃⁻ was achieved in EET-dependent anammox. These findings are promising in the context of implementing EET-dependent anammox process for energy-efficient treatment of nitrogen.

Microbial electrolysis cell (MEC) is an anaerobic biological process for the conversion of organics in wastewater into renewable energy in the form of hydrogen or methane. However, MEC cannot be used as a standalone technology for urban wastewater treatment, and post-treatment or integrated processes are required to meet water reuse and discharge limits. Recent advances in material science and the discovery of new microorganisms capable of extracellular electron transfer to the electrodes have widened the integration opportunities of MEC in mainstream and side-stream urban wastewater treatment. This review addresses recent developments in the integration of MEC with other processes such as membrane filtration, anaerobic ammonium oxidation and anaerobic digestion, as well as discusses current challenges and new integration opportunities.