Anaerobic ammonium oxidation (anammox) is widely viewed as a high-efficiency, low-carbon route for nitrogen removal, yet many mechanistic details have remained unresolved—especially how the process proceeds when nitrite is absent and when the anammox cells lack a complete cytochrome c conduit. In a new study, Zheng and colleagues report that anammox can still oxidize ammonium through extracellular electron transfer (EET) supported by redox-active compounds embedded in extracellular polymeric substances.
The work proposes a cooperative division of labor among multiple bacterial partners. γ-Proteobacteria identified as Zeimonas sp. and Actinobacteria related to Candidatus ATN2 were found to produce quinones, while Planctomycetes represented by Candidatus CAADGN01 were enriched for phenazine synthesis. These two classes of molecules function as electron shuttles, enabling electrons generated at the cell exterior to move through the biofilm matrix toward anammox metabolism.
Central to the coupling is the ability of the anammox bacteria to connect ammonium oxidation with EET under nitrite-free conditions. The authors describe coordinated upregulation of hydroxylamine oxidase and additional oxidoreductases, alongside increases in intracellular multi-haem cytochrome c components. Even without a continuous cytochrome c pathway, the redox mediators appear to bridge the functional gap between extracellular electron sources and intracellular electron handling.
This metabolic arrangement is not one-sided. In return for providing electrochemical support, the anammox organisms secrete vitamins and amino acids that promote the growth of their symbiotic partners. The results therefore frame EET not merely as an electron-hopping process, but as a metabolically reciprocal ecosystem interaction.
To assess whether such partnerships occur beyond controlled experiments, the team performed global metagenomic analysis of 7,412 environmental samples. They confirmed widespread co-occurrence of anammox bacteria with redox-mediator-producing symbionts, especially in artificial settings such as bioreactors and wastewater treatment facilities.
Overall, the study links redox mediator-based cooperation to a practical pathway for anammox-driven nitrogen removal via EET in the absence of nitrite. By suggesting that mediator-enhanced systems could be engineered for more robust wastewater performance, the findings point toward a feasible strategy for next-generation biological treatment.
Subject of Research: Anaerobic ammonium oxidation (anammox) via extracellular electron transfer (EET) using redox mediators
Article Title: Redox mediator-based bacterial cooperation for anammox extracellular electron transfer.
Article References: Zheng, R., Chen, B., Kong, L. et al. Redox mediator-based bacterial cooperation for anammox extracellular electron transfer. Nat Water (2026). https://doi.org/10.1038/s44221-026-00683-0
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s44221-026-00683-0
Keywords: anammox; extracellular electron transfer; redox mediators; quinones; phenazines; extracellular polymeric substances; metagenomics; wastewater treatment
