Anaerobic ammonium oxidation (anammox) plays a crucial role in efficient, low-carbon and sustainable nitrogen removal. Here we show that anammox bacteria, in cooperation with symbiotic bacteria and without a complete cytochrome c conduit, oxidize ammonium via extracellular electron transfer (EET) in the absence of nitrite through redox mediators embedded in extracellular polymeric substances. 污-Proteobacteria (Zeimonas sp.) and Actinobacteria (Candidatus ATN2) produce quinones and Planctomycetes (Candidatus CAADGN01) synthesize phenazines as electron shuttles. The anammox bacteria couple ammonium oxidation and EET by upregulating hydroxylamine oxidase, oxidoreductases and intracellular multi-haem cytochrome c. In return, they secrete essential vitamins and amino acids to support the growth of symbiotic bacteria. Global metagenomic analysis of 7,412 samples verified the widespread co-occurrence of anammox bacteria and redox mediator-producing symbiotic bacteria mainly in artificial ecosystems, such as bioreactors and wastewater treatment plants. This study reveals redox mediator-based cooperation for anammox with EET and the results imply a feasible strategy of redox mediator-enhanced EET in wastewater treatment.
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