Accelerating mechanism of carbon dots-modified biocarrier on anammox process using metagenomic and metatranscriptomic techniques

• CDs-modified biocarrier improved nitrogen removal efficiency in anammox system. • CDs acted as an intracellular electron shuttle in iCDs-mediated anammox process. • CDs increased the abundances and expression of genes related with anammox. • CDs enhanced glycogen metabolism and the endogenous DNRA pathway of Ca. Brocadia . • CDs facilitated quorum sensing of Ca. Brocadia and its mutualism bacteria. The rates of anammox and endogenous dissimilatory nitrate reduction to ammonium (DNRA) under autotrophic conditions are usually low due to the slow metabolism rates of anammox bacteria. Carbon dots (CDs) as novel electron mediators can accelerate extracellular and intracellular electron transfer of bacteria. Herein, for the first time, the effect of CDs-modified biocarrier on anammox and endogenous DNRA processes in the absence of additional organic carbon sources was investigated. Results showed that the immobilized CDs (iCDs) could significantly promote the removal rates of NH 4 + -N (4.58%∼57.32%), NO 2 − -N (7.17%∼42.90%) and TN (7.53%∼58.48%) of the anammox reactor in the nitrogen loading rate range of 1.044∼1.74 kgN/m 3 /d. During this process, the iCDs could be slowly released and partly enter cells, resulting in the enhanced tightly bound extracellular polymeric substances conductivity and specific anammox activity of biofilm, especially, key enzymes activities of anammox process. Further metagenomic and metatranscriptomic sequencing analysis showed that iCDs addition markedly increased the relative abundances of hzsA/B/C , hao , hdh , nxrA genes related to anammox and narH and nrfA genes related to DNRA in Ca. Brocadia , their expressions in Ca. Brocadia were also remarkably upregulated. Meanwhile, the addition of iCDs increased the relative abundances and expressions of genes associated with signaling molecules, accelerating biofilm formation and associating AnAOB and their mutualism bacteria ( Desulfobacillus and UBA12294 ) more tightly. These findings revealed that iCDs accelerated nitrogen removal via mainly increasing anammox and DNRA rates of Ca. Brocadia , which provides a new strategy to enhance the nitrogen removal of anammox process under autotrophic conditions.