Anammox activity improved significantly by the cross-fed NO from ammonia-oxidizing bacteria and denitrifying bacteria to anammox bacteria

Nitric oxide (NO) has been suggested as an obligate intermediate in anaerobic ammonium oxidation (anammox), nitrification and denitrification. At the same time, ammonia-oxidizing bacteria (AOB) and denitrifying bacteria (DNB) are always existed in anammox flora, so what is the role of NO produced from AOB and DNB? Could it accelerate nitrogen removal via the anammox pathway with NO as an electron acceptor? To investigate this hypothesis, nitrogen transforming of an anammox biofilter was analyzed, functional gene expression of anam-mox bacteria (AnAOB), AOB and DNB were compared, and NO source was verified. For anammox biofilter, anammox contributed to 91.3 % nitrogen removal with only 14.4 % of AnAOB being enriched, while DNB was dominant. Meta-omics analysis and batch test results indicated that AOB could provide NO to AnAOB, and DNB also produced NO via up-regulating nirS/K and down-regulating nor. The activation of the anammox pathway of NH4++NO -> N2 caused the downregulation of nirS and nxr in Ca. Kuenenia stuttgartiensis. Additionally, changes in nitrogen transforming pathways affected the electron generation and transport, limiting the carbon metabolism of AnAOB. This study provided new insights into improving nitrogen removal of the anammox system.