Endogenous mechanism of microbial functional gene and exogenous nitrogen removal factors driven by sustainable iron-nitrogen cycling

Feammox and nitrate-dependent Fe(II) oxidation (NDFO) play an important role in the iron-nitrogen cycle, respectively. To further understand the intrinsic molecular biological mechanism of Feammox with NDFO and improve the total nitrogen (TN) removal efficiency of the culture system, functional gene endogenous development and exogenous factors affecting nitrogen removal driven by the iron-nitrogen cycle were analyzed. Principal component analysis (PCA) showed that the three different ecological groups all formed a similar stable iron-nitrogen bacteria community structure. For 16S rRNA and qPCR, the dominant bacterial genera of Feammox coupled with NDFO were closely related, the amount of nxrB, nirS, and nosZ gene are 1-2 orders of magnitude larger than the amoA, narG, and AMX gene, which are the most important endogenous factors affecting nitrogen removal in the system. And an interesting result is that amoA, nxrB, narG, nirS and nosZ genes were all significantly positively correlated with each other (p < 0.05), indicating that the nitrogen cycle function genes changed synchronously, which was consistent with the changes in the microbial genera. The experimental process was optimized by changing environmental factors such as pH, organic carbon and extracellular electron shuttle to improve nitrogen removal. The results indicated that TN removal efficiency (67.05–76.12%) was greatly increased compared with the previous experiments (18.62–19.38%), which was the highest in pH = 8 groups (COD/TN = 3) because pH of the enrichment system stabilized to approximately 8. Moreover, the added 9,10-anthraquinone-2,6-disulfonate (AQDS) has improved TN removal efficiency to 67.27–87.75% through its extracellular electron transfer function, which was higher than that of control groups (without AQDS). This study provides a new scientific perspective and sustainable practical support for nitrogen pollution removal in the natural ecosystems or wastewater treatment systems.