Dissolved organic nitrogen molecular transformation and microbial mechanisms in a completely autotrophic nitrogen removal over nitrite (CANON)-based algal-bacterial symbiotic system.

Sunlight-driven CANON-based algal-bacterial symbiotic system (CANON-AB) achieves efficient nitrogen removal through algal oxygen production and assimilation. However, dissolved organic nitrogen (DON), as a major fraction of residual total dissolved nitrogen, remains poorly understood in CANON-AB. This study investigated DON transformation and microbial mechanisms in CANON-AB compared with conventional CANON. After 140  days operation, CANON-AB increased average dissolved inorganic nitrogen (DIN) removal efficiency from 83.6 % to 98 %, reducing effluent nitrate from 27.4 ± 8.3 to 0.7 ± 0.8  mg/L. Meanwhile, DON concentration decreased by 54.8 % in CANON-AB (1.4 ± 0.2  mg/L) compared with CANON (3.1 ± 0.2  mg/L). FT-ICR MS revealed that algae integration reduced DON molecular diversity from 1396 to 723 formulas, with preferential removal of carbohydrates, condensed aromatics, tannins, and sulfur-containing DON molecules. Metagenomic analysis demonstrated enhanced DON mineralization potential in CANON-AB, with DON transformation genes enriched by 1.1-2.4 times higher compared with CANON. These results reveal a coupled mineralization-assimilation mechanism driving DON removal in CANON-AB, providing new insights into nitrogen cycling in sustainable wastewater treatment.