Optimizing Nitrogen Removal Through Coupled Simultaneous Nitrification-Denitrification and Sulfur Autotrophic Denitrification: Microbial Community Dynamics and Functional Pathways in Mariculture Tailwater Treatment

This study investigates the nitrogen removal pathways and microbial community dynamics in a novel system coupling simultaneous nitrification and denitrification (SND) with sulfur autotrophic denitrification (SAD) for the treatment of mariculture tailwater. High-throughput sequencing and predictive functional analysis were employed to examine microbial compositions and their functional roles across varying carbon-to-nitrogen (C/N) ratios. The results revealed that SND occurred in the aerobic stage, with Nitrosomonas and Nitrospira facilitating nitrification, while Denitromonas and Paracoccus drove denitrification. In the anaerobic stage, SAD was the primary nitrogen removal process, with sulfur metabolism supported by Thiobacillus and Desulfobacteria. Increasing C/N ratios enriched denitrifying bacteria, enhancing nitrogen removal performance, but reduced nitrifying activity. Functional gene analysis demonstrated the upregulation of denitrification genes (napAB, nirS, norBC, nosZ) with higher carbon inputs, while sulfur metabolism genes (sqr, soxB, dsrAB) confirmed the critical role of sulfur cycling in SAD. The integration of SND and SAD pathways, supported by carbon addition, achieved efficient nitrogen removal, while promoting sulfur bioavailability. Under C/N ratios of 1.2, the nitrate nitrogen (NO3−-N) removal efficiencies reached 93.48%, respectively, while the total nitrogen (TN) removal efficiencies were 95.06%. Ammonia nitrogen (NH4+-N) removal efficiency consistently exceeded 95%, stabilizing at 99.00% in the steady-state operation. This research provides a comprehensive understanding of the microbial and functional mechanisms underlying SND–SAD systems, offering an innovative solution for sustainable mariculture tailwater management.