Salinity-driven trade-offs between nitrogen removal and microbiome dynamics in Fe-C-CWs toward saline aquaculture tailwater management.

Salinity-driven nitrogen removal mechanisms in iron-carbon CWs (Fe-C-CWs) remain poorly understood for aquaculture tailwater management. Through a 155-day trial under four salinities (designated as S0, S10, S20, and S30), result showed that S20 achieved optimal removals of total nitrogen (84.9 ± 3.1 %), nitrate (81.8 ± 2.5 %), and ammonium (79.3 ± 3.0 %), significantly outperforming other groups (P < 0.05). Metagenomics revealed that S20 significantly enriched denitrifying taxa (Halothiobacillus, Prolixibacter) and upregulated nitrogen cycling genes (nirB, nrfA, nrfH, hao) and iron cycling genes (feoA, feoB), highlighting the functional synergy between microbial composition and biogeochemical cycling processes. Dual isotope signatures (δN/ δO) first applied in Fe-C-CWs confirmed salinity-mediated pathway shifts: nitrite oxidation dominated in saline groups, especially in S20 versus reduction in S0. Enzymatic profiling substantiated the concurrent operation of nitrification, denitrification, and anammox pathways across all groups, with activities exhibiting significant salinity-dependent modulation. S20 demonstrated remarkable enzymatic potentiation, where core nitrogen-cycling enzymes including nitrite oxidoreductase (NXR: 8.79 ± 0.67 U/g), nitrate reductase (NAR: 18.13 ± 1.19 U/g), and nitrite reductase (NIR: 6.74 ± 0.47 U/g) showed 16.00∼32.18 % higher than S0 (P < 0.01). This enzymatic synergy suggests salinity-optimized coupling between dissimilatory iron reduction and nitrogen transformation processes. Ecological network analysis revealed significant interactions among microbial phyla, particularly between Proteobacteria and Planctomycetota. This study demonstrates that S20 can enhance interaction between Fe-C matrix and microorganisms, thereby improving the efficiency of Fe-C-CWs in removing nitrogen pollutants from aquaculture tailwater. These findings offer theoretical insights for further understanding the internal operational mechanisms of the Fe-C-CWs.