Abstract
Saline aquaculture tailwater challenges conventional constructed wetlands (CWs) with their limited phosphorus (P) removal capacity. To address this, iron-carbon constructed wetlands (IC-CWs) were developed and operated under four salinity gradients (0, 10, 20, and 30) for 155 days to investigate the effects of salinity on P removal and associated microbial mechanisms. The results showed that salinity critically influenced long-term P removal, with the system at salinity 20 (S20) achieving the highest total phosphorus (TP) removal efficiency (78.80 ± 6.01%). Enhanced P removal was primarily attributed to the upregulation of phosphate transport genes (pstS, 14.25-fold increase) and elevated activity of key enzymes (AKP and ACP) in phosphorus-accumulating organisms (PAOs). However, high salinity (30) suppressed microbial metabolic functions. Metagenomic analysis revealed that salinity stress reshaped microbial community structure, with Bacteroidota abundance increasing 10-fold in S20 compared to S0 (control). This phylum harbored the phnE gene, significantly promoting organic phosphorus mineralization. Additionally, iron release increased with rising salinity, and the relative abundance of the phnE gene in Bacteroidota was highest in the S20 group, indicating a close association between iron release and PAOs as well as organic P mineralization genes. The quadratic polynomial model revealed that iron release under high salinity followed nonlinear kinetics, with passivation layer rupture promoting iron-phosphorus precipitate desorption in later stages. These findings provide a theoretical basis for optimizing salinity parameters to enhance chemical-biological P removal synergy, offering a promising strategy for saline aquaculture wastewater treatment.