Sulfate-driven organic phosphorus mineralization stimulates endogenous phosphorus release in the effluent-receiving...

Seasonal algal blooms in effluent-receiving rivers are being exacerbated by often-overlooked sulfate discharge from wastewater treatment plants. Sulfate inputs can alter sulfur-iron-phosphorus (S:Fe:P) ratios, regulating the contributions of dissimilatory sulfate reduction (DSR) and dissimilatory iron reduction (DIR) to P release. However, how and to what extent DSR and DIR participate in P release under sulfate input remains unknown. Diffusive gradients in thin films (DGT), pathway-specific inhibitors, and microbiological analyses were combined to investigate P release pathways under different S:Fe:P ratios and to quantitatively differentiate the relative contributions of DSR and DIR to P release. It was found that elevated S:Fe:P ratios shifted the primary labile P release from the DIR-driven zone (20-80 mm) to the DSR-driven zone (80-120 mm). Inhibitor experiments further confirmed that sulfate-mediated P release was dominated by DSR (95.4%) in the actual effluent-receiving river. P fractions and metagenomic analyses identified that DSR-driven P release was governed by organic P (OP) mineralization, as evidenced by decreased OP fractions (e.g., NaOHNRP) and increased abundance of OP-mineralizing genes (e.g., phoD). These findings underscored that DSR-driven OP pools in deep sediments (80-120 mm) served as key sources of sulfate-mediated P release in effluent-receiving rivers and highlighted the importance of controlling sulfate discharge to mitigate eutrophication.