Enhancing sulfide-based denitrification under dual high-salinity and sulfide stress: Metabolic adaptation via...

This study demonstrated enhanced sulfur autotrophic denitrification (SAD) under dual high-salinity (3% Na₂SO₄) and sulfide (142.5 mg L^-1 S^2-) stress using biostimulants. Screening of ten biostimulants (1 mg L^-1) revealed compatible solutes (glycine betaine, trehalose, and mannitol) as optimal enhancers. At a higher level of 100 mg L^-1, the addition of all compatible solutes significantly boosted TN removal rates >3-fold, with mannitol achieving the highest (9.38 mg L^-1 h^-1) (p < 0.05). Short term of dosage tests further showed better TN removal at higher trehalose/mannitol concentrations (10-100 mg L^-1), while glycine betaine peaked at 50 mg L^-1. Furthermore, metagenomic analysis revealed potential mechanisms for resisting the dual stress of high salinity and sulfide. Those mechanisms act through osmotic protection (via specific transporter genes and elevated extracellular polymeric substances), carbon provision boosting TN removal via heterotrophic denitrification (nirS), and enzymatic antioxidant defense (SOD1/NQO1). The addition of exogenous compatible solutes critically mitigates dual stress from high salinity and sulfide toxicity, demonstrating significant potential for SAD system enhancement.