Mechanism-anchored ecological risk assessment of phthalates in coastal waters: Bridging environmental fate and molecular toxicity via an AOP framework.

The environmental fate and ecological risks of phthalate esters (PAEs) in coastal ecosystems remain poorly understood, particularly regarding their toxicological mechanisms on primary producers. This study systematically investigated the occurrence of 16 PAEs in seawater, suspended particulate matter (SPM), and sediments of Qingdao coastal waters, identifying sediments as the primary sink due to a strong "scavenging effect" in the semi-enclosed bay. The mean concentration of Σ16PAEs in seawater was 866.0 ng/L, dominated by diisobutyl phthalate (DIBP) and dibutyl phthalate (DBP). Ecological risk assessment based on the species sensitivity distribution (SSD) model revealed a critical divergence: although butyl benzyl phthalate (BBP) exhibited the highest inherent toxicity, DIBP and DBP posed the most significant individual risks driven by their high exposure levels. Crucially, a cumulative risk assessment utilizing the concentration addition (CA) model demonstrated that the mixture risk (RQmix > 1) consistently exceeded the high-risk threshold (RQmix > 1), highlighting a pervasive ecological threat. Furthermore, a mechanism-anchored hybrid toxicity model was developed to unravel the toxicity pathway within an adverse outcome pathway (AOP) framework. SHAP analysis identified the disruption of superoxide dismutase (SOD) activity as the dominant KE triggering growth inhibition. Molecular docking and dynamics simulations further explored a putative molecular initiating event (MIE): bulky PAEs (BBP/DBP) act as competitive inhibitors by stably binding to the hydrophobic pockets of antioxidant enzymes (CAT/APX), forming a persistent blockade. This study establishes a "Mechanism-Anchored Risk Assessment" framework, bridging the gap between macroscopic environmental monitoring and microscopic molecular toxicity.