Comparative Toxicity of Four Industrially Important Nanoparticles to the Marine Diatom Phaeodactylum tricornutum: Temporal Dynamics, Hazard Classification, and Phenotypic Response Patterns

Engineered nanoparticles (ENPs) are increasingly released into marine environments, raising concerns about their potential ecological risks. In this study, we evaluated the toxicity of four widely used ENPs—CuO, ZnO, TiO2, and SiO2—to the marine diatom Phaeodactylum tricornutum over a 7-day exposure. Using a range of endpoints, including cell abundance, growth kinetics, chlorophyll a content, the integral inhibition index (IAUC), and effective concentrations (EC10, EC50), we established a toxicity ranking of CuO > (TiO2 ≈ ZnO) > SiO2 based on IAUC and final cell counts. Each nanoparticle exhibited a distinct phenotypic response pattern: CuO showed cumulative and irreversible toxicity (IAUC = 57.3% at 5 mg/L); TiO2 caused acute toxicity, with cell counts dropping to 29.8% of the control by day 3, followed by partial recovery (59.7% by day 7); ZnO induced delayed hormesis at 0.5 mg/L (125.1% of the control at day 7) but moderate inhibition at higher concentrations; and SiO2 displayed an algistatic, partially reversible effect (IAUC = 37.1%), likely due to physical adhesion and shading. Based on EC50 values, CuO, ZnO, and TiO2 were classified as “Toxic” (GHS Acute Category 2), while SiO2 was borderline. The lowest EC10 values were recorded for CuO (0.21–0.33 mg/L), indicating high chronic hazard. Our findings underscore the importance of integrating acute toxicity data with temporal dynamics and phenotypic response patterns for a reliable environmental risk assessment of ENPs in seawater.