Toxicity assessment of atmospheric particulate-bound polycyclic aromatic hydrocarbons on marine phytoplankton

Particulate-phase polycyclic aromatic hydrocarbons (PAHs), known for their stability and significant toxicity, can undergo long-range atmospheric transport and deposit into oceans. However, their impact on phytoplankton remains controversial. We extracted PAH-containing mixtures from aerosol particulate matter (AP-PAHs) and assessed their toxicity on coastal diatom Skeletonema costatum under environmentally relevant exposure levels (0.2-1.25 μg/L). Toxicity was evaluated by comparison with a PAH mixture (standard-PAHs) present in AP-PAHs, based on the 16 priority PAHs identified by the U.S. Environmental Protection Agency (EPA). AP-PAH exposure exhibited significantly greater toxicity, with a half-maximal inhibitory concentration (EC₅₀) of 0.62 μg/L, nearly half that of standard-PAHs (1.22 μg/L). Chlorophyll (a + b) and carotenoid concentrations generally declined across all PAH treatments, while pigment content per cell increased by up to 179 % at 1 and 1.25 μg/L AP-PAH loadings relative to standard-PAHs, suggesting free pigment accumulation and photodynamic damage. AP-PAH exposure led to progressive disruption of the photosynthetic system with increasing loadings-initially targeting core proteins of Photosystem II (PsbM, PsbO, PsbQ, PsbU, Psb27, and Psb28) at 0.2 μg/L, and extending to Photosystem I (PsbA, PsbB) at 1.25 μg/L. No comparable gene expression changes were observed in standard-PAH treatments. These findings suggest that the standard EPA PAHs cannot fully capture the toxicity of aerosol-bound mixtures, underscoring the role of unidentified compounds with similar polarity. Given the comparable AP-PAH derived EC₅₀ and natural PAH concentrations in seawater, and increased sensitivity observed in natural phytoplankton, our results highlight overlooked ecological risks associated with atmospheric PAH deposition in marine environments.