Transcriptomic and physiological analyses reveal the toxic effects of inorganic filters (nZnO and nTiO₂) on scleractinian coral Galaxea fascicularis

The effects of sunscreen on scleractinian corals have garnered widespread attention; however, the toxic effects and mechanisms remain unclear. This study investigated the toxicological effects of two common inorganic filters used in sunscreens, nano zinc oxide and titanium dioxide (nZnO and nTiO₂), on the reef-building coral Galaxea fascicularis, focusing on the phenotypic, physiological, and transcriptomic responses. The results showed that after exposure to 0.8 mg/L of nZnO and 30 mg/L of nTiO₂ for 48 h, all coral polyps exhibited retraction. Zn and Ti ions were detected in coral tissues at concentrations of 67.18 and 24.87 μg/g, respectively, indicating the accumulation of nZnO and nTiO₂ in coral tissues. The zooxanthellae density, Fv/Fm, and chlorophyll-a content decreased significantly. The activity of antioxidant enzymes showed an increasing trend. Meanwhile, glutamine synthetase and glutamate dehydrogenase activities exhibited a decreasing trend. The health status of corals was impacted as a result of nZnO and nTiO₂ stress. Transcriptomic analysis showed that the toxicity mechanisms of nZnO and nTiO₂ differed in corals. Following exposure to nZnO, differentially expressed genes (DEGs) in corals were mainly enriched in signaling pathways related to immune response. The genes related to innate immunity, such as MASP1, MUC5AC, TLRs, and C2, were significantly upregulated, indicating that nZnO exposure induces an innate immune response in corals. Meanwhile, following nTiO₂ exposure, the upregulated DEGs were mainly enriched in signaling pathways related to transporter activity. In contrast, the downregulated DEGs were mainly enriched in energy metabolism pathways, indicating that nTiO₂ disrupted the energy supply of corals, thereby leading to an increased demand for nutrient transport. This study reveals the toxic effects of nZnO and nTiO₂, and their mechanisms of action on scleractinian corals, providing a reference for further assessing the toxicity of sunscreen on corals.