Unveiling zinc oxide nanoparticle toxicity in Symbiodinium kawagutii: Proteomic insights and coral reef implications

Zinc oxide nanoparticles (nZnO), widely used as inorganic UV filters, are increasingly released into coastal waters, yet their mechanistic impacts on coral symbionts remain insufficiently resolved. Here, we evaluated nZnO toxicity in Symbiodinium kawagutii, a critical symbiotic alga in coral reefs, using proteomic profiling and cellular analyses. Results reveal that nZnO exposure significantly suppresses algal growth, disrupts photosynthesis, and induces oxidative stress, leading to cellular damage. Proteomic data indicate downregulation of essential photosynthetic proteins and upregulation of stress response proteins, reflecting profound metabolic disruptions. Cells also increased extracellular polymeric substance (EPS) production; together with evidence of surface association and zinc accumulation, this suggests a defensive response that may modulate particle-cell interactions and Zn²⁺ dynamics. Collectively, these results indicate that nZnO can impair key physiological functions of S. kawagutii that underpin coral-algal symbiosis, supporting the need to incorporate nanomaterial contaminants into coastal monitoring and ecological risk assessment frameworks.