ABSTRACT                      

Microplastic (MP) pollution poses a growing threat to coral reef ecosystems, yet its direct effects on symbiotic Symbiodiniaceae remain poorly characterized. In this study, we investigated the size-dependent toxicity and underlying mechanisms of polystyrene microplastics (PS-MPs; 0.1, 1, and 10 μm) on three Symbiodiniaceae species-Cladocopium goreaui, Durusdinium trenchii, and Symbiodinium natans-representing distinct ecological niches. Among the tested sizes, 10 μm PS-MPs induced the most pronounced toxicity, significantly reducing algal growth rate, photosynthetic efficiency, and Chlorophyll a (Chl-a) content. These effects were accompanied by increased cell volume and enhanced accumulation of carbohydrates and lipids. Notably, S. natans exhibited partial recovery at later stages, whereas C. goreaui showed persistent inhibition. Exposure to 10 μm PS-MPs also elicited marked oxidative stress in all species, as evidenced by elevated superoxide dismutase (SOD) and malondialdehyde (MDA) levels, along with enhanced production of extracellular polymeric substances (EPS) and soluble proteins. Morphological analyses revealed PS-MP adhesion to algal surfaces, leading to membrane disruption, chloroplast damage, and stimulated EPS secretion. Transcriptomic profiling demonstrated size- and species-specific responses: under 0.1 μm PS-MPs, D. trenchii upregulated oxidative phosphorylation and the TCA cycle, whereas S. natans responded to 1 μm PS-MPs by activating purine metabolism and oxidative phosphorylation. In contrast, 10 μm PS-MPs downregulated ribosomal and photosynthetic genes, while upregulating fatty acid biosynthesis and antioxidant defense pathways. Collectively, these findings reveal that PS-MP toxicity is both size- and species-dependent, providing mechanistic insights into how MPs disrupt coral-algal symbiosis and undermine reef ecosystem resilience.