ABSTRACT  

Coral reef ecosystems are increasingly threatened by environmental stressors, with microplastics (MPs) emerging as a pervasive and harmful contaminant. As essential symbionts of reef-building corals, Symbiodiniaceae are vital to coral health; however, their physiological responses to MPs remain largely unexplored. This study investigates the toxic effects of polystyrene microplastics (PS-MPs) of five distinct colors (red, yellow, green, blue, and white) on Cladocopium goreaui, a dominant coral symbiont. Exposure to 5 μm PS-MPs at 20 mg/L significantly inhibited algal growth, with blue and white PS-MPs exhibiting the strongest suppression. Growth rates decreased by up to approximately 36.0 % compared to controls over a 20-day exposure period. Although C. goreaui upregulated photosynthetic pigment content to compensate for reduced light availability due to MP aggregation, a substantial portion of the generated energy was diverted to mitigate oxidative stress. Transcriptomic analysis revealed that PS-MPs exposure downregulated key genes involved in biosynthetic pathways (e.g., peptide/amide formation) and primary metabolism (e.g., nitrogen assimilation, lipid metabolism). The pronounced toxicity of blue and white PS-MPs was attributed to their strong suppression of fatty acid metabolism and ribosomal function. These findings highlight the role of color disparity in modulating MP toxicity and offer new insight into the physiological and molecular responses of coral symbionts to MP pollution, with implications for coral reef health and resilience.