Combined toxicity of microplastics and copper on Goniopora columns

As microplastics (MP) become ubiquitous, their interactions with heavy metals threatens the coral ecosystem. This study aimed to assess the combined toxicity of MP and copper (Cu) in the environment of coral. Goniopora columna was exposed to polyethylene microplastics (PE-MP) combined with Cu²⁺ at 10, 20, 50, 100, and 300 μg/L for 7 days. Polyp length and adaptability were recorded daily, and coral samples were collected at 1, 3, 5, and 7 days to analyse zooxanthellae density and antioxidant activity. Tissue observations and the analysis of MP and Cu²⁺ accumulation were conducted on the 7th day. After 1 day of exposure, PE-MP combined with different concentrations of Cu²⁺ significantly decreased polyp length and adaptability compared with PE-MP alone. Simultaneously, a significant increase in malondialdehyde (MDA) content, lead to coral oxidative stress, which was a combined effect with PE-MP. After 3 days of exposure, PE-MP combined with Cu²⁺ at >50 μg/L significantly reduced zooxanthellae density, damaging the coral's symbiotic relationship. In antioxidant enzyme activity, superoxide dismutase (SOD) activity decreased significantly after 1 day of exposure. After 3 days of exposure, glutathione peroxidase (GPx) activity significantly increased with Cu²⁺ at >20 μg/L. After 5 days of exposure, PE-MP combined with different concentrations of Cu²⁺ significantly reduced catalase (CAT), glutathione (GSH), and glutathione transferase (GST) activity, disrupting the antioxidant enzyme system, and acting antagonistically to PE-MP alone. Tissue observations revealed that the PE-MP combined with Cu²⁺ at >50 μg/L caused severe mesenteric atrophy, vacuolar, and Cu²⁺ accumulation in the coral mesenteric compared with PE-MP alone. The results suggest that combined exposure of PE-MP and copper leads to more severe oxidative stress, disruption antioxidant enzyme system, tissue damage, and Cu²⁺ accumulation, resulting in a significant maladaptation of corals to the environment.