Unraveling individual and combined toxicity of microplastics and tetracycline at environment-related concentrations to coral holobionts

Coral holobionts constitute the foundational organisms of coral reef ecosystems. As an emerging pollutant, the projected accumulated levels of microplastics (MPs) are expected to continue increasing. Meanwhile, due to their properties, MPs can absorb multiple other marine pollutants, such as antibiotics (ATs). However, the co-toxicity mechanism of MPs and ATs to coral holobionts remains to be explored. Here, using Zoanthus sociatus as a model organism, we investigate the individual and combined toxicity of MPs and tetracycline (TC) at environment-related concentrations to coral holobionts. Microbiomics indicate that MPs and TC increase coral holobionts bacterial species richness while concurrently reducing the microbial community structure stability. The key metabolites and enzyme activity results demonstrated that the impacts of MPs and TC on corals encompassed antioxidant capacity, detoxification capability, immune function, and lipid metabolism. Transcriptomics shows that MPs and TC disrupt coral-algae relationships mainly through host nutrition limitation and inhibition of symbiotic algae carbon/nitrogen metabolism, respectively. A synergistic effect between MPs and TC has also been observed. In contrast, coral holobionts have shown adaptability through activating coral-symbiodiniaceae-bacteria interactions, mainly including: 1) enhancing the abundance of BMCs (beneficial microorganisms for corals); 2) enhancing host lipid accumulation; 3) immunoregulation; 4) symbiotic regulation. Overall, our findings provide new insights into the co-toxicity of MPs and TC, and highlight those MPs and TC at current environment concentration and predicted for most oceans in the coming decades, can ultimately cause coral bleaching.