Interactive effects of temperature and microplastic size on biofilm formation and Vibrio harveyi infection in Korean rockfish Sebastes schlegelii.

Environmental changes affect microplastic (MP) biofilms, but how the resulting bacterial adsorption influences infection rates and physiological responses in fish remains unclear. This study investigated the combined effects of temperature (18 °C, 22 °C, and 26 °C) and MP size (102 and 220 μm) on biofilm formation and the subsequent effects on Vibrio harveyi infection and physiological responses in Korean rockfish Sebastes schlegelii. Biofilms were formed on MPs over 21 days under different temperature conditions, followed by a 5-day co-exposure experiment with V. harveyi. Biofilm formation markedly increased with rising temperature and was more pronounced on smaller MPs owing to their larger specific surface area, with the highest levels observed in the 26 °C small MP group. Correspondingly, the abundance of V. harveyi on MPs and its accumulation in fish tissues were markedly elevated, indicating enhanced bacterial transport and internalization. Co-exposure to MPs and V. harveyi induced oxidative stress, as evidenced by increased reactive oxygen species production in the liver, along with upregulation of antioxidant enzymes. Immune responses were also activated, with marked increases in the mRNA expression levels of interleukin-1 beta (il-1β), tumor necrosis factor alpha (tnf-α), and c-c chemokine ligand 25 (ccl25), particularly under high-temperature and small MP conditions. In situ hybridization further confirmed increased il-1β expression in liver tissues. Overall, elevated temperature and smaller MPs promoted biofilm formation, facilitating bacterial infection and inducing oxidative stress and immune responses. These findings suggest that climate-driven warming and MP pollution may synergistically increase disease risks in marine fish, with important implications for aquaculture and marine ecosystem health.