Microplastic and Vibrio harveyi co-exposure induces oxidative stress in big-belly seahorse Hippocampus abdominalis

Microplastics in marine environments can act as vectors for pathogenic bacteria, increasing the risk of infection and physiological stress in marine organisms. Seahorses, which inhabit coastal ecosystems, may be vulnerable to the combined effects of microplastics and bacterial pathogens such as Vibrio harveyi. Therefore, the aim of this study was to investigate the effect of microplastic and bacteria co-exposure on big-belly seahorse (Hippocampus abdominalis). Microbeads (MBs) were introduced via water exposure, while V. harveyi was administered by injection. Seahorses were exposed to microbeads (MBs; concentration; 50 beads/L; size, 0.2 and 1.0 μm), V. harveyi (1 × 10 CFU/mL), and a combination of both for 4 days to evaluate oxidative stress responses. Tissue sampling was performed at 0, 1, 2, and 4 days. Notably, MB accumulation in the gills and intestine was significantly higher in the group co-exposured to MB and bacteria than in the single exposure groups. V. harveyi copy number in the liver was significantly higher in the co-exposure groups, especially in the Vh + MB1.0 group. Compared with the control group, MB and V. harveyi co-exposure significantly upregulated antioxidant response, including superoxide dismutase, catalase, and 1-cysteine peroxiredoxin mRNA and protein levels. Additionally, hydrogen peroxide levels and DNA damage were elevated in the Vh + MB1.0 group, indicating enhanced oxidative stress and genotoxicity. In conclusion, MB and V. harveyi co-exposure induces oxidative stress and DNA damage in seahorse, suggesting potential ecological risks associated with simultaneous exposure to microplastics and marine pathogens in coastal ecosystems.