UV-aged microplastics exacerbate cadmium-induced intestinal barrier dysfunction, innate immune impairment, and Vibrio harveyi colonization in big-belly seahorses, Hippocampus abdominalis.

Ultraviolet (UV) aging alters the surface physicochemical properties of microplastics (MPs) and may increase their capacity to transport toxic metals in aquatic environments. This study investigated whether UV-aged high-density polyethylene MPs exacerbate cadmium (Cd) toxicity by impairing intestinal barrier function and innate immunity and increasing susceptibility to Vibrio harveyi infection in the big-belly seahorse, Hippocampus abdominalis. MPs were UV-aged for 5 or 10 days and administered alone or in combination with Cd for 5 days. UV aging increased specific surface area, shifted zeta potential toward more negative values, and enhanced Cd adsorption, with the strongest effects observed after 10 days of aging. Combined exposure, particularly to 10-day-aged MPs and Cd, decreased intestinal ctnnb1 and sdc2 expression and goblet cell abundance while increasing intestinal and plasma diamine oxidase and D-lactate levels, indicating impaired epithelial integrity and increased intestinal permeability. Hepatic G-type lysozyme mRNA expression was increased, whereas plasma lysozyme concentrations decreased, indicating disruption of innate immune homeostasis. Following V. harveyi challenge, the 10-day-aged MP/Cd group exhibited the highest hepatic bacterial burden and Brown-Hopps-positive signals. These findings indicate that UV aging can enhance the capacity of MPs to modulate Cd toxicity, linking intestinal barrier disruption with impaired host defense and increased susceptibility to bacterial infection. Such interactions may increase disease vulnerability in wild seahorses inhabiting contaminated coastal habitats, highlighting the potential importance of aged MPs as modifiers of contaminant-related ecological risk and seahorse conservation.