Synergistic Impacts of Co‐Exposure to Microplastics and <i>Vibrio harveyi</i> on the Immune and Stress Responses of the Big‐Belly Seahorse <i>Hippocampus abdominalis</i>

Microplastics (MPs) originating from synthetic polymers can act as vectors for harmful microorganisms and pollutants, exacerbating ecological risks. Vibrio harveyi, commonly found in seawater, is a major opportunistic pathogen causing vibriosis in marine fish. MPs and pathogenic bacteria such as V. harveyi have increasingly been recognized as co-contaminants in marine environments. This study investigated the physiological responses of the big-belly seahorse Hippocampus abdominalis exposed to MPs and V. harveyi, both individually and in combination. Seahorses were exposed to small (0.2 μm) and large (1.0 μm) polystyrene MPs (50 beads/L) and V. harveyi (1 × 10³ CFU/mL) for 4 days under controlled conditions. Biochemical parameters and molecular analyses were conducted to assess hepatic function, immune regulation, oxidative stress, and apoptosis-related responses. MP exposure promoted V. harveyi proliferation within seahorse tissues, with higher bacterial copy numbers in the kidney than in the liver, indicating the kidney's active immune role. Plasma biochemical indices (alanine aminotransferase, aspartate aminotransferase, and glucose) and immune-related genes, including lysozyme G, interferon regulatory factor 8, and interleukin 10, were significantly elevated in co-exposure groups compared with individual exposures. Expression of heat shock protein 75 and caspase 8 was also upregulated, suggesting enhanced oxidative stress and activation of apoptotic pathways. These findings indicate that MPs and V. harveyi exert synergistic physiological stress, disrupting immune homeostasis and promoting apoptosis in seahorses. This study provides mechanistic insights into combined MP-pathogen toxicity and establishes the big-belly seahorse as a sensitive bioindicator for complex marine pollution.