Laboratory-simulated marine heatwave enhances physiological damage to mussels exposed to titanium dioxide nanoparticles by disrupting the gut-hepatopancreas axis

The aggregation state of nano-TiO₂ in the environment is altered under marine heatwaves (MHWs), thus affecting its bioavailability and toxicity to the marine organisms. Here, we investigated the toxic mechanisms and effects of nano-TiO₂ on gut-hepatopancreas axis health of Mytilus coruscus exposed to 25 and 250 μg/L of nano-TiO₂ under laboratory-simulated MHW. Compared with the control conditions or post-MHW cooling phase, prolonged MHW exposure significantly inhibited digestive function, decreased immune-related enzymes activities, and caused neurotoxicity in the mussels. 16S rRNA analysis demonstrated that high concentration nano-TiO₂ and combined exposures decreased the abundance of Bacteroidota while increased the Proteobacteria. Additionally, the elevated pro-inflammatory bacteria released endotoxin lipopolysaccharide (LPS), which activated Toll-like receptor 4 (TLR-4) in the hepatopancreas and induced hepatopancreatic inflammation by downregulating nuclear factor-kappa B (NF-κB) signaling pathway and detoxification-related genes. Furthermore, nano-TiO₂ and MHW exposure dysregulated the glutathione system, decreased the levels of antioxidation-related genes, and induced the accumulation of ROS and lipid peroxide (LPO) contents, thus causing severe oxidative damage and hepatopancreatic cell apoptosis. These findings demonstrate that nano-TiO₂ and MHW induce hepatopancreatic inflammation and cell damage, which are strongly associated with the gut lesions and disrupted gut-hepatopancreas axis homeostasis.