Ecophysiological Responses Of Mussel Mytilus Galloprovincialis To CuO And CeO₂ Nanoparticles Under Ocean Acidification

Approximately 1/3 of CO₂ emitted by anthropogenic activities was absorbed by the ocean,leading to a continuous decrease in pH and carbonate concentration of global surface seawater,known as ocean acidification（OA）.Apart from posing direct threats to marine organisms,ocean acidification also could alter the environmental behaviors of marine pollutants to indirectly change their toxic effects on marine organisms.Marine bivalves are widely recognized as model organisms to examine the changing environmental factors in coastal ecosystems.In this study,we determined the toxic effects and potential risks of two typical metal-based nanoparticles CuO NPs and CeO₂NPs on the mussels Mytilus galloprovincialis under ocean acidification scenarios.The main contents included the following parts:（1）The mussels M.galloprovincialis were exposed to two seawater pH values（8.1and 7.6）,three metal-containing contaminants(Cu²⁺ions,100 nm nCuO,and 500 nm-5μm bCuO),and three metal-contaminant exposure concentrations（0,25μg Cu/L,and50μg Cu/L）for 28 days to investigate the toxic effects to mussels caused by the three typical environmental contaminants Cu²⁺ions,nCuO and bCuO under seawater acidification condition.After that,a 56-day recovery experiment was conducted to assess the adaption ability of mussels to the unstable marine environment.The results showed that the solubility of nCuO and bCuO in seawater was minimal,and seawater acidification did not affect the solubility of nCuO and bCuO in seawater.However,seawater acidification significantly increased the copper concentration in the digestive glands of mussels under lower Cu²⁺exposure conditions（25μg Cu/L）,while decreasing that under higher Cu²⁺exposure conditions（50μg Cu/L）.After 14 days,under pH 7.6condition,exposure to Cu²⁺/nCuO/bCuO all increased the clearance rates in M.galloprovincialis,while exposure to Cu²⁺with seawater acidification inhibited the clearance rates.The respiration rate of M.galloprovincialis fluctuated with exposure time,presented as an overall trend of activation followed by inhibition.Combined with seawater acidification,exposure to CuO NPs had more significant effects than exposure to Cu²⁺,and the variation in clearance rate and respiration rate confirmed the environmental stress induced the mussels M.galloprovincialis to change energy metabolism strategy.Combined exposure to OA and CuO NPs/Cu²⁺,oxidative stress damages were found in both digestive glands and gills in M.galloprovincialis.The gills are more sensitive and vulnerable to environmental perturbations.The recovery experiment revealed that exposure to OA and Cu²⁺/nCuO/bCuO had residual effects on oxidative stress damage in M.galloprovincialis.Although the oxidative damage in mussels cannot be fully repaired,mussels could maintain antioxidant defense homeostasis by regulating antioxidant enzyme activities.A comparative analysis of the differences in the potential toxicity risk to M.galloprovincialis caused by OA and Cu²⁺/nCuO/bCuO exposures using the weight of evidence（WOE）models.We found that CuO NPs caused lower toxicity risk in M.galloprovincialis thanCu²⁺whether in single or combined exposure,while nCuO had higher hazard risks than bCuO.（2）The mussels M.galloprovincialis were exposed to two seawater pH values（8.1and 7.6）and three CeO₂ NPs exposure levels（0,10,and 100μg/L）for 30 days to determine the effects of ocean acidification and/or CeO₂ NPs exposure on bioaccumulation,hemocyte immune defense,physiological responses,and oxidative stress in M.galloprovincialis using traditional physiological ecological techniques.Results showed that the digestive gland was the main targeted organ of M.galloprovincialis to CeO₂ NPs accumulation,and seawater acidification had no significant effect on the bioaccumulation of CeO₂ NPs.Exposure to OA and CeO₂ NPs had no significant effect on the clearance rate and condition index in M.galloprovincialis.However,both stressful conditions enhanced the respiration rate of M.galloprovincialis separately.Additionally,co-exposure to OA and CeO₂ NPs significantly increased the mitochondrial membrane potential,while the co-exposure might inhibit the phagocytosis of hemocytes in M.galloprovincialis.And the single CeO₂ NPs exposure inhibited the production of reactive oxygen species（ROS）and increased apoptosis rates of the hemocytes of M.galloprovincialis.Seawater acidification and CeO₂ NPs exposure caused little oxidative stress to M.galloprovincialis,and the antioxidant defense response of the digestive gland and mantle in M.galloprovincialis under OA and CeO₂ NPs exposure conditions was tissue-specific.According to the integrated biomarker response index,the potential toxicity risk of OA to M.galloprovincialis was found to be more severe than that of CeO₂ NPs.This study demonstrates that the bio-toxic effects of CuO NPs and CeO₂ NPs on mussels M.galloprovincialis primarily depended on the self-nature of the nanomaterials but not the dissolved metal ions.Co-exposure of OA and Cu²⁺/CuO NPs/CeO₂ NPs would affect M.galloprovincialis on multiple levels,including the individual,tissue,and molecular levels.We concluded that the potential risks of M.galloprovincialis are Cu²⁺>CuO NPs>CeO₂ NPs according to our results.Overall,these findings might provide useful information for assessing the marine environmental risk of nanomaterials in light of complex worldwide climate changes.