Ecotoxic Effects Of Titanium Dioxide Nanoparticles Under Seawater Acidification In The Thick Shell Mussel Mytilus Coruscus

With the rapid development of economic construction and the continuous improvement of urbanization level in coastal areas,various pollutants produced by industry,agriculture and daily life will inevitably flow into the ocean,and the ocean is becoming the gathering place of almost all pollutants.As a type of widely used and less toxic nano-material,titanium dioxide nanoparticles?nano-TiO₂?is easy to coagulate and deposit on the surface of sediment in water environment.Comparing with other organisms,filter-feeding bivalves living in intertidal zone have a higher risk of exposure to nanoparticles.Besides,global climate change has also led to multiple environmental stresses on marine ecosystems,and ocean acidification?OA?is one of them.In view of the fact that pH value is the key factor affecting the dispersion and aggregation of nanoparticles,and that OA and nano-TiO₂ pollution exist simultaneously,it is urgent to clarify the complex ecological toxic effects of nanoparticles on marine organisms under OA.Based on integrated ecotoxicology,the synergistic ecotoxic effects of nano-TiO₂ under seawater acidification conditions on the thick shell mussel Mytilus coruscus were comprehensively analyzed.Hemocyte parameters were evaluated in the mussels M.coruscus exposed to two types of nano-TiO₂(25nm&100nm:0.1,1,10 mg L^-1,respectively)for21 days by using low cytometry?FCM?assays.Throughout the whole experiment,the total hemocyte count?THC?,phagocytosis activity?PA?,esterase activity?EA?,lysosomal content?LC?,mitochondrial number?MN?and mitochondrial membrane potential?MMP?of the mussels were significantly decreased as size decreased and concentration increased,while the hemocyte mortality?HM?and the production of reactive oxygen species?ROS?were significantly induced.In general,size-and concentration-dependent toxicity was pronounced with 25 nm-TiO₂ and highest concentration(10 mg L^-1)being the most toxic.Physiological toxicity was investigated in the mussel M.coruscus exposed to three concentrations of nano-TiO₂(25 nm,0?control?,2.5,10 mg L^-1)under two pH values?7.3,8.1?control??for 14 days.We found that clearance rate?CR?,food absorption efficiency?AE?,respiration rate?RR?,O:N ratio and scope for growth?SFG?were significantly reduced by nano-TiO₂,whereas ammonia excretion rate?ER?rate and faecal organic weight rate were increased.OA exerted lower effects on CR,RR,ER and O:N ratio than nano-TiO₂.Most physiological functions of M.coruscus were more severely impaired by the combination of nano-TiO₂ and OA suggesting a synergistic effect on mussels.Antioxidant indexes of gills and digestive glands as well as immune parameters of hemocytes were also investigated in the mussel M.coruscus exposed to different concentrations of nano-TiO₂(25 nm,0?control?,2.5,10mg L^-1)and two pH levels?7.3,8.1?control??for 14 days.Mussel responses were also investigated after a 7 days recovery period?pH 8.1 and no nanoparticle?.A decrease in superoxide dismutase?SOD?activity and glutathione?GSH?content as well as an increase in malondialdehyde?MDA?level were observed in gill of mussels exposed to high concentration nano-TiO₂(25 nm,10 mg L^-1).A different response was observed in the digestive gland as SOD,catalase?CAT?and GSH levels increased as nano-TiO₂ concentration increased.These effects were more severe in mussels kept at low pH?7.3?as compared to normal pH?8.1?.Gills of the thick shell mussel M.coruscus are more sensitive to the combined stressors of nano-TiO₂ and low pH?7.3?as compared with digestive glands.A 7 days recovery period was not sufficient to fully restore SOD,glutathione peroxidase?GPx?,glutathione S-transferase?GST?,GSH and MDA levels to levels before exposure to nano-TiO₂ and low pH?7.3?.Nano-TiO₂ influenced the immune functions of the thick shell mussel M.coruscus more severely than ocean acidification.THC,PA,EA and LC significantly decreased under low pH?7.3?and high nano-TiO₂ concentration(25 nm,10 mg L^-1),whereas HM and the production of ROS obviously increased.Slight recovery from the combined stresses was observed for THC,HM,PA and LC,but significant carry-over effects of nano-TiO₂ and low pH were still observed.In summary,the present study found that nano-TIO₂ would inhibit physiological activities,reduce antioxidant capacity and weaken immune defense of the thick shell of mussel M.coruscus.The combination of low pH?7.3?and high concentration nano-TiO₂(25 nm,10 mg L^-1)had negative effects on mussels,and these effects still acted for some time even though the mussels were already out of such stressors.