Effects Of Ca²⁺ On The Accumulation And Toxicity Of TiO₂ Nanoparticles And Cadmium In Daphnia Magna

With the rapid development of nanotechno logy,nanometer material products are widely used in various fiekds.The wide use of nanoparticle materials results in the release of nanoparticles into the environment,raising a potential environmental risk.The natural environment is not a sinple system,a variety of pollutants(e.g.,inorganic and organic pollutants)have already existed.When nanoparticles enter the environment,how they may interact with these conventional pollutants and how these interactions may affect the behavior and effects of each pollutant remain unclear.Although In this thesis,we examined the effects of Ca on the bioavailability and toxicity of polyacrylic acid coated TiO2 nanoparticles(PAA-TiO2-NP)and Cd when these two pollutants were applied separately or in combination.A brief summary of this research writes as follows.1.Calcium plays versatile roles in aquatic ecosystems,and its effects on the uPtake of polyacrylate-coated TiO2 nanoparticles(PAA-TiO2-NP)by the cladoceran Daphnia magna were investigated.When the ambient Ca concentration in the experimental medium([Ca]dis)was low,PAA-TiO2-NP were well dispersed and were distributed across the daphnids;the concentration was highest in the gut and thoracic limbs.The direct internalization of PAA-TiO2-NP was visualized by synchrotron radiation-based micro X-ray fluorescence spectroscopy,transmission electron microscopy,and scanning electron microscopy.More intriguingly,Ca induced PAA-TiO2-NP uptake as a result of the increased Ca influx.When[Ca]dis was high,a substantial aggregation of PAA-TiO2-NP was observed.The micron-sized aggregates were concentrated only in the gut independently of the Ca influx.Overall,it was found for the first time that the well-dispersed nanoparticles and their micron-sized aggregates were taken up by D.magna through different routes,and Ca had multple effects on nanoparticle bioaccumulation.2.Despite the ever-increasing research on nanotoxicokigy,how engineered nanoparticles(NP)may influence the bioavailability and toxicity of other pollutants remains obscure,especially for NP with good dispersibility.In the present study,effects of polyacrylate-coated TiO2-NP(PAA-TiO2-NP)on Cd accumulation and toxicity in D.magna were investigated at two Ca levels(0.2 and 2.0 mM).PAA-TiO2-NP were well dispersed with an average size of 20 nm at low Ca level,but formed micron-sized aggregates at high Ca level Although PAA-TiO2-NP ameliorated Cd toxicity at both Ca levels,the underlying mechanisms were completely different.At high Ca level,the metal-NP complex was actively ingested by D.magna.Cd accumulation was induced and both pollutants were concentrated in the gut.Although Cd accumulation was induced under this condition,it was dominantly concentrated in the gut together with the nanoparticles and would thus have little biological effects.At tow Ca level,the metal-NP complex was accumulated either by passive drinking or endocytosis.Although both pollutants were distributed all over the body of D.magna,Cd accumulation was suppressed as a result of its quick dissociation from PAA-TiO2-NP during endocytosis.The Ca-dependent route of Cd accumulation was further evidenced by its distinct pattern of efflux from D.magna at low and high Ca leveL Overall,Ca reversed PAA-TiI2-NP effects on Cd bioaccumulation.This should not be neglected when evaluating the environmental risks of nanoparticles.