Acceleration Of Polystyrene Microplastics On The Toxicity And Accumulation Of Nano-ZnO In Daphnia Magna

As a kind of widely used artificial nano-metal oxide,Zinc Oxide nanoparticles（nano-ZnO）have toxic effects on aquatic organisms,and could be accumulated in organisms along the food chain after entering into aquatic environment.Due to the complexity of the aquatic environment,the toxic effects of nano-ZnO can be affected by coexisting pollutants.Microplastics（MPs）,as a new global contaminant of concern,the toxic effects and bioaccumulation of nano-ZnO are still unclear.Daphnia Magna,an important link in the food chain,is often used as a model organism for toxicity testing.In this study,the toxicity of nano-ZnO and PS MPs single system on Daphnia magna（acute toxicity,oxidative stress,digestive physiology,etc.）were investigated.Furthermore,the effect of the biological accumulation of zinc in D.magna by PS MPs,were studied.In addition,the joint effect and damage of intestinal microstructure of D.magna by nano-ZnO and PS MPs combined system were explored.The main research conclusions of this paper are as follows:（1）The single nano-ZnO system has toxic effects on the D.magna with the EC₅₀of 0.70 mg/L.nano-ZnO inhibited the swimming speed,movement distance and heart rate of D.magna.nano-ZnO induced oxidative stress in D.magna,and inhibited the digestive enzyme activities.The accumulation of nano-ZnO in the D.magna also increased with the increase of the concentration.nano-ZnO mainly accumulated in the digestive tract and intestinal tract in D.magna,and at high concentrations,nano-ZnO adhered to the antennae,limbed and body surfaces of D.magna,thereby inhibiting the activities and physiological activities of D.magna.（2）PS MPs with two different particle sizes both had toxic effect on D.magna.The EC₅₀of 3μm and 0.1μm PS MPs on D.magna were 61.45 mg/L and 21.71 mg/L,respectively.Both sizes of PS MPs reduced the swimming speed,movement distance and heart rate of D.magna,and induced oxidative stress and reduced the activity of digestive enzymes in D.magna.The distribution of PS MPs of the two particle sizes in D.magna,are similar,at low concentrations,it mainly accumulates in the gut of the D.magna,with the increase of the concentration,not only the accumulation in the gut of the D.magna gradually increases,but also in the antennae,thorax,appendages and body surfaces.（3）In the combined system of nano-ZnO and 3μm and 0.1μm PS MPs,the EC₅₀of nano-ZnO is 0.60 mg/L and 0.38 mg/L,respectively.The combined toxicity evaluation of nano-ZnO and PS MPs with two particle sizes were synergistic Compared with single nano-zno,the swimming speed and moving distance of D.magna in the composite system decreased with the increase of nano-zno concentration,the addition of PS MPs made the swimming speed and the moving distance of the D.magna decrease.The ROS level and SOD enzyme activity in the complex system were higher than those of the single nano-ZnO system.However,the lipid peroxidation levels in the D.magna were not significantly different between the complex system and the single system.The activities of trypsin and amylase in the complex system were lower than those of the single nano-ZnO system.Compared with the single system,the lipase activity at low concentrations（0.2,0.4 mg/L）showed a downward trend.At higher concentrations（0.8 mg/L）,the lipase activity increased slightly compared with the single system,but the lipase activity increased slightly.The activity was still lower than the blank control.（4）PS MPs promoted the bioaccumulation of Zn in D.magna but reduced the discharge of Zn.When the concentration of nano-ZnO was 0.4 mg/L,the two PS MPs with 0.1μm and 3μm increased the Zn bioaccumulation by 13.3%and 7.4%of the control,respectively,while reduced the discharge of Zn by 18.3%and 8.7%,respectively.The results showed that PS MPs with small particle size facilitated the bioaccumulation of Zn by D.magna more than large particle size.nano-ZnO,PS MPs and their combined systems damaged the ultrastructure（including microvilli,nuclei,mitochondria,etc.）In addition,it was found that in the nano-ZnO and PS MPs combined system,the dissolution rate of Zn²⁺was lower than that of the single nano-ZnO system,and PS MPs promoted the agglomeration behavior of nano-ZnO.