Toxic Effects Of ZnO ENPs On Dunaliella Salina Under Ocean Acidification

Since the industrial revolution,a large amount of CO₂ has been emitted into the atmosphere by human social production activities,of which about 1/3 has been absorbed by the ocean,causing ocean acidification.Ocean acidification not only directly affects the stability of marine ecosystem,but also indirectly changes the toxic effects of pollutants on marine organisms by changing their environmental behavior.With the widespread use of nano products,it is inevitable that a large number of engineered nanoparticles(ENPs)will enter the marine environment,which will have a negative impact on marine organisms and marine ecosystems.At present,a large number of studies focus on the effects of ocean acidification or ENPs alone on marine organisms,but in the actual marine environment,marine organisms are under the combined stress of ocean acidification and ENPs,and the effects on marine organisms are likely to be more complex and severe than when they are exposed to single conditions,The coupling effect of ocean acidification and ENPs on marine organisms has attracted much attention.However,there are few studies on the toxic effects of ENPs on marine organisms under ocean acidification.Therefore,in this study,We selected ZnO ENPs(<50 nm)as the experimental material,Dunaliella salina as the experimental organism,and setted one control group and five experimental groups as follows:control group(pH=8.2,0 mg/L),ion group(pH=8.2,=0.70 mg/L),nano group(pH=8.2,=1 mg/L),acidification group(pH=7.4,0 mg/L),acidification ion group(pH=7.4,=0.92 mg/L),acidified nano group(pH=7.4,=1 mg/L),in which the Zn²⁺concentrations of seawater ion group and acidified ion group were set according to the experimental results of ZnO ENPs ion release.In this paper,the environmental behaviors of ZnO ENPs in seawater under normal and acidified conditions were studied;The effects of ZnO ENPs on the growth,metabolism and cell morphology of Dunaliella salina;Effects of ZnO ENPs on antioxidant system of Dunaliella salina under ocean acidification.The results are as follows(1)The average hydraulic diameters of ZnO ENPs in pH=8.20 and pH=7.47seawater after 96 h were 813.87 nm and 733.23 nm,respectively,which indicated that ZnO ENPs would agglomerate rapidly in seawater,and acidification would promote the dispersion of ZnO ENPs.This is mainly because the p I of ZnO ENPs in seawater is 10.27,the pH of acidification group is farther from the isoelectric point,the surface charge of acidification group is higher,the repulsive force between electric double layers is stronger,and the agglomerative property is weakened,The dispersion between particles is better;When the pH value of ZnO ENPs was 8.20 and 7.47,the release concentration of Zn²⁺was 0.70 mg/L and 0.92 mg/L after 96 h,respectively,indicating that the ion release ability of ZnO ENPs was enhanced under acidification conditions.(2)Acidification of seawater enhanced the potential activity of photosystem II of Dunaliella salina,promoted the growth of algal cells and increased the content of chlorophyll;Zn²⁺and ZnO ENPs inhibited the potential activity of PS II and the growth of Dunaliella salina cells.When chlorophyll was stressed,chlorophyll b and carotenoids would respond and play the role of scavenging ROS,and acidification enhanced the toxic effect of ZnO ENPs on Dunaliella salina;Under acidification conditions,ENP_ion and ENPparticle contributed 29%and 71%to the growth inhibition of algal cells,42%,58%and 29%,71%to the chlorophyll-a content and carotenoid inhibition,and 7%and 93%to the potential activity of photosystem II,respectively,The inhibition of ZnO ENPs on Dunaliella salina growth is mainly due to the size effect of ENPs rather than the ion effect.(3)At pH=8.20,compared with the control group,the total EPS of Dunaliella salina exposed to Zn²⁺and ZnO ENPs increased by 37.1%and 53.2%,respectively.At pH=7.47,compared with the control group,the total EPS of Dunaliella salina exposed to Zn²⁺and ZnO ENPs increased by 4.5%and 25.1%,respectively.(4)Dunaliella salina is oval in shape with two flagella at the front.Zn²⁺can cause the rupture and deformation of Dunaliella salina cell membrane,loose and broken thylakoid lamellar structure,apoptosis and cavity formation of some organelles;ZnO ENPs can lead to serious rupture of Dunaliella salina cell membrane,serious oxidation of thylakoids;Acidification enhanced the damage of ZnO ENPs on the microstructure and ultrastructure of Dunaliella salina cells.(5)Under acidification,the activities of SOD and cat of Dunaliella salina decreased,the content of MDA increased,and the inhibition of antioxidant defense system enhanced;Under seawater conditions,the activities of SOD and cat in Dunaliella salina cells exposed to ZnO ENPs were inhibited,resulting in the increase of MDA content.However,under acidification conditions,the inhibition effect of ZnO ENPs on the activities of antioxidant enzymes in Dunaliella salina cells was greater,and the oxidative damage was stronger;Under acidification condition,the contribution rates of the ion effect and size effect of ZnO NPs to the SOD,cat and MDA of Dunaliella salina cells were 65%and 35%,76%and 24%,76%and 24%,respectively,which indicated that the ion effect of ZnO ENPs was the main one to inhibit the antioxidant system of Dunaliella salina under acidification condition.In conclusion,ocean acidification enhanced the inhibition of ZnO ENPs on the growth of Dunaliella salina,the structural damage of Dunaliella salina cells and oxidative stress,mainly because the hydraulic diameter of ZnO NPs decreased and the ion release increased under acidification conditions.Acidification enhanced the ion effect and size effect of ZnO ENPs on Dunaliella salina.