Persistent Toxicity And Mechanisms Of Silver Nanoparticles On Daphnia Magna

Silver nanoparticles are one of the most widely used nanomaterials,which can enter the water bodies through various routes and have potential impacts on ecological and environmental health of water bodies.Although there have been a lot of reports about the toxic effects of silver nanoparticles on aquatic organisms,the persistent toxic effects of silver nanoparticles on aquatic organisms are still unknown.In this study,Daphnia magna was used as the experimental organism,and the 24-hour exposure stage and 24-hour recovery stage were set respectively,two silver nanoparticles（AgNPs-l0nm and AgNPs-70nm）were selected and their silver ion supernatants（ctrl10nm and ctrl-70nm）were set as control group.By comparing the toxic effects of silver nanoparticles on Daphnia magna during the exposure stage and recovery stage,the persistent toxic effects and mechanism of silver nanoparticles on Daphnia magna were revealed.The main results are as follows:（1）The persistent toxic effect of silver nanoparticles on Daphnia magna was revealed by the method of measuring heartbeat rate and hopping frequency.The results showed that when Daphnia magna exposed to silver nanoparticles for 24 h was placed in clean medium for 24 h,its heartbeat rate and hopping frequency could not return to normal level,and even decreased continuously in high concentration group（13 μg/L）.These results indicate that silver nanoparticles have persistent toxicity to Daphnia magna.（2）The persistent accumulation effect of silver nanoparticles in Daphnia magna was revealed by the experiment of absorption and elimination of silver nanoparticles in Daphnia magna.The results showed that when Daphnia magna exposed to silver nanoparticles for 24 h was transferred to clean medium for 24 h,the silver accumulation in Daphnia magna in RAgNPs-10nm group and RAgNPs-70nm group（R denotes the recovery stage）decreased significantly,from 16.34 ± 1.59 μg/g and 26.41 ± 9.93 μg/g to 4.98 ± 1.91 μg/g and 9.14 ± 2.34 μg/g,but it was not completely eliminated.However,Daphnia magna exposed to silver ion supernatant for 24 hours was transferred to clean medium for 24 hours,there was no significant change in silver accumulation in Daphnia magna in Rctrl-10nm and Rctrl-70nm groups,from 3.91±1.71 μg/g and 4.52 ± 2.23 μg/g to 3.75 ± 2.41 μg/g and 3.03 ± 0.72 μg/g,indicating that silver ion is difficult to remove in Daphnia magna.（3）The persistent metabolites effect and toxic mechanism of silver nanoparticles on Daphnia magna was revealed by metabolomics technology.The results showed that AgNPs-10nm and AgNPs-70nm can induce significant changes in 53 and 54 differentially expressed metabolites,which mainly interfere with pathways related to the lipid metabolism in Daphnia magna,such as arachidonic acid metabolism,linoleic acid metabolism and alpha-linolenic acid metabolism.In the recovery stage,the number of differentially expressed metabolites induced by AgNPs-10nm and AgNPs-70nm increased significantly,reaching 71 and 110 respectively.In addition to interfering with lipid metabolism related pathways,it also affected pathways related to the digestive system,such as protein digestion and absorption,mineral absorption.