Study On Toxic Effects Of Polystyrene Microplastics On Early Developmental Stages Of The Marine Medaka Oryzias Melastigma

Microplastics(MPs)are defined as plastic spheres,fragments,fibers and films with diameter less than 5 mm.MPs are widely distributed in marine environment and have become one of the hot-spot issues of marine pollution.MP pollution is a potential threat to marine organisms,while studies of the toxic effects of MPs on aquatic organisms,especially marine fish,are still in a preliminary stage in terms of its mechanisms.The core issues,such as effective processes and mechanisms are still unclear.The toxic effects of different sizes(0.05,0.5 and 6.0 μm)and concentrations(102,104 and 106 particles/L)of PS-MPs on the early developmental stages(including embryo and larva)of O.melastigma and its possible mechanisms were investigated.This study revealed that the toxic effects of PS-MPs with environmentally relevant concentrations on the marine medaka embryos were size-dependent,while the mechanism of toxic effects of PS-MPs on the marine medaka larvae was proposed.The present study provides technical support for assessment,monitoring and early warning of the ecological risk of MPs in marine environment,and are of great theoretical significance for further understanding of the effects of MPs on the growth and development of marine fishes.The main results are as:1.Toxic effects of PS-MPs on the embryos of marine medakaNo obvious lethal effects were observed on the marine medaka embryos exposed to different sizes of PS-MPs(0.05,0.5 and 6.0 μm)with a concentration of 106 particles/L for 20 d.The hatching time and hatching rate of embryos were decreased when the embryos exposed to PS-MPs with a size of 0.05 μm,although no statistical significance was observed.The hatching time and hatching rate were significantly shortened of the embryos exposed to 0.5-and 6.0 μm-sized PS-MPs,and size-dependent toxicity was found in the embryos exposed to PS-MPs with the environmentally relevant concentrations:the larger sizes of MPs have greater deleterious effects on the embryos,and more impurities attached on the surface and chorion villi of the embryos exposed for 9 d.The heartbeat rates of the embryos increased with the extension of exposure time,and the earlier development stages of the embryos were more sensitive to PS-MPs.Compared with the control,no significant changes were observed in the relative expression levels of cyp1a1,immune and inflammatory genes of the embryos exposed to different sizes of PS-MPs.However,the relative expression levels of cat were significantly down-regulated of the embryos exposed to 6.0 μm PS-MPs for 9 d.In addition,the chorion plays an important protective role in the marine medaka embryos under PS-MPs exposure,the PS-MPs with larger size than that of the chorion pore hardly entering into the embryos,which attach on the surface or villus of the embryos.2.Toxic effects of PS-MPs on the larvae and the speculated mechanismsPS-MPs accumulation can be found in the digestive tract of marine medaka larvae exposed to different concentrations of PS-MPs(102,104 and 106 particles/L)with a size of 6.0 μm for 14 d,while the numbers of PS-MPs accumulated in the larvae were positively correlated with the concentrations and exposure time.However,no obvious mortality was observed of the larvae.The body lengths of the larvae exposed to a high concentration of PS-MPs(106 particles/L)for 14 d were significantly lower than that of the control,which indicates that high concentration of PS-MPs inhibited the growth of larvae,and medium(104 particles/L)and high(106 particles/L)concentrations of PS-MPs induced hyperactive swimming behavior of the marine medaka larvae.This might be the result of PS-MPs stimulation on the larvae and therefore caused inflammatory response.In addition,under the exposure of medium and high concentrations of PS-MPs,the epithelial cells and villi in the intestinal appeared to be partially sloughed off,while the hepatocytes showed vacuolization as well as nuclear and cytoplasmic alterations.The relative expression levels of mucin,jak,stat-3 and nf-κb were significantly up-regulated,which suggest that PS-MPs exposure may lead to activation of the signaling pathways such as JAK/STAT and NF-κB.In summary,the exposure of larvae to PS-MPs with environmentally relevant concentrations caused body length decrease,abnormal swimming behavior,intestinal and liver tissue damages.PS-MPs exposure inhibited the expression of mucin genes,induced inflammatory and immune responses and activated JAK/STAT and NF-κB signaling pathways,which may eventually affect the normal development of the marine medaka larvae.