Toxicity Of Polyvinyl Chloride Microplastics On Marine Medaka(Oryzias Melastigma) Embryos

With the widespread application of plastic products,a large amount of plastic waste will inevitably enter the marine environment.Marine plastic pollution has become a major global environmental issue alongside global climate change,ozone depletion,and ocean acidification.Among them,plastic fragments with a particle size of less than 5 mm or granules（microplastics）are new pollutants that have attracted worldwide attention.Due to its small particle size,stable chemical properties,and strong hydrophobicity,it is easily ingested by marine organisms,accumulates and migrates in different tissues of the organism,and occurs in the marine food chain.The existence of living organisms has a negative impact and may even threaten the stability and health of the entire marine ecosystem.However,the current research on microplastics mainly focuses on the toxic effects and toxic mechanisms of high-concentration commercial microplastic standards on marine organisms,while the environmental characteristics of microplastics on marine organisms have been seldom studied,and its mechanism of toxicity is unclear.Therefore,in this study,the embryos of the marine model organism Oryzias melastigma were used as experimental organisms,and primary polyvinyl chloride microplastics（microplastic standard）and secondary polyvinyl chloride microplastics（broken microplastic）were used as experimental materials,set up the control group（without polyvinyl chloride microplastics）,the LPMP group（low concentration primary polyvinyl chloride microplastics group,1×10³particles/L）,and the HPMP group（high concentration primary polyvinyl chloride microplastics group,1×10⁶particles/L）,LSMP group（low concentration secondary polyvinyl chloride microplastics group,1×10³particles/L）and HSMP group（high concentration secondary polyvinyl chloride microplastics group,1×10⁶particles/L）.In this paper,the environmental behaviors of surface and morphology,suspensibility,average hydraulic diameter,and Zeta potential of primary and secondary polyvinyl chloride microplastics under seawater conditions are studied.Under the exposure of polyvinyl chloride microplastics average heart rate,incubation time,incubation rate,malformation rate,and malformation type ratio of marine medaka embryos.Finally,by measuring the surface oxygen flow of marine medaka embryos and observing the interaction between microplastics and embryos,the mechanism of toxicity of polyvinyl chloride microplastics to marine medaka embryos was elucidated.The findings are as follows:（1）The shape of primary polyvinyl chloride microplastics is more regular,mainly spherical,and the surface is smooth.Compared with primary microplastics,secondary polyvinyl chloride microplastics have an irregular shape,with a rougher,matte surface,obvious irregularities,and prominent edges.Under seawater conditions,the hydraulic diameter of primary and secondary microplastics increased,and both agglomerated.The average hydraulic diameter of the secondary microplastics is smaller than that of the primary microplastics,indicating that the primary microplastics have stronger agglomeration.The primary and secondary microplastics settled more in the first 30 min,reaching more than 50%,and the later settling tended to be stable,and the secondary microplastics had a higher content in the water layer;the primary and secondary polyvinyl chloride microplastics isoelectric points(p H_zpc)are 10.28 and 10.52,respectively,while the p H of natural seawater is 8.20,farther away from the isoelectric point of the secondary microplastics,thereby increasing the negative charge on the surface of the particles,resulting in a reduction in the hydraulic diameter of the particles and an improvement in suspension.Therefore,the secondary microplastics have smaller hydraulic diameter and stronger suspension performance.（2）After 25 days of exposure,it was found that the primary and secondary polyvinyl chloride microplastics did not significantly affect the mortality and hatchability of marine medaka embryos,which indicates that the polyvinyl chloride microplastics are less toxic and have no lethal effect;compared with the control group,the secondary microplastics group and the low concentration primary microplastics group significantly inhibited the hatching time of marine medaka embryos,and the secondary microplastics also significantly increased the average heart rate of embryos on the 5th day;in addition,it was also found that exposure to high-concentration secondary polyvinyl chloride microplastics increased the malformation rate of marine medaka embryos.Types of malformations include spinal deformity,pericardium edema,yolk sac edema,fin rot,heart stretch and craniofacial deformity.The types of malformations were significantly higher than those in the control group,the primary microplastics group,and the low concentration secondary microplastics group.The above results show that the secondary microplastics have a more negative effect on the medaka embryos than the primary microplastics group.（3）After 24 hours of exposure,it was found that the oxygen absorption flow rate of marine medaka embryos in the primary microplastic experimental group was significantly lower than that in the control group and the secondary microplastic experiment group,and a large number of primary microplastics were observed on the surface of the marine medaka embryos.It is indicated that a large amount of nascent microplastics adsorbed on the surface of marine medaka embryos and caused hypoxia in marine medaka embryos.It was also observed that the secondary microplastics attached to the surface of marine medaka embryos were irregular in shape,angular,and relatively sharp,causing mechanical damage to the surface of marine medaka embryos.In summary,the impact of secondary microplastics on the hatching of marine medaka embryos and the deformity rate and type of deformities are greater than those of primary microplastics,which indicates that the effect of secondary microplastics on the early development of marine medaka is greater than that of primary microplastics.This is mainly due to the irregular shape of the secondary polyvinyl chloride microplastics and the smaller hydraulic diameter,which causes mechanical damage to the medaka embryos,resulting in shortened hatching time and increased deformity rate.The effect of primary microplastics on marine medaka embryos is mainly due to the sedimentation performance of primary microplastics is greater than that of secondary microplastics.A large number of primary microplastics are adsorbed on the surface of embryos and cause hypoxia in marine medaka.Therefore,the mechanical damage of secondary polyvinyl chloride microplastics has a more negative impact on the medaka embryos than the negative effects caused by hypoxia caused by the adsorption of primary microplastics on the surface of embryos.