Combined Toxic Effect Of Microplastics And Heavy Metals(Cd,Pb,Zn)on Marine Medaka

In recent years,microplastics(MPs)have attracted more and more attention because of their universality and toxicity.However,water environment pollution is not only caused by the microplastics,but frequently accompanied by a variety of toxic pollutants.Currently,studies have shown that microplastics are vectors for heavy metals(HMs)contamination from the marine environment,which may affect the bioavailability of heavy metals and cause higher environmental risks.However,the specific toxic effects and mechanisms are still unclear.Therefore,in this study,polystyrene microplastic and typical heavy metals(Cd,Pb,Zn)at environmentally relevant concentrations(ERCs)in the ocean were exposed to five-month-old adult marine medaka(Oryzias melastigma)for one month.Then,the individual and combined toxicogenetic effects of MPs and HMs were investigated.The distribution pattern of HMs and MPs in marine medaka,especially in intestinal and gonad,was observed,and their toxicities to the fish offspring were also elucidated.Finally,the molecular toxic mechanism of MPs and HMs was clarified.The main conclusions of this study are as follows:(1)Polystyrene MPs were mainly distributed in the intestinal of marine medaka,and were completely metabolized within 48 h without feeding.The highest concentrations of Cd,Pb and Zn were found in the intestinal,followed by the accumulation of Cd and Pb in the liver.In addition,MPs were observed to increase the accumulation of Zn in the intestinal.(2)After one month of exposure to environmentally relevant concentrations of MPs and HMs,significant alterations of intestinal microbiota were found.Exposure to the MPs reduced the diversity and abundances of intestinal microbiota.This study showed that the MPs-HMs treatment caused a higher pollution load on the gut of the marine medaka,and triggered more significant changes of specific species and gut function in the males.(3)During the gonadal development,HMs and MPs-HMs treatments resulted in empty follicles(EF)and follicular atresia(FA),and altered the gene expression levels related to hypothalamic-pituitary-gonadal(HPG)axis.In short,this study demonstrated that the reproductive disturbance was mainly caused by the HMs,but the combination of MPs and HMs did not strengthen the risk to the gonad development of marine medaka.(4)During the growth of marine medaka larvae(22dpf),exposure to the HMs and MPs-HMs affected the survival and hatching ratios of the larvae,and the MPs treatment showed no significant effect.Both the MPs and HMs treatments changed the expression levels of immune-related genes in larvae,and the MPs and MPs-HMs treatments induced the suppression of immunity,while exposure to HMs enhanced the inflammatory response in the larvae of marine medaka.These results showed that there was certain impact of ERCs of microplastics on bio-toxicity of heavy metals.While the impact was complicated and could not merely be defined with positive or negative one,it was necessary to carry out further study to explore the mechanisms.