Combined Toxic Effects Of Microplastics And Cadmium On The Sea Cucumber Apostichopus Japonicus

Microplastics(MPs)and the heavy metal cadmium(Cd)are common pollutants found in marine sediments.Sea cucumber(Apostichopus japonicus),which feeds on sediments,is often exposed to the coexistence of these two pollutants.The combined toxic effect of MPs and Cd on A.japonicus has not been previously reported.This study explores the combined toxic effects of Cd and MPs on A.japonicus at both environmentally relevant concentrations(Cd: 0.50 mg/kg;MPs: 1000/kg)and high concentrations(Cd: 50 mg/kg;MPs: 100,000/kg).The study analyzes the effects of dietary exposure to the pollutants on body weight,Cd tissue accumulation,antioxidant enzyme activity,digestive enzyme activity,intestinal microbiota,and intestinal gene expression in A.japonicus.The main results of this study are as follows:(1)Effects of microplastics and cadmium stress on physiological characteristics of sea cucumbersAfter a 30-day stress experiment,the basic physiological indexes of A.japonicus were measured.The results showed that at environmental concentrations,MPs and Cd stress had less impact on A.japonicus.When A.japonicus was exposed to combined stress of MPs and Cd,the presence of MPs amplified the toxic effect of Cd.This in turn caused adverse effects such as decreased specific growth rate,increased activity of oxidative stress enzymes,and decreased activity of digestive enzymes.The group with the highest specific growth rate was the low-concentration MPs group(0.64±0.26 %/d),while the smallest group was the high-concentration MPs-Cd combined group(0.24±0.15 %/d).The Cd content of the respiratory tree was the highest,with the highest value appearing in the highconcentration Cd group(6.37±1.99 μg/g).In the high-concentration MPs-Cd combined group,MPs and Cd showed an antagonistic effect on the inhibition of A.japonicus lipase activity,but showed a synergistic effect on trypsin activity.(2)Changes of intestinal microorganisms in sea cucumbers under microplastic and cadmium stressThe study used 16 S rRNA to determine the gut microbes of sea cucumbers.The results showed that Cd and MPs could significantly alter the composition and diversity of the gut microbiota of A.japonicus,in which the abundance of Lactobacilli,β-bacteria,and Retrichia significantly changed.The intestinal community richness of the environmentally relevant concentration Cd group was lower than that of the other groups.The biosynthesis of phenylpropanol and the biosynthesis pathways of flavonoids and flavonols in sea cucumber intestine were significantly up-regulated in the high-concentration MPs group.In contrast,the isoquinoline alkaloid biosynthesis and flavonoid biosynthesis pathways were down-regulated in the high-concentration Cd and high-concentration MPs-Cd combined groups.(3)Intestinal mRNA response mechanisms of sea cucumbers under microplastic and cadmium stressThis study represents the first analysis of the transcriptome information of A.japonicus gut after exposure to both Cd and MPs.Under Cd stress,the expression of FASassociated death domain protein(FADD)in the intestine of A.japonicus was upregulated,while the expression of intestinal heparan sulfate 2-O-sulfotransferase increased in both the environmentally relevant concentration Cd group and the high-concentration Cd group.Under MPs stress,the expression imbalance of NLR family inflammasome and CASP6 genes led to an inflammatory response in the intestine of A.japonicus.The combined toxicity of MPs and Cd is not a simple additive effect,as MPs and Cd exhibit antagonistic effects during fatty acid metabolism,mainly manifested in inconsistent expression of SCD1 protein.MPs and Cd also exhibit antagonistic effects in lipid and carbohydrate metabolism pathways.TOLLIP and SELE genes may be key immune response genes of A.japonicus to Cd and MPs stress.Under the environmental concentration,microplastics and cadmium had less toxic effects on A.japonicus,and the effects of microplastics and cadmium alone or in combination on A.japonicus were not significant.However,under the stress of high concentrations of microplastics and cadmium,the combined stress of microplastics and cadmium inhibited the growth of A.japonicus.The presence of high concentrations of microplastics reduced the accumulation of cadmium in the respiratory tree tissues of A.japonicus.Microplastics and cadmium have a synergistic effect on the increase of oxidative stress-related enzyme activities in A.japonicus and the change of intestinal microbial function in A.japonicus.Microplastics and cadmium showed antagonism in the inhibition of lipase activity,as well as in the process of fatty acid metabolism and pathways of lipid metabolism and carbohydrate metabolism.