Enrichment And Toxic Effects Of Microplastics In Holothuria Atra

Due to its small size,lower mass,and difficult decomposition in nature,microplastics can easily enter the marine environment through atmospheric deposition or escape from water treatment systems.Microplastics can absorb other typical pollutants,which pose a potential threat to marine ecosystem and human health.The widespread existence of microplastic particles in the marine environment has already been proven by many studies,but the occurrence characteristics of microplastics in echinoderms were yet to be researched extensively.Therefore,we took Holothuria atra,a representative species of sea cucumbers in tropical coral reefs,as the research object and studied the accumulation and toxic effects of microplastics in H.atra.The contents and main conclusions of the study are summarized as follows:1.Characteristics of microplastics pollution in wild sea cucumber H.atra and its habitat in Yunlong Bay,WenchangThe microplastic pollution characteristics in H.atra and the surrounding environment were investigated in July and October 2019,January and May 2020 in Yunlong Bay,Wenchang.The results showed that microplastics were found in all samples of habitats and tissues,and the abundance decreased first and then increased from July 2019 to May 2020.Microplastics with a small size of 1-1000μm were dominant in every season.The proportion of small size microplastics in sediment and intestine was highest in October and January,and lowest in July and May,respectively.The proportion of small size microplastics in respiration tree and surface seawater was highest in July and lowest in October.Fiber microplastics were the most common in all the samples.The proportion of fiber microplastics in the intestine and surface seawater was the lowest in July and highest in January.The proportion of fiber microplastics in the respiration tree was the highest in July and lowest in January.The proportion of fiber microplastics in sediment was highest in May and lowest in January.The microplastics in the various samples were mainly transparent.The proportion of transparent microplastics in intestine and environmental sample was lowest in July,and highest in January and May,respectively.The proportion of transparent microplastics in the respiration tree was highest in July and lowest in October.The main components of microplastics in all the samples were cellophane,polyethylene terephthalate,polycarbonate,and polypropylene,and among of which the proportion of cellophane was highest in all the samples.The abundance of microplastics in the H.atra was higher than that in the environment（P﹤0.05）.The abundance of microplastics in the respiratory trees was positively correlated with that in surface seawater,but not significantly（Pearson:r=0.372,P﹤0.05）,and there was no significant correlation between the abundance of microplastics in intestines and sediments（Pearson:r=-0.273;P﹤0.05）.2.Effects of microplastics on immunity and digestive physiology of H.atraTo investigate the effects of microplastics on the immunity and digestion of sea cucumber,H.atra was exposed to four different concentrations（0,10²,10⁴,and 10⁶ items/L）of polystyrene microplastics.The results showed that microplastics exerted significant effects on the total coelomocyte count,phagocytic activity of coelomocytes,and activities of lysozyme（LZM）,acid phosphatase（ACP）,and superoxide dismutase（SOD）in coelomic fluid（P<0.05）.No significant impact on alkaline phosphatase（AKP）activity was observed.An increase in microplastic exposure led to a sustained increase in the phagocytic activity of coelomocytes.Meanwhile,the total coelomocyte count and the activity of ACP increased as the microplastic concentration increased from 0 items/L to 10⁴ items/L,and decreased at 10⁶items/L.In addition,the activities of LZM and SOD increased as the microplastic concentration increased from 0 items/L to 10² items/L,and decreased from 104 items/L to10⁶ items/L.The amylase（AMS）activity in the intestine of H.atra was significantly affected by the concentration of microplastics（P<0.05）.However,no significant effect of microplastic concentration was observed on trypsin and lipase activities（P>0.05）.The AMS activity increased and reached the maximum level when the microplastic concentration increased to 10⁴ items/L and then decreased at 10⁶ items/L.The trypsin activity increased continuously as the microplastic concentration increased,while the lipase activity in the three concentration groups of microplastics was lower than that in the control group.Thus,an increase in the microplastic exposure induced an immune defense in H.atra,while high concentrations of microplastics exposure may damage the morphological structure of coelomocytes,thereby leading to decreased resistance of H.atra and disrupting its physiological processes.Meanwhile,H.atra tended to choose nutrient materials that are easy to be digested,such as amylum and protein,rather than lipids.Such reaction in digestion enables H.atra to quickly gain the energy needed to adapt to environmental changes.3.Effects of microplastics on respiration and metabolism of H.atraTo investigate the effects of microplastics on the respiration and metabolism of sea cucumber,H.atra was exposed to four different concentrations（0,0.0004,0.04,0.4 mg/L）of polystyrene microplastics.Results showed that the respiratory cavity volume of H.atra decreased after 48 h exposure to microplastics compared with that without microplastics.The contraction time of the respiratory cavity of the H.atra increased in the 0.0004 and 0.04mg/L treatments,which may be related to the blockage of the respiratory tree branches.Correspondingly,the oxygen consumption rate of 0.4 mg/L treatment was higher than that of the control group after exposure except for 6 hours.The results showed that microplastics exerted significant effects on the activities of Na⁺/K⁺-ATPase and Ca²⁺/Mg²⁺-ATPase in respiratory tree of H.atra（P<0.05）.No significant impact on Ca²⁺-ATPase and Mg²⁺-ATPase activity was observed.With the increased of microplastics concentration,the activity of ATPase increased first and then decreased.A low concentration of microplastics exposure made sea cucumber produce more ATPase to maintain osmotic balance and cope with the rapid consumption of ATP caused by metabolism and substance conversion,while a higher concentration of microplastics exposure may damage cell membrane or the structure of ATPase protein,thereby leading to a decrease in enzyme activity.