| Microplastics(MPs),small plastic particles less than 5 mm in diameter,are considered as ubiquitous pollutants in the marine ecosystem.Despite many reports on the effects of MPs on marine animals,their effect on sea cucumbers is still unclear.With the increasing of MPs in the ocean,especially from anthropogenic activities,together with their hazardous properties,their effect on sea cucumbers need to be clarified.To know whether these hazardous materials can influence the biological functions and the development of sea cucumbers and whether the ingestion of these materials by sea cucumbers has implications for food web since the internal viscera of sea cucumbers can be consumed by humans.In this thesis,MPs ingestion has been quantified,for the first time,in the sea cucumber Apostichopus japonicus(A.japonicus)from the field.Then,MPs concentrations and shapes that were detected from the field were used to investigate MPs effect on the physiology and growth performance of A.japonicus in the laboratory.The main results are as follow:1.Microplastic ingestion by the farmed sea cucumber Apostichopus japonicus inChinaMPs were isolated from the sea cucumber A.japonicus that was collected from eight farms along the Bohai Sea and the Yellow Sea in China.Accordingly,the ingested MPs by A.japonicus ranged from 0 to 30 particles intestine-1,while MPs abundance in the sediment ranged from 20 to 1040 particles kg-1 of dry sediment.The ingested microplastics did not correlate with the animal body weight but were site-dependent(Pearson:r=0.767),suggesting that sea cucumber might serve as a sentinel for microplastic pollution monitoring in the sediment.After filtering the coelomic fluid,the extracted microplastics from the coelomic fluid ranged from 0 to 19 particles animal-1.Thus,we speculated that MPs may transfer to the coelomic fluid of sea cucumber.This study revealed that A.japonicus ingested MPs as suitable with their mouth open,and A.japonicus might serve as a sentinel for microplastic pollution monitoring in the sediment.Besides,MPs might transfer to the coelomic fluid of A.japonicus.2.Heavy metals in sediment,microplastic,and sea cucumber Apostichopusjaponicus from farms in ChinaMPs adsorb pollutants from the ocean.Such pollutants may transfer to the biota after MPs ingestion.To determine whether there is a relationship between MPs,A.japonicus,and hazardous pollutants such as heavy metals in the field,eight heavy metals(As,Cd,Cr,Cu,Mn,Ni,Pb,and Zn)were measured in the sediment,the isolated microplastics from the sediment and the body wall of sea cucumbers from farms in China.Accordingly,the heavy metal concentrations in the sediment were below the class I upper limit of Chinese sediment quality guidelines.Among heavy metals,the median concentrations of Cd and As were higher in the body wall than in the corresponding sediment.Additionally,the median concentrations of Cd,Pb,and Zn were higher on the microplastics than in the corresponding sediment.Furthermore,there was no significant correlation among heavy metals in sediment,sea cucumber and microplastics.The results indicate that heavy metals associated with the MPs are not an indicator of the heavy metal concentrations in the sea cucumber or the sediment.3.The sea cucumber Apostichopus japonicus ingests suspended particles from thewater through the mouthAn unexpected pathway of MPs particles was detected in A.japonicus under MPs exposure.A.japonicus ingested particles from the water through the mouth,which revealed previously undiscovered nutritional characteristics of A.japonicus.Particle ingestion by the sea cucumber increased with increasing particle concentrations in the water(Pearson:r=0.808).The removal rates of particles ranged from 0.3-0.9 L-1 h-1.4.Microplastic fibres transfer from the water to the internal fluid of the seacucumber Apostichopus japonicusThe previous field study showed that MPs in the form of microfibres(MFs)might transfer to the coelomic fluid of A.japonicus.Therefore,the possibility of MPs transferring to the coelomic fluid of A.japonicus was examined in the laboratory.MFs were captured during their transfer from the water to the coelomic fluid through the respiratory tree.A.japonicus ingested in the MFs along with the water during respiration;the MFs got stuck in the respiratory tree or transferred to the coelomic fluid.The transferred MFs increased during 72 h of exposure and persisted for 72 h after the transfer to clean water.Among the immunity indices,lysozyme(LZM)levels increased in response to the transferred MFs,which confirms the defensive role of LZMs against strange substances.Additionally,non-significantly decreased levels of total antioxidant capacity(T-AOC),malondialdehyde(MDA),peroxidase(POD)and phenol oxidase(PPO)were observed at 24 h and 48 h post-exposure,suggesting minimal oxidative imbalance.Furthermore,there were no significant changes in the speed and the total distance moved by A.japonicus post-transfer.This study revealed that MFs transfer and accumulate in the coelomic fluid of A.japonicus.5.Molecular responses of the respiratory tree during microfiber transferrinprocess in the sea cucumber Apostichopus japonicusTo examine the molecular responses of the respiratory tree during the MFs transferring process,time course RNA-Seq technique was used to construct a large-scale dynamic expression profile in the respiratory tree of A.japonicus in response to microplastic exposure.A.japonicus was exposed to MPs(2 MP m L-1)at several time points of 0 h(control),0.5 h(T1),2 h(T2),4 h(T3),8 h(T4),16 h(T5).Differentially expressed genes(FDR<0.05 and|log2FC|>1)increased with time exposure,which accounted for 238,225,757,183,and 885 at T1,T2,T3,T4,and T5 respectively.Gene ontology analysis showed that the DEGs associated with cellular,metabolic,and single-organism processes in the category‘biological process';and the DEGs associated with the cell,cell part,organelle in the category‘cellular component'were notably abundant,also catalytic activity in the category of molecular function was enriched.The DEGs were mapped the KEGG database,and apoptosis pathways were mostly and significantly enriched among treatments.Accordingly,MFs exposure altered gene expression and induced the apoptosis process in all treatments.Besides,microplastic exposure can suppress the metabolic pathways by down-regulating gene expression,which increased by time exposure of microplastics.6.Chronic exposure to microplastic ingestion in the sea cucumber ApostichopusjaponicusTo examine whether the ingestion of MPs can affect the growth performance of A.japonicus,the chronic exposure to MPs ingestion along with the sediment in A.japonicus was investigated for 60 days,with two size categories.The initial weight of big animals was 83.23±1.85 g(mean±SD)and small animals initial weight was 15.47±1.73 g(mean±SD).The sea cucumbers were exposed to environmentally relevant MPs concentrations of 0.6 MF g-1 and 1.2 MF g-1.A concentration of 10 MF g-1 was also used to examine whether increasing MPs in the sediment can affect the sea cucumber.After60 d of exposure,the specific growth rate(%/d)in the small group of the control compared to the treatments was 0.99±0.17,0.96±0.18,0.92±0.15,and 0.83±0.02(Mean±SE)respectively;there was no significant difference between groups(P>0.05).The faeces production rate(g ind./d)was 0.6±0.06,0.7±0.06,0.5±0.07,and 0.6±0.08(Mean±SE)respectively;there was no significant difference between groups(P>0.05).While the specific growth rate(%/d)for the big group of the control compared to the treatments was 0.63±0.11,0.50±0.15,0.51±0.08,and 0.49±0.14(Mean±SE)respectively;there was no significant difference between groups(P>0.05),and the feces production rate(g ind./d)was 2.27±0.32,2.00±0.16,1.98±0.49,and 2.02±0.35(Mean±SE)respectively;there was no significant difference between groups(P>0.05).Therefore,there was no significant effect of thechronic exposure of MPs on the growth performance of sea cucumbers.The specific growth rate of the big or small animals was not significantly different from the control as well as the faeces production rate. |
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