The Characteristics Of Microplastics In Mussels And Their Combined Toxic Effects With Typical Pollutants

Microplastics?particles less than 5 mm?pose a potential threat to the ecological environment and human health due to their wide distribution,difficulty in degradation,easy adsorption of pollutants in the environment and easy access to organisms.Microplastic pollution was listed as a new environmental issue and challenge facing the world by the United Nations Environment Program in 2011.Although a large number of studies have confirmed that microplastics are widely present in various media in the ocean,there is currently no uniform standard technical method for monitoring and ecological risk assessment of marine microplastics.Seabirds and sea turtles have been widely used to monitor plastic fragments larger than 1 mm.However,there is currently no effective indicator species for the monitoring of smaller size of microplastics.Mussels have been used as biological indicator species for traditional contaminants for more than 40 years,and many indoor simulation experiments have shown their sensitivity to microplastics.In addition,the important role of mussels in the food chain and their status as one of the main seafood products have made them excellent indicator species for assessing the ecological and healthy risks of marine microplastics.In this thesis,mussels are used as model organisms,and the following studies have been carried out.In order to establish the analysis method of microplastics in bivalves and explore whether bivalves in China have suffered from microplastic pollution,nine kinds of common marine bivalves from Chinese fishery markets were investigated firstly.The abundance,type,size and chemical composition of microplastics were characterized.The results showed that the number of microplastics in 9 kinds of bivalves could reach2.1-10.5 items/gram soft tissue,and Scapharca subcrenata contained the highest abundance of microplastics?about 10 items/g?.The investigated bivalves contained many different types of microplastics including fibers,fragments,and spheres.Fiber was the dominant type among the microplastics detected,accounting for more than half of the total number of microplastics in 8 bivalves.Interstingly,the number of spheres was the most in Alectryonella plicatula,accounting for 60%.The most common size range was 5-250?m,which accounted for 33-84%in nine species.The results of this study indicated that the investigated bivalves contained high levels of microplastic contamination and many species was reported for the first time,posing a potential threat to human health.In the above study,we established a method for the separation and identification of microplastics in bivalves,and then further studied the characteristics of microplastics in mussels with known sources and their environmental correlations.An investigation was conducted on mussels?Mytilus edulis?sampled or purchased from China and U.K.It involved 22 sites from China and 16 sites from U.K.The results showed that the mussels contained many different types of microplastics including fibers,fragments,films and spheres.The samples from these two countries had similar characteristics in terms of type and size distribution.Fiber was the most common type,followed by fragment.Among the microplastics detected,the most common size range is below 250?m.The difference is that cellophane?CP?,polyethylene terephthalate?PET?and polyester?PES?were the dominant polymertypes in Chinese samples,while PES,rayon,Polyethylene?PE?and polypropylene?PP?were the main polymers in British samples.In terms of abundance,the abundance of microplastics in mussels from China was significantly higher than that from U.K.by weight.There is no significant difference in abundance of microplastics in mussels between two countries by individual.By comparation,the abundance of microplastics in wild mussels was significantly higher than that in cultured mussels from both China and U.K.The abundance of microplastics in mussels from heavily polluted areas was significantly higher than that in mussels from slightly polluted areas.The artificially shelled mussels purchased from supermarkets contained significantly higher levels of microplastic contamination than live mussels.The types of microplastics in mussels were consistent with that in living waters.The proportion of smaller particles?less than 250?m?in mussels was higher than that of water,indicating mussel's selectivity for smaller sized microplastics in seawater.In general,the microplastic pollution in mussels from China and U.K.maintained at a relatively stable level,and was closely related to the environmental interference degree caused by human activities.The microplastics in mussels can reflect the microplastic characteristics in their living waters to a certain extent.Hence,it is proposed to use mussels to monitor marine microplastic pollution.This study conducted a large-scale investigation on microplastics in mussels and revealed its environmental relevance for the first time in the world.An international comparison study was conducted under the same research method for the first time.In addition,scanning electron microscope?SEM?was applied to distinguish microplastics from diatoms for the first time,which is important for the identification of microplastics.We have characterized the microplastics in mussels through previous field investigations,and the assessment of ecological and health risks requires us to explore the toxicological effects associated with them through indoor simulation experiments.Toxicological studies can be used to find indicators sensitive to microplastics in mussels for environmental monitoring and ecological risks can be assessed through toxic effects at the same time.In the real environment,microplastics ingested by organisms contain or adsorb a variety of pollutants,thereby posing a potential risk to marine organisms and human health.In order to explore this issue,we conducted the following experiment.Mussels?Mytilus edulis?were exposed to CdCl₂ and Polyvinyl chloride?PVC?microparticles.Lysosomal membrane stability?LMS?and the differential expression of three genes including metallothionein isoform 20-IV?MT-20?,catalase?CAT?and pyruvate kinase?PK?were selected as biological indicators to test whether microplastics have an effect on the absorption,accumulation and toxicity of heavy metals in organisms.After seven-day exposure,the tissue sections showed microplastics entering the gills and digestive glands.However,microplastics had no effect on the bioavailability of cadmium.The LMS of the two exposed groups with cadmium chloride was significantly lower than that of control group,while the PVC alone exposed group did not have a significant effect.The expression levels of MT-20in the digestive glands of mussels in the two exposed groups involved in cadmium chloride increased significantly,but there was no significant difference between the two groups,indicating that PVC particles didn't affect the toxicity of cadmium.There were no significant differences between the expression of CAT and PK genes in all experimental groups.In general,PVC particles did not have a significant effect on mussels,nor did they significantly affect the absorption,accumulation and toxicity of cadmium.Since biomarkers sensitive to microplastics were not found and microplastics did not affect the bioavailability and toxicity of heavy metals in the above study,it was necessary to select different biological indicators and continue to explore the combined toxic effects of microplastics and other typical pollutants.Hence,another typical organic pollutant,estrogen,was selected to continue to explore the relevant toxicological effects.Mussels?Mytilus edulis?were exposed to estradiol-17??E2?and microparticles?polyvinyl chloride+polystyrene?.Condition index?CI?of mussels,cell activity of haemocytes and the differential expression of Estrogen receptor gene?ER2?in the gonad were selected as biological indicators to test whether microplastics have an effect on the toxicity of estrogen in organisms.Both microplastic isolation experiments and tissue sections showed that microplastics entered the tissues such as gills and digestive glands.Neither microplastic nor E2 exposure had a significant effect on the condition index of the mussels.The expression of ER2 in the gonads was not significantly affected by E2 or microplastics.The cell activity of haemocytes in both microplastic exposed groups was lower than that of the control group,but only the combined exposure group had significant differences.This indicated that the mixing of the two contaminants would lead to enhanced toxicity.Therefore,the combined toxicity of estrogen and microplastics may have a synergistic effect.This chapter provides a scientific basis for the selection of microplastic-specific indicators and their ecological risk assessment.In this thesis,our investigations of microplastic pollution in mussels and the exploration of related combined toxic effects have confirmed that microplastics are widely existent in mussels and the corresponding combined toxic effects can not be ignored,revealing the ecological risks and human health risks of microplastics in mussels.Meanwhile,this study has shown that microplastic pollution in mussel is closely related to the environment and it can reflect the characteristics of microplastics in environmental waters to some extent.It is therefore proposed to monitor marine microplastics with mussels.This study provides a theoretical basis for the selection of indicator species of marine microplastic pollution,and provides a scientific basis for establishment of biomonitoring and ecological risk assessment methods for microplastics,making an important contribution to the maintenance of our marine ecosystem's safety and the formulation of relevant policies and laws.