| Microplastic(MP,sizes<5 mm)is an emerging environmental pollutant and has received increasing attentions around the world.In recent years,occurrence of microplastics has been widely reported in various marine environmental matrices.Microplastics can be accumulated in body tissues of organisms and cause adverse effects,thereby bringing threats to the ecosystem and human health.Previous studies have shown that microplastics were detected in organisms at every trophic level of marine food web.However,knowledge is still limited about loading,transport and elimination characteristics of microplastics within body tissues of organisms.In this thesis,two typical kinds of marine organisms(macroalgae and mussels)that are closely related to human health were chosen as study objects.In combination with methodologies of field investigation,laboratory analysis and simulation experiments,this thesis aims to investigate microplastic pollution features in macroalgae and its related seaweed products.Meanwhile,kinetics of microplastics in different tissues of mussels and possible new routes of microplastic uptake by mussels were also investigated in the same way.Firstly,this thesis investigated occurrence of plastic debris including microplastics in 5 species of macroalgae commonly found in beaches of Chinese eastern coast.These species were consisted of Gracilaria lemaneiformis,Chondrus ocellatus,Ulva lactuca,U.prolifera,and Saccharina japonica.Results showed that there were 5 loading patterns of different sizes of plastics(macroplastics,25 mm–1 m;mesoplastics,5–25 mm;microplastics,1 ?m–5 mm)in macroalgae,including twinning,wrapping,attachment,embedment and entrapment by epibionts.This indicated that marine macrophyte had complicated and diverse pathways for loading plastics in the coastal environment.According to direct observations through the non-digestion method,all 3 size classes of plastics were found in the macroalgae:macroplastics,0–201.5 items/kg dry weight;mesoplastics,0–1178 items/kg dry weight;microplastics,0–355.6 items/kg dry weight.These plastics were dominated by fibers(52.2%)in shape,1-5 mm(39.6%)in size and polystyrene(36.5%)in composition.According to indirect observations through the digestion method,only 2size classes of plastics were identified in the macroalgae: mesoplastics,0 to 888.9items/kg dry weight;microplastics,148.1 to 5889 items/kg dry weight.These plastics were prevailing by fibers(71.5%)in shape,1–5 mm(52%)in size and polyester(29.3%)in composition.Additionally,similarity and principle component analysis results indicated that environmental factors could have significant impacts on the distribution of characteristics of plastics loaded by macroalgae.Secondly,to further investigate microplastic pollution features in primary marketed seaweed products,this thesis studied microplastics in four types of seaweed products which have relatively large production volume in Chinese markets.They were composed of dried nori,dried kelp,dried sea asparagus,and dried agar weed.Results showed that microplastics were ubiquitous in all analyzed seaweed products.Microplastics were detected in 38 out of 39 brands of commercially packaged seaweed products(occurrence rate: 97.4%).The abundance of microplastics in four types of seaweed products ranged from 330 to 3000 items/kg dry weight.Among them,the highest average microplastic abundance occurred in dried nori,with 1800±700 items/kg dry weight.Additionally,characteristics of microplastics in commercially packaged nori products were compared with those in factory-processed nori products.Results suggested that an increased size fraction of large microplastics(1-5 mm)was observed along with development of nori processing stages(33.2%rose to 53.3%).Compared with commercial products,the proportions of polypropylene,polyethylene and poly(ethylene-propylene)copolymers increased(4.8% rose to 34.9%),whereas that of polyester decreased in factory-processed products(19.3% down to 10.5%).Also,according to results of field investigation and laboratory simulation on nori products,abundance and chemical composition of microplastics in nori were strongly related to microplastic concentration and polymer type in their ambient environments.Futhermore,the removal characteristics of microplastics loaded in four types of dried seaweed products were compared after immersing them in laboratory simulated cold and hot water experiment.This indicated that using different cooking methods might affect pollution levels of microplastics in seaweed food and cause different levels of hazard to human health.Thirdly,to explore kinetic characteristics of microplastic uptake and elimination by marine animals,mussels were used to carry out a field transfer experiment and another laboratory microplastic exposure and then both laboratory and platform synchronous depuration experiment,respectively.In first experiment,characteristics of microplastics in mussels and in their surrounding waters were monitoring during the period of mussel transplantation from Gouqi Island(open sea area)to Xiangshan Bay(intracontinental port area).Results showed that fiber was the major shape of microplastics(77.4-88.9%),and polyester was the most frequently detected polymer(25-32.9%)in both mussels and waters.However,sizes distribution of microplastics was significantly different between them,1.0-2.0 mm microplastics(29.2%)were primary size fractions in waters,whereas microplastics with sizes<0.5 mm(40.4%)dominated in mussels.This finding suggested that mussels were more likely to take up small sized microplastics.In the second experiment,the distribution of polyester microfibers in different tissues and organs of mussels were analyzed during short-term exposure and short-term depuration as well as short-term exposure and long-term depuration.Results showed that short-term laboratory and platform depuration(3days)had significant differences in removal efficiency of polyester microfibers accumulated in gills and the digestive tract of mussels(clearance rates in gills:87-100%;clearance rates in the digestive tract: 3-83%).Only after a long-term platform depuration(30 days),mussels could utterly remove the enriched polyester microfibers from the digestive tract(clearance rate: ?99%).These results indicated that mussels had obvious tissue differences of time and rate to remove accumulated polyester microfibers.Furthermore,more results of laboratory exposure experiments showed that when exposure concentrations of microfibers were above 1000 fibers/L,microfibers with composition of polyester and polyacrylonitrile,sizes of 500–1000?m,and surface weathering characteristics,were more likely to be accumulated in the digestive tract of mussels in comparison with gills of mussels.This finding further indicated that mussels had tissue differences of taking up different types of plastic microfibers.Lastly,to further explore a possible new route of microplastics entering marine animals,byssus which is a biopolymer secreted by mussels was chosen as studied object.Microplastic pollution features on mussel byssus were investigated by combing field investigation with laboratory simulation methods.In the field investigation,microplastics were widely found in the byssus of mussels,and abundances of microplastics were 0.85-1.02 items/individual mussel and 3.69-9.16items/g byssus.Three types of man-made microplastics(polystyrene microbeads,polyamide microfragments,and polyester microfibers)were used for exposure to newly formed byssus of mussels in laboratory simulation experiment.Results showed that microplastics not only adhered to the surface but fused with the mussel byssus,for example,10 ?m polystyrene microbeads were incorporated into the mussel byssus.This finding suggested that microplastics can be accumulated in tissues and organs of animals through non-ingestion route,in addition to the general assumption that animals take up microplastics through ingestion.In conclusion,this thesis illuminated that loading patterns of plastic debris,especially microplastics in macroalgae were complex and diverse,revealing the prevalence of microplastics in seaweed products and related potential health risks;it uncovered that uptake and clearance characteristics of microplastics in different tissues and organs of mussels,discovering a new route of microplastics entering tissues of mussels from the environment;it summarized and discussed similarities and differences in the intake,loading patterns and transport characteristics of microplastics between marine plants and animals.Together,this thesis not only provides new cognitions for exploring ecotoxicological effects of microplastics in future studies,but also provides scientific basis for ecological and health risk assessments of microplastics. |
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