| Microplastics(MPs)are a new type of environmental pollutant that has received much attention in recent years,posing serious potential risks to aquatic organisms and human health.Rivers are the main sites of microplastic discharge in urban and rural areas,and especially the interaction between microplastics in rivers and freshwater organisms makes rivers an important site for environmental impacts of microplastics to occur.However,at present,domestic and international studies on the distribution of microplastics in rivers are mostly limited to densely populated urban areas,and less attention has been paid to the current status of microplastic pollution in rivers in remote areas.With the rapid increase of population in recent years,the ecological pollution of Lhasa River has been intensified,and its microplastic pollution status and distribution characteristics deserve attention.Spirodela polyrrhiza is a widely distributed freshwater higher plant in China,which has the advantages of easy culture,simple structure,and fast growth rate,and is often used as a model organism for ecological restoration and toxicity detection.Metal-doped plastic particles are playing an advantage in the environmental tracking and quantitative study of microplastics,providing an effective means to study the ecotoxic effects of microplastics.In this study,the distribution pattern of microplastics in Lhasa River was firstly analyzed by field sampling survey,and its environmental risk was assessed;subsequently,we investigated a series of ecotoxic effects of adding and removing small-size europium(Eu)metal-doped polystyrene plastic particles(0.1 μm)on the growth index,photosynthetic pigment content and antioxidant enzyme activity of the model aquatic organism,Spirodela polyrrhiza,controlled experiments in the laboratory.Finally,laser confocal microscopy and scanning electron microscopy were used to track and verify the surface adsorption of europium metal-doped polystyrene plastic particles and the distribution and migration of europium metal-doped polystyrene plastic particles within the Spirodela polyrrhiza,and inductively coupled plasma mass spectrometry was applied to quantify the internalization and excretion of europium metal-doped polystyrene plastic particles by Spirodela polyrrhiza after the addition and removal of microplastic stress.Main results of these studies are as follows:(1)The abundance of microplastics in the surface water and sediment of the Lhasa River ranged from 0.17-1.30 n/L and 0.20-0.57n/g,respectively.Compared with other rivers,microplastic pollution in the Lhasa River was at a relatively low level.Among the detected microplastics,fibrous,transparent,and small-size particles dominated in surface water and sediment,and the qualitative Raman spectroscopy results showed that polypropylene(PP)and polyethylene(PE)were the main components of microplastics in the Lhasa River.The main sources of microplastics in the Lhasa River may be domestic sewage discharge,domestic waste,and agricultural film use.The risk assessment of microplastic pollution in the Lhasa River was carried out by polymer risk index and pollution load index,and it was found that the pollution load index of the Lhasa River was Class I,which was at a low level.Due to the presence of highly toxic polyvinyl chloride(PVC),the polymer risk index hazard level was Class Ⅲ.(2)In the indoor control experiment,the growth indexes(plant length,plant number,and fresh weight)of Spirodela polyrrhiza were all inhibited by microplastic exposure,and the effect of high concentration of microplastic on the growth indexes of Spirodela polyrrhiza was higher than that of low concentration.Adding and removing microplastic stress resulted in a decrease in the photosynthetic pigment content of Spirodela polyrrhiza compared to the control group,but the effect was not significant.With the increase in exposure time,the antioxidant enzyme activities were inhibited under the high concentration treatment,while the antioxidant enzyme activities showed a trend of increasing and then decreasing under the low concentration treatment.After the removal of microplastic stress,the plant length,plant number,and fresh weight of Spirodela polyrrhiza increased;the photosynthetic pigment content of Spirodela polyrrhiza did not change significantly under both low and high concentration treatments;the antioxidant enzyme activity of Spirodela polyrrhiza showed a trend of increasing and then decreasing with the increase of self-purification time.(3)The results of laser confocal microscopy showed that the fluorescence intensity of polystyrene plastic particles in the roots and leaves gradually increased with increasing exposure time.After the removal of microplastic stress,the fluorescence intensity in leaves significantly decreased with increasing time,while the fluorescence signal in roots did not change significantly.Scanning electron microscopy confirmed that microplastic particles were found on the surface and cross-section of Spirodela polyrrhiza plants after the addition and removal of microplastic stress.The dense microplastic particles were adsorbed on the root surface,followed by the lower surface of the leaf,while the least microplastic particles were found on the upper surface of the leaf.Microplastics accumulated mainly in the intercellular spaces of the cortex and vascular cells,suggesting that microplastics may enter the vascular bundles via the plastid extracellular pathway and be transported upward into the leaves with water and nutrients.ICP-MS analysis showed that the internalized microplastic content of Spirodela polyrrhiza was significantly higher in the high concentration treatment(212.75 μg/g)than in the low concentration treatment(7.67 μg/g),and decreased by 49.55-51.38% after 72 h of the removal of microplastic stress.In conclusion,although microplastic pollution in Lhasa River is at a low level compared with other rivers,we need to monitor the status of microplastic pollution in Lhasa River for a long time due to the detection of highly toxic microplastics such as polyvinyl chloride.So as to provide a basic reference for local microplastic pollution management.In addition,microplastics enter the Spirodela polyrrhiza through internalization and have a series of growth and physiological effects on Spirodela polyrrhiza.In order to cope with microplastic stress,the internalized microplastic particles were released when microplastic stress was removed,thus reducing the accumulation of microplastics in the body. |
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