| Microplastics have attracted widespread attention in recent years,however,related researches on microplastics mainly focused on marine ecosystem,and there is less relevant data on the impact of microplastics on freshwater systems.Multiple studies have shown that freshwater aquatic organisms can ingest microplastics,but study on the long-term effects of continuous exposure are still few.When the ambient temperature rises,the metabolic rate of organisms will increase,which will enhance feeding and exacerbate the biological effects of pollutants.Cladocerans are the key biological components of aquatic ecosystems,they act as predators of producers such as algae and prey of fish and shrimp.They carry the energy flow between producers and higher trophic levels in the aquatic ecosystem food chain.Moreover,the cladocerans are more sensitive to changes in environmental pollutants and ambient temperature,and fluctuations in their individual parameters and population dynamics will also affect the abundance of higher trophic organisms.In this thesis,the model organism Daphnia magna was selected as the experimental material,and the changes of its tolerance were explored through the response of survival and reproduction and the expression of key enzymes under the influence of different microplastics concentration,particle size and experiment temperature.The main results are as follows:1.The influence of particle size and concentration of microplastics on the fitness of D.magnaThis experiment explored the survival and reproduction of D.magna under different size（70 nm,1μm,and 10μm）and concentration(0,0.5,1,and 2 mg·L-1)of polystyrene（PS）microplastic.During the 21-day experiment,the average life span,net reproduction rate,generation time,and intrinsic growth rate of population were calculated.The results showed that the particle size of microplastics affected its toxic effect on D.magna.High concentrations of microplastics significantly inhibited the survival and reproduction of D.magna,and reduced its fitness.Furthermore,microplastics with small particle sizes were more harmful.This result provided a reference for a deep understanding of the toxic effects of different sizes of microplastics on aquatic organisms in the environment.2.Effects of microplastics on fitness and life history parameters of D.magna at different temperaturesIn this experiment,MP（PS）with smaller particle size（70 nm）and D.magna size（10μm）were selected at different concentrations(0,0.5,1,2 mg·L-1)and different temperatures（15,20,and 30℃）respectively to explore the effects of MP,temperature and their interactions on the survival and reproduction of D.magna.During the 21-day experimental period,the average lifespan,net reproduction rate,generation time,and intrinsic rate of population were calculated.The results showed that both low temperature and high temperature affected the survival and reproduction of D.magna to different extents.Microplastics with large particle size at low temperature and small microplastics particle size at high temperature are more harmful.Then,small particle size（70 nm）microplastics was used to study the life history parameters of D.magna at different temperatures（20,25,and 30℃）.The results showed that the mortality of D.magna increased significantly at high temperatures,and the offspring body length and offspring number showed different trends at different temperatures,reflecting different life history strategies of the mother at different temperatures.3.Molecular characteristics and expression regulation of thioredoxin reductase in large scale cormorants under microplastics stressThe present study firstly identified TRxR in D.magna（Dm-TRxR）and found that the full-length c DNA was 1862 bp long,containing an 1821 bp open reading frame.Homologous alignments showed the presence of conserved catalytic domain CVNVGC and the seleocysteine（Se Cys）residue（U）located in the N-and C-terminal portions.Subsequently,the expression of Dm-TRxR,together with permease,arginine kinase（AK）,was investigated by approach of quantitative real-time PCR after exposure to four（1.25μm）polystyrene（PS）micropalstics levels:0（control）,2,4 and 8 mg·L-1for10 days.Dm-TRxR,permease and AK m RNA were significantly upregulated after exposure to 2,4 mg·L-1 of microplastics,but then declined at presence of 8·mg L-1microplastics.The gene expression results suggested that oxidative defense,energy production and substance transportation out of cellular were significantly regulated by microplastic exposure.Collectively,the present study enriched our knowledge regarding the biological effects of microplastic pollution on zooplankton,and built a foundation for freshwater environmental studies on mechanistic and biochemical responses to microplastics.In summary,the exposure of microplastics had a deleterious effect on the survival and reproduction of D.magna,and this effect was enhanced with the increase of microplastics concentration.For microplastics with different particle sizes,the survival and reproduction parameters of D.magna had different responses.At the same concentration,the toxic effects of nano-level microplastics on D.magna was stronger,and the effects was enhanced when temperature rose to 30°C,while the toxic effects of micro-level microplastics was opposite to that of nano-level microplastics:the toxic effects of microplastics was stronger when temperature was reduced to 15℃.In addition,the expression of key redox and energy-related enzymes in D.magna changed under microplastics exposure stress,indicating that exposure caused oxidative damage and cytotoxicity.At the same time,D.magna under microplastics exposure stress responded to and regulated the toxic effects of microplastics through reproduction strategies and corresponding changes in key enzymes in the body.This study will improve our understanding of the biological effects of zooplankton microplastic pollution,enrich research data on microplastics in freshwater ecosystems,and help to accurately assess the exposure risks of microplastics in freshwater ecosystem. |
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