Research On The Toxicological Effect Of Nanoplastics On Chlorella Pyrenoidosa

Recently years,owing to the widespread use of plastics in industrial,agricultural,pharmaceutical,packaging,and other fields,their adverse impact on environmental protection and ecologically sustainable development has been gradually realised by numerous scholars worldwide.Furthermore,plastic fragments are gradually disassembled into tiny particulates,which are caused by the effects of physical chemistry and biological function in the long term,and are universally described as microplastics(MPs,<5 mm in size)and nanoplastics(NPs,<100 nm in size).Growing evidence suggests that the negative influence of nanoplastics on bioaccumulation and toxicity in aquatic organisms might be worse than that of microplastics.Nanoplastics have smaller particle size,larger specific surface area,and stronger adsorption capacity than microplastics.Therefore,future research should focus on the role of nanoplastics,owing to their direct or indirect threats,particularly to aquatic organisms and human health.Primary producers as microalgae is the base of aquatic food-chain,thereby shoulders critical role with the stability of integral aquatic ecosystem.Chlorella pyrenoidosa is a common bloom green alga,which often coexists with nanoplastics in seriously eutrophic waters.Therefore,it is of great significance to study the effects of nanoplastics on C.pyrenoidosa.Polystyrene(PS)plastics,as the most common plastic subtype,are one of the dominant materials for light manufacturing,such as electronic and electrical appliances.As a result,PS-NPs(80 nm)and C.pyrenoidosa were chose as target pollutants and tested species,respectively.The effects of acute exposure,chronic exposure and pulse exposure of PS-NPs on the physiology and biochemistry of C.pyrenoidosa were studied.Transcriptome analysis(RNA-seq)was used to further reveal the toxic mechanism of nanoplastics on C.pyrenoidosa at the molecular level.The results of this research will be important in the assessment of further risks of nanoplastic exposure to terrestrial ecosystems.They are also expected to provide fundamental data on the possible impacts of nanoplastics on the aquatic environment.The main research results of this paper are as follows:(1)This study compared the toxic effects of PS-NPs on C.pyrenoidosa under acute exposure,chronic exposure and pulse exposure.The results showed that the maximum inhibition rates of algal cell growth in 10,20,30,40 and 50 mg/L PS-NPs acute exposure treatment were 10.78%,11.87%,13.78%,24.78%and 27.73%,respectively.Furthermore,the inhibition and promotion effects exist in the exposure of nanoplastics to C.pyrenoidosa under these three exposure modes.C.pyrenoidosa will rely on its detoxification ability to resist the damage of nanoplastics under acute and chronic exposure;However,under pulse exposure exposure,the detoxification ability of C.pyrenoidosa gradually weakened,indicating that the detoxification ability of algal cells is limited.Nanoplastics have a cumulative toxic effect on algal cells and exert an irreversibly negative effect on aquatic organisms.Hence,the toxic effect of nanoplastics on C.pyrenoidosa depends on the mode,frequency,concentration and duration of exposure.(2)In current study,the interaction between nanoplastics and C.pyrenoidosa was analyzed through microscopic observation and physiological and biochemical experiments.It is found that some nanoplastics directly adhere to the surface of algal cells.These behaviours will lead to shading effect,thus affecting chlorophyll synthesis and reducing the ability of algal cells to absorb and convert light energy.Such impairment directly interferes with the photosynthetic electron transport between Q_Aand Q_B and reduces the light energy conversion efficiency of the PSII system.Furthermore,the reduction of electron transfer promotes the production of reactive oxygen species(ROS).The reaction of ROS with phospholipids on the cell membrane results in lipid peroxidation and increased malondial dehyde(MDA)content,which affects the fluidity and permeability of the cell membrane and damages the structure and function of cells.In addition,the excessive production of ROS also causes cell antioxidant processes,as shown by the promotion of the synthesis of antioxidant enzymes such as,superoxide dismutase(SOD)and catalase(CAT).On the other hand,it hinders the transmission and transportation of nutrients,and the energy supply for algae cells to maintain normal operation is limited.The above two effects will lead to poor fluidity and permeability of cell membrane,damage the cell membrane structure,collapse of cell membrane skeleton,distortion and deformation of cells,weakening or even loss of selective permeability of cell membrane,and affect the normal growth and metabolism of algal cells.(3)Our current research found that a large number of heterogeneous-aggregation will be formed between Nanoplastics and algal cells.The initial stage of heteroaggregation formation involves the physical damage of algal cells,such as cell wall structure changes and cell membrane damage and makes the microalgae inactive.With the extension of the exposure time,the abundant presence of hetero-aggregation,wherein wrapped dead cells are separated from other cells,leads to the reduction of nanoplastic concentration in the supernatant solution and limits the damage to other healthy cells.Furthermore,this result may be explained by the detoxification phenomenon in which plastic particles attach to cells and thus plastic concentration decreases in the media.New divided cells do not find these particles in suspension and grow better.(4)Transcriptome analysis showed that nanoplastics had a significant effect on gene expression of C.pyrenoidosa.We found 573 differentially expressed genes(DEGs)in the 10 mg/L PS-NPs concentration group,including 259 upregulated and 314downregulated genes,and 2915 DEGs in the 50 mg/L PS-NPs concentration group,including 1875 upregulated and 1040 downregulated genes.A total of 468 genes were co-differentially expressed in the 10 and 50 mg/L PS-NPs concentration groups.Gene Ontology(GO)enrichment showed that 10 mg/L PS-NPs treatment group had significant effects on the following pathways of algal cells:amino acid activation,aminoacyl t RNA synthase and ligase,acetic acid metabolism and threonine metabolism;50 mg/L PS-NPs treatment group had significant effects on the following pathways of algal cells:nucleic acid catabolism,meiosis,cell cycle,chromosome organization,cell response to DNA damage stimulation,nuclear division,organelle division and DNA repair.Kyoto Encyclopedia of Genes and Genomes(KEGG)enrichment showed that under 10 mg/L PS-NPs treatment group,the non-homologous end-jioning pathway of algal cells was significantly up-regulated and the aminoacyl t RNA synthesis pathway was significantly down regulated.Under 50 mg/L PS-NPs treatment group,the pathways significantly up-regulated by algal cells are:DNA replication,homologous recombination,mismatch repair,nucleotide excision repair,non-homologous end-jioning;The significantly down regulated pathways are aminoacyl t RNA synthesis and photosynthesis.Gene Set Enrichment Analysis(GSEA)enrichment showed that under10 mg/L PS-NPs treatment group,algal cell pathway showed an upward trend:vacuole,organelle division,nuclear division and meiosis;Under 50 mg/L PS-NPs treatment group,the pathways significantly up-regulated by algal cells include organelle division,nuclear division,meiosis,recombination repair,telomere maintenance,telomere tissue and DNA modification.(5)Transcriptome analysis revealed the interaction mechanism between nanoplastics and C.pyrenoidosa.The results showed that the inhibitory effect of nanoplastics on C.pyrenoidosa was mainly to inhibit the protein synthesis,metabolism and photosynthetic pathway of algal cells,and damage the DNA of algal cells,which led to the mechanism of DNA repair.The detoxification ability of C.pyrenoidosa give the credit to defense the damage of nanoplastics by regulating ion balance,ion transport,metabolic reaction and membrane vesicle transport to improve intracellular osmotic pressure,accelerating the degradation of damaged proteins,organs and autolysis of damaged cells,and promoting cell proliferation.