Ecotoxicity And Mechanism Of Microplastics On Daphnia Magna Under Global Warming

Microplastics(plastic particles<5mm in size)are ubiquitous in the natural environment and pose a serious threat to organisms,ecosystem and even human health.The ecotoxicity of microplastics has been widely studied,but most of the existing microplastics exposure experiments have used concentrations well above actual environmental concentrations,which can overestimate the true ecological risk of microplastics.In addition,the ecological risk assessment of microplastics often ignores the effects of environmental factors,specifically global warming.Global warming and pollutants can interact,on the one hand,global warming can alter the ecotoxicity of pollutants,and on the other hand,pollutants can alter the adaptation of organisms to warming by affecting their heat tolerance.The present paper investigated the ecotoxicity of environmentally relevant concentrations of microplastics to Daphnia magna,a key species in freshwater ecosystems,under six temperature treatments(2 mean temperature,MTX 3 daily temperature fluctuations,DTF)through a full-factorial exposure experiment,and proposed to illustrate the mechanisms of microplastics’s ecotoxicity at low concentrations under global warming through transcriptomics.The main conclusions of this paper are as follows:(1)Elevated MT,DTF and thermal evolution changed the ecotoxicity of microplastics.Exposure to low concentrations of microplastics had little effect on the test organism Daphnia magna(except for a slight reduction in heartbeat rate)at standard temperatures(20℃ MT&0℃ DTF),but microplastics significantly altered life history(earlier maturity,increased fecundity and intrisinc growth rate)and behavioural indicators(increased swimming speed)of Daphnia magna at elevated MT and DTF.In additional,high-latitude Daphnia magna could buffer the effects of microplastics at elevated MT through thermal evolution under global warming by using the "space-for-time" substitution method.(2)Long-term microplastics exposure at low concentrations improved the heat tolerance of Daphnia magna.Microplastics exposure at environmentally relevant concentrations increased heat tolerance,energy reserves(Ea),cellular energy allocation(CEA)and cytochrome P450 enzyme activity,and decreased energy consumption(Ec of Daphnia magna.This could be explained by hormesis,in which low concentrations of microplastics exposure may have postive effects on organisms.The increased heat tolerance induced by microplastics may be related to the energy changes of Daphnia magna,which adapts to global warming by increasing Ea and decreasing Ec to obtain more CEA to defend extreme temperature,thus increasing its heat tolerance.(3)Exposure to microplastics mainly affected the immune,growth and energy metabolism of Daphnia magna under global warming.The number of differentially expressed genes(DEGs)induced by microplastics increased significantly with increasing DTF at 20℃ MT(0℃ DTF:22 DEGs,5℃ DTF:322 DEGs,10℃ DTF:839 DEGs).KEGG Pathway analysis revealed that exposure to low concentrations of microplastics at 20℃ MT&0℃ DTF may enhance cellular resistance to apoptosis through up-regulation of bircs genes,thus enhancing the immunity of organisms and better facilitating their ability to face exogenous stress;exposure to microplastics at 20℃MT&5℃ DTF affectd the growth and development of Daphnia magna through altering protein digestion and absorption pathways;exposure to microplastics at 20℃MT&10℃ DTF affectd the sex and microplastics tolerance of Daphnia magna through genes involved in the biosynthetic pathways of steroid and terpenoid skeletons.Notably,the number of DEGs induced by microplastics decreased significantly with increasing DTF at 24℃ MT(0℃ DTF:540 DEGs,5℃ DTF:38 DEGs,10℃ DTF:246 DEGs).KEGG pathway analysis revealed that microplastics exposure at both 0℃ and 5℃ DTF altered the energy metabolism of Daphnia magna by up-regulating far genes in the keratin,folinic acid and wax biosynthesis pathways and promoting the production of long-chain wax esters;exposure to microplastic at 24℃ MT&5℃ DTF also altered the immune function of Daphnia magna by affecting antigen processing and presentation,lysosomal,phagosomal and apoptotic pathways;exposure to microplastics at 24℃ MT&10℃ DTF altered energy metabolism of Daphnia magna by affecting the nitrogen metabolism pathway.The present paper systematically illustrated the ecotoxicity and mechanism of microplastics on Daphnia magna under global warming,and predicted for the first time that high-latitude Daphnia magna could buffer the effects of microplastics by thermal evolution under global warming scenarios by using the "space-for-time" substitution method.In addition,this paper revealed that microplastics increased the heat tolerance of organisms,which implied that the presence of microplastics enhanced the ability of organisms to adapt to global warming,and revealed the underlying mechanisms from the perspective of energy change.Lastly,the paper revealed the target genes and metabolic pathways of microplastic exposure under global warming through transcriptomics,and found that microplastics mainly affected the immunity,growth and energy metabolism of organisms.Taken together,this paper provides a new theoretical basis and data support for the assessment of the real ecological risk of microplastics in a warming climate through the study of the ecotoxicity and mechanism of microplastics on Daphnia magna under global warming.