Effect Of Microplastics On Ecosystem Functioning:Microbial Nitrogen Removal

Approximately 300 million tons of microplastics enter the ecosystem each year,resulting in a wide variety of microplastics with varying abundance in the water bodies and sediments of the freshwater ecosystem.Microplastics not only provide survival carriers for microorganisms,but also impact the health of aquatic organisms.Therefore,microplastics may further affect the relative ecosystem functions.As one of the important functions of the freshwater ecosystem,microbial denitrification can effectively alleviate the eutrophication of water bodies and play an important role in the nitrogen cycle;microplastics may impact microbial denitrification,which may be more complicated due to the participation of macroinvetebrates.However,few studies focus on the effect of microplastics on the microbial denitrification in freshwater ecosystems and the mechanism is still unkonw.We use the microcosmic system to do tne research,firstly,we analyzed the effect of different types（PLA,PVC,PE）and different concentrations（0.1%wt.,1%wt.and 10%wt.）of microplastics on the microbial denitrification（denitrification and anammox process）in water bodies and sediments.Secondly,we studied the impact of different concentrations of PE（0.1%wt.,1%wt.）on microbial denitrification with the participation of macroinvetebrates.In this study,by analyzing water quality indicators,microbial community structure,denitrification functional genes（nir S,hzs B）,and denitrification and anammox rates,this study explored the effects of different concentrations and types of microplastics on microbial denitrification in freshwater ecosystems.The research results will provide a theoretical basis for the impact of environmental microplastics on ecosystem functions,and have important practical significance for guiding the prevention of environmental and ecological risks of microplastics pollution.The main conclusions are as follows:Microplastics have different effects on the soluble inorganic nitrogen in the overlying water in the water and sediment systems.The concentration of NH₄⁺-N（the main form of inorganic nitrogen）in the water system is about 15 times that of the sediment system,and the concentration of NO₃^--N is about 3/5 that of the sediment system,compared with the sediment,the ammonia oxidation rate of the water system may be lower;（2）The impact of microplastics on the N cycle varies with the type and concentration of microplastics:in all treatment of the water system,except the 10%wt.PLA significantly promotes the process of microbial denitrification,other different types and concentrations of macroplastics have no effects on microbial denitrification;（3）In the sediment system,low concentration（0.1%wt.）of microplastics increase the rate of microbial denitrification.In different types of microplastics,the values are:PLA(46.91 nmol N g^-1h^-1),PE(41.82 nmol N g^-1h^-1),PVC(32.26 nmol N g^-1h^-1),which are1.55,1.38 and 1.07 times that of the control group;1%wt.PLA and PE increased the rate of microbial denitrification(45.69 and 42.00 nmol N g^-1h^-1),which are 1.51 and 1.39times that of the control group,while 1%wt.PVC inhibited microbial denitrification(28.60 nmol N g^-1h^-1);high concentration（10%wt.）PLA and PE increased the rate of microbial denitrification,but the promotion effect of high concentrtions of PE was weakened,the vales are 52.75 and 31.08 nmol N g^-1h^-1 respectively,which are 1.75 and1.03 times that of the control group.PVC significantly inhibited microbial denitrification(22.23 nmol N g^-1h^-1),and its denitrification rate dropped by 26%.（4）PE microplastics promote the microbial denitrification of freshwater sediments by increasing the number of denitrification and anammox bacteria genes,therefor,which increases the rate of microbial denitrification and reduces the concentration of NO₃^-.However,as the concentration of PE microplastics increases,the rate of microbial denitrification is reduced;the denitrification rates of 0.1%wt.and 1%wt.PE are 40.63nmol N g^-1h^-1 and 26.27 nmol N g^-1h^-1 respectively,which are 1.1 times and 0.59 times that of the control group;（5）The chironomid larvae significantly promotes the process of microbial denitrification,which increased the gene abundance of denitrifying bacteria(9.39E-03copies g^-1ml^-1),and the denitrification rate was 2.23 times that of the control group(36.97nmol N g^-1h^-1);（6）In the treatments chironomid larvae and microplastics co-existing,the increases of nitrogen removal rates measured in the treatments with microplastics and chironomids co-existing compared to the control,were less than the sum of their in dividual effects.The promotion of the chironomid larvae on the microbial denitrification is affected by the the side effects of microplastics,which is partially offseted,especially at high concentrations of 1%wt.PE,the offsetting effect is more obvious.