Effects Of Microplastics On Microbial Communities And Nitrogen And Phosphorus Release In Sediment

The Pengxi River has been one of the tributaries where the occurrence of blooms is more serious in the Three Gorges Reservoir area,and excessive nutrient contents are the main cause of eutrophication in the water column,especially nitrogen and phosphorus content.Sediments,as a sink or source of various nutrients and pollutants in freshwater ecosystems,store large amounts of nitrogen,phosphorus,and pollutants,and play an important role in the geochemical cycling of these nutrients and pollutants.Under the condition that exogenous inputs have been effectively controlled,nutrient concentrations in the water column are highly correlated with sediments.Microplastics,as an emerging pollutant,have become the focus of many researchers because they can serve as carriers for other pollutants in the environment and transports them over long distances.With the mass production and use of plastic products,the risk of microplastics entering aquatic and terrestrial ecosystems is increased.Microplastics entering aquatic environments can be decomposed and deposited in sediments.Nevertheless,most studies to date have focused on soil and marine ecosystems,and few have focused on the mechanisms of the environmental behavior of microplastics to nutrients in freshwater lake sediment systems,especially on the cycling of sediment endogenous phosphorus.Therefore,this paper researches the problems,and the specific contents and results are as follows:（1）Through field investigation studies,the effects of the change of water level regulation of the Three Gorges Dam on the variation of microplastic characteristics,nitrogen and phosphorus,and crustal metal contents in the sediments of the tributary backwaters and the interrelationship between the three during low and high water periods.In this study,we investigated the variation of nitrogen and phosphorus form,crustal metal content,and microplastic characteristics in the sediment-water system of the tributary backwaters during low and high water periods,and analyzed their variation and the relationship between microplastic characteristics and nitrogen,phosphorus,and metal content in the sediment under the changed hydrological conditions of impoundment.Our results showed that the water level changes of the dam resulted in changes in the hydrological conditions of the tributaries in the basin,reduced water velocity,and increased sediment residence time,which significantly affected the nutrient,metal content,and microplastic characteristics of the sediment-water system.Therein,the water level changes significantly changed the average contents of NO^2-,NO^3-,NH₄⁺,TN,PO₄^3-,and TP in the overlying water of the tributary Gaoyang Lake section.The main component of the phosphorus form in the sediments during low and high water periods was HCl-P with an average content of 42.16%and 50.78%,respectively.Followed by Bio-P with an average contribution of 36.33%and 31.03%,respectively.The Na OH-IP and Na OH-OP contributed the highest contribution to bioavailable Bio-P with 21.11%and 56.80%in the low water period and 18.10%and57.24%in the high water period,respectively.In addition,TN content in the sediment significantly decreased in the high water period,and the average content during low and high water periods was 668.38±149.38 and 467.35±187.88 mg/kg,respectively.（2）Microplastic types in the sediment during low and high water periods were mainly nylon（RY）,polyethylene（PE）,polypropylene（PP）,polyethylene terephthalate（PET）,polyamide（PA）,polyvinyl alcohol（PVA）,and cellophane（CP）.The average abundance of microplastics during low and high water periods ranged from 0.20×10²～3.40×10³ n/kg and 0.20×10²～8.40×10³ n/kg,respectively.The color and shape of microplastics in the sediments during the low water period were mainly black,transparent,linear,and fibrous,while the high water period,they were mainly blue,transparent,fragmented,and linear.The size range of microplastics in the sediments during the low water period concentrated in the range of 0.1 to 2 mm,accounting for95.6%,while the high water period was mainly less than 1 mm.Pearson and PCA analysis showed that the effect of microplastics on phosphorus form and metal concentration during low and high water periods depended on their color,shape,and size instead of microplastic abundance.The response of metals and their reactive oxides to microplastic characteristics also affect the cycling of endogenous phosphorus in the sediment.The results of this study help to understand the relationship between changes of microplastic characteristics,nutrients,and metals in the tributary sediment-water system under the different water level states of the Three Gorges Dam.The study also supports future systematic evaluation of the environmental chemical behavior of microplastics in the ecosystem.（3）Indoor-controlled experiments were conducted to elucidate the mechanism of the effect of microplastics in sediments on nutrient release.In the later period of this study,we conducted microcosm experiments with 0.5%（w/w）polyvinyl chloride（PVC）,polylactic acid（PLA）,and polypropylene（PP）microplastics were added to the sediment through indoor incubation experiments,and the eco-function of the bacterial community were predicted and assessed via 16S r RNA gene sequencing.The results showed that microplastics significantly affected the microbial community composition and nutrient cycling in the sediments.PVC and PP microplastics can promote microbial nitrification and nitrite oxidation,while PP can significantly promote alkaline phosphatase（ALP）activity and the abundance of the phosphorus-regulation（pho R）gene.PLA microplastics had the potential to promote the abundance of microbial phosphorus transporter（ugp B）,nitrogen fixation（nif D,nif H,and nif X）,and denitrification（nir S,nap A,and nor B）genes and inhibit nitrification,resulting in massive accumulation and release of ammonia nitrogen.Although PLA microplastics inhibited the activity of ALP and the abundance of the organophosphorus mineralization（pho D）gene,it could enhance dissimilatory iron and sulfite reduction,which may promote the release of sedimentary phosphorus.Our findings may help understand the mechanisms of nitrogen and phosphorus cycles and microbial communities driven by microplastics in sediments and provide a basis for future assessments of the environmental behavior of microplastics in freshwater ecosystems.