Characterization Of Bacterial Compositions Of Biofilm On Microplastics And Associated Environmental Factors In The Wetland Soil

The accumulation of persistent microplastics(MPs)in the environment had raised increasing concerns due to their potential impacts on human and ecosystem health.Researches in MPs have primarily focused on the distribution and effects of MPs in marine ecosystems,there were rudimentary understanding of ecological impacts of MPs in soil.Despite that numerous studies have indicated that MPs are capable of adsorbing organic and inorganic pollutants,there are growing interests in the development of biofilms on MPs.MPs may served as a matrix on which microorganisms could colonize,proliferate and interact,forming biofilms with specific compositions and functions.However,the taxonomic compositions and functional traits of biofilms on MPs,and the factors influencing the development of biofilms remained poorly characterized.To address this knowledge gap,this thesis aims to investigate the bacterial compositions and functional genes involved in nitrogen cycling and arsenic biotransformation of biofilm on MPs,and to depict the environmental factors affecting the biofilm in wetland soil.The major results of this thesis were summarized as follows:(1)The bacterial compositions of biofilm on MPs and the abundance of nitrogen cycling genes(NCGs)on three types of MPs(Polyethylene,Polystyrene and Polyvinyl Chloride)were characterized by 16S rRNA gene sequencing and high-throughput quantitative PCR(HT-PCR)after in situ incubation of MPs in mangrove soil.The results showed that MPs provided a matrix for microorganisms and recruited unique microbial assemblies.Bacterial community structures on MPs were significantly different from those of mangrove soils.Proteobacteria,Bacteroidetes,Chloroflexi,Planctomycetes and Acidobacteria were dominant on MPs.Bacterial community structures on three types of MPs showed higher dissmilarity in bulk soils than those in rhizosphere soils,indicating significant rhizospheric effects of mangrove plant on the formation of bacterial biofilm on MPs.The relative abundance of hzsA,amoA,nasA,nirS,nirK and nosZ gene on MPs was significantly higher than those in the surrounding soil,and the abundance of NCGs on polystyrene was higher than those on polyvinyl chloride and polyethylene.(2)A microcosm experiment was conducted to investigate the impact of tide on the biofilm formation on three types of MPs(polyvinyl chloride,polystyrene,and polyethylene).The results showed that bacterial community diversity on three types of MPs increased gradually with the extension of tidal time(0,7,14,21,28,and 35 days),indicating that longer culture time may lead to the formation of more mature biofilm on MPs.The time-lag regression analysis showed that the variation of bacterial communities on three types of MPs were positively correlated with tide time,indicating that bacterial communities on three types of MPs that were undergoing a directional change.The bacterial community structures on three types of MPs were significantly different with the change of tide time,demonstrating that the simulated tidal treatment affected the enrichment of microorganisms on MPs.In addition,the relative abundance of NCGs on MPs was higher than that of soil.Compared with other tide time,the relative abundance of NCGs on three types of MPs was the highest at the 14th day.(3)The formation of biofilm on MPs could be affected by MP properties and environmental factors.The effects of salinity on the bacterial compositions of biofilm on MPs and the relative abundance of NCGs were explored.The results showed that the bacterial community structures on three types of MPs were significantly different under different salinity(0,10,20,and 30 ppt),indicating that soil salinity significantly affected the bacterial compositions of biofilm on MPs.Structural equation model(SEM)indicated that bacterial compositions of biofilm on MPs was directly affected by soil salinity,and that changes in soil bacterial communities caused by salinity also contributed indirectly to the development of biofim on MPs.Soil salinity had a positive correlation with the bacterial diversity of biofilm on polyethylene and polystyrene,but had a negative correlation with the bacterial diversity of biofilm on polyvinyl chloride.The relative abundance of NCGs on three types of MPs was significantly higher than that in soil under different salinity conditions.Increased salinity resulted in elevated NCGs abundance on polyethylene and polystyrene,while the relative abundance of NCGs on polyvinyl chloride decreased with increased salinity.(4)Heavy metals are frequently detected and accumulated in agricultural soil,seriously threatening human health.The absorption of heavy metal on MPs was largely affected by environmental conditions.We investigated the effect of pH on the adsorption of arsenic on MPs(polyvinyl chloride),as well as the bacterial community compositions and the abundance of arsenic biotransformation genes on MPs.The results showed that bacterial community structures on three types of MPs were significantly different under different pH conditions(pH 4.62,pH 6.5,and pH 7.46)and two arsenic content conditions(13 mg.kg-1 and 74 mg.kg-1),indicating that soil pH and arsenic content significantly affected the bacterial compositions of biofilm on MPs.SEM indicated that soil pH and arsenic concentration could directly affect the diversity of bacterial communities on MPs,and also exhibited indirect effects by affecting soil bacterial communities.Soil pH had stronger impact on bacteterial community structures on MPs than soil arsenic content.The change of soil pH affected the content of total arsenic,arsenic species on MPs and the abundance of arsenic biotransformation genes on MPs,with highest concentration of total arsenic and As(DMA)on the MPs detected when soil pH was 6.5.The relative abundance of arsenic biotransformation genes on MPs was higher than those in the surrounding soil when soil pH was 4.62.Taken together,these results provided comprehensive characterization of phylogenetic compositions and functional traits on MPs,and indicated that types of MPs,pH,salinity and arsenic strongly affect the development of biofilm on MPs,which could support further assessment of ecological effects of MPs on ecosystem functions.