Formation Of Biofilm On Microplastics And Its Influences On Physicochemical Properties Of Microplastics In The Coastal Sea

Microplastics pollution has been a hot spot environment issue all over the world.Due to their small sizes and large specific surface area,microplastics could serve as carriers for pollutants and pathogens/invasive species.It has been reported that the microbial colonizers attaching on the surfaces of microplastics mainly assemblaged in the form of biofilms.Formation of biofilms could change physicochemical properties of microplastics,to some degree which influences its environmental fates further.So far,an increasing number of researches on biofilm microbial communities have been carried out.However,little is known about the processes of biofilm formation on microplatics and its influences on physicochemical property of microplastics.In this study,an offshore fishery area of Northern Yellow Sea located in Yangma Island,Yantai,Shandong Province was chosen as the site for field experiment.Polyethylene(PE)films,polypropylene(PP)pellets,as well as expanded polystyrene(EPS)spherules,with the sizes ranged from 3 to 5 mm,were selected as model microplastics.A buoyant nested cage was designed and applied in the microplastics immersing field experiment to investigate the dynamic influences of biofilm formation on the physicochemical properties of three different types of microplastics types at different seawater depths(2 m,6 m and 12 m).The dynamic relations between morphology,biomass,intensity and thickness of biofilm and exposure time and depths were analyzed.Then,biofilm DNA extraction methods from two types of soft microplastics were optimized.At different seawater depths,The dynamic influences of biofilms information on hydrophobicity and funtional groups of microplastics and diameters,densities and surface roughness of EPS with exposure time were illuminated.These datum preliminarily threw light on interactions between marine microplastics and biofilms,providing scientific evidences that biofilms formation affected fates,weathering and degradation of microplastics in coastal environment.The main conclusions of this research are as follows:(1)Based on literature and field investigation,combining a shell breeding cage and stainless stell barrel,a testing apparatus with low cost,environmentally friendly features and simple use,was effectively set up and it could be fit for experiments in different seawater depths.Then,a field experiment,microplastics immersion in seawater,has been carried out at scallop cultivation area of Yangma island inYantai city.(2)Lots of marine microorganisms could attach quickly on surfaces of different microplastics which were exposed to offshore fishery area at different seawater depths.After 30 day,75 day and 135 day of exposure in seawater,lots of marine micoorganisms including cocci and bacilli have attached on the surfaces of three microplastics types at different seawater depths(2 m,6 m and 12 m).In terms of PE films,except for that biofilm biomass at 12 m depth significantly increased after 135 day of exposure,biofilm biomass of PE films showed no significant differences among three exposure depths and time.Additionally,Laser scanning confocal microscop was used to measure the structure and thickness of biofilm.The result indicated that the intensity and thickness of biofilm on PE film decreased with increasing depths,but increased with increasing exposure time.As to EPS,during exposure time,biofilm biomass of EPS significantly increased with increasing exposure time at different three depths,but had no significant differences among three depths.After 135 day of exposure,biofilm biomass of PP pellets increased significantly more than before twice at all depths.Totally,biofilm biomass of PP pellets slightly increased with increasing depths.(3)Methods of extracting biofilms DNA toward different plastic types(PE films and EPS spherules),sizes(1~3 mm and 3~5 mm)and amount(10 pieces,20 pieces and 30 pieces)were optimized.Taking DNA concentration and purity into consideration,concentrations of biofilm DNA from two soft types of microplastics namely PE films and EPS spherules extracted by FastDNA? kit were significantly higher than by MOBIO PowerSoil? kit.Further,this method was applied to extracting biofilm DNA from PE films at different depths.The result of high-throughput sequencing indicated that bacterial richness and diversitysignificantly increased with increasing seawater depths.The bacterial community composition from PE films was similar at the level of phylum,class and genus at different seawater depths.The phylum of Proteobacteria and Bacteroidete and the class of Gamma-proteobacteria,Alpha-proteobacteria and Flavobacteria showed a dominance.However,the bacterial richness at the level of genus significantly changed with seawater depths.(4)The physicochemical properties were changed by biofilm formation,including roughness,hydrophobicity and functional groups.The colour of PE film turned dark with increasing exposure time and transparent with increasing depths.After 75 day of exposure,carbanyl group spectrum appeared,showing that PE films have been weathered.After 135 day of exposure,microplastic fragments occurred on the surface of PE films.Moreover,the hydrophobicity of PE film significantly decreased with increasing exposure time.The colour of EPS spherules turned darker with increasing exposure time and depths.The diameter of EPS significantly decreased with increasing exposure depths,while it was in opposition for the density.The relationship between diameter of EPS and exposure time showed no significant differences,but the density indicated the increasing tendency with exposure time.In fact,there were no signals of weathering on the surfaces of EPS and PP,but roughness of PP pellets increased with increasing exposure time.