Study On The Interaction Between Microplastics And Microorganisms In Elevated Aquaculture Ponds

In recent years,microplastics pollution in coastal area has attracted much attention in the field of environmental science.Microbiota living in the aqueous environment are easy to colonize and form biofilms on the surface of microplastics due to their durability,large specific surface area and strong hydrophobicity.The formation of biofilms may change the physicochemical properties of microplastics,further affecting their fate in the environment.Recent studies have shown that the discharge of offshore aquaculture wastewater is one of the major sources of microplastic pollution in coastal water.However,studies on the interaction between microplastics and microorganisms and their secreted extracellular polymer substances（EPS）in offshore aquaculture zones are still scarce.In this context,aquaculture industry related plastics,including polyethylene（PE）,polystyrene（PS）and polyvinyl chloride（PVC）,and biodegradable polylactic acid（PLA）were selected as the target microplastic in the present study.Microcosmic experiment and field exposure test in the mariculture pond for shrimp feeding were carried out.Firstly,difference of the morphological characteristics and microbial community structure of microplastics-associated biofilm in microcosmic test and exposure test were compared and analyzed.Then the morphological characteristics of microplastic surface biofilm and the dynamic change of microflora with the growth of shrimps were elucidated.Finally,the influence of biofilm formation on the physical and chemical properties of microplastics was investigated.The obtained results could provide a theoretical basis for the analysis of the environmental behavior and ecological effects of microplastics in coastal seawater of tropical region.The main research results of this paper are as follows:（1）Bacillus,streptococcus,cocci/ovoid（spherical）,filamentous and boat-shape algae inhabited on the surface of microplastics exposure to the mariculture pond,while bacillus and coccus were the dominant microorganisms in the microcosmic test.The total amount of biofilm on the surface of microplastics and the abundance and diversity of microbial community in the in-situ exposure test were significantly higher than those in the microcosmic test.Actinobacteria,Floptomyces,Chloroflexo＿bacteria,Cyanobacteria,Spirochetes,Ignavibacteriae and Parcubacteria were found only in the microplastic biofilm of in-situ exposure test.This revealed the limitations of laboratory microcosmic experiments,and in-situ exposure experiments are more conducive to comprehensive understanding of microplastic-associated biofilms in the natural environment.（2）During the growth of shrimp,cracks appeared on the surface of PE and PS,and more grooves and micropores appeared on the surface of PLA.The microflora structure on microplastics also changed,for example,the abundance of Proteobacteria on PE and PLA decreased,while the abundance of Bacteroidetes on PLA increased.On the whole,the biomass,abundance and diversity of bacterial community and EPS of microplastic surface biofilm were significantly increased with the growth of shrimps.The EPS of biofilm was mainly composed of high molecular weight polypeptides and proteins（related to microbial activity）,rather than humic-like DOMs.（3）The adhesion process between microplastics and EPS is partially reversible.The absorption peak strength of–OH and–CH₂ on PE was increased,which was mainly attributed to the lipids and polysaccharides in EPS.Due to the smooth surface of PS and PLA,the biofilm formed during the growth of shrimps is also easy to detach from the MPs.The O,N and Si elements were also detected on the surface of MPs after the formation of biofilm,which provided evidence for the presence of proteins,polysaccharides,lipids and diatoms on MPs.