| Due to the durability of plastic products,most synthetic polymers have poor degradability in the marine environment.However,when their structure becomes fragile,they can be decomposed into small particles under physical or chemical action,that is microplastics are produced.Microplastics usually refer to plastic particles with a maximum size of less than 5 mm.Because of its small particle size,floatation and slow degradation characteristics,microplastics are ubiquitous in marine environment and do harm to marine organisms.At present,microplastic pollution has become one of the main problems facing the marine environment.Microplastics are easy to be aged under the influence of chemical,physical and even biological processes in the environment,in which solar radiation is the main factor leading to the aging of microplastics.In addition,as a relatively new kind of pollutants in marine ecosystem,microorganisms can adhere to microplastics surface to form biofilm.Microplastic biofilm can provide important ecological advantages for attached microorganisms,such as providing nutrients and forming protective barriers.Because the floatability and persistence of microplastics are stronger than most natural substrates,they may become the carrier for the transportation of some pollutants,toxic,or harmful organisms.In this paper,fragment,pellet and fiber polypropylene(PP)microplastics were used as microbial attachment materials,and some microplastics were treated with UV irradiation to obtain aged PP microplastics.Microplastics samples were incubated in Yantai coastal marine,estuarine and river habitats for enclosure experiments in spring and summer respectively,and the samples were incubated at each station for 6 weeks.The key environmental factors affecting biofilm growth rate and flora composition were discussed through the analysis of biofilm morphology,microbial community characteristics,carbon source utilization capacity and antibiotic resistance genes.We try to apply the research results to the screening of plastic degrading bacteria,so as to apply them to the relevant contents of plastic treatment.This research can provide data support for the ecological risk assessment of microplastics in the marine environment,and provide a scientific basis for the pollution control of marine micro plastics.The main research results of this paper are as follows:(1)Microplastics samples were irradiated with UV for 15 days(UVC,4 × 60W).The change of element content on microplastic surface was characterized by electron spectroscopy.The results show that the content of oxygen on the surface of microplastics increases significantly after aging,indicating that UV irradiation can cause the generation of oxygen-containing groups.The results of FTIR showed that there were carbonyl additional vibration peaks on the surface of aging microplastics,indicating that the oxidation effect occurred on the surface of microplastics.The results of scanning electron microscope show that accelerated ultraviolet irradiation will produce small cracks and folds on the microplastics surface and increase the roughness.In addition,the effect of biofilm formed in field environment on the virgin microplastics showed that the surface oxygen carbon ratio of microplastics attached with biofilm increased with the extension of culture time,so the formation of biofilm will also accelerate the aging process of microplastics.(2)The content of microplastics biofilm biomass increases with the increase of incubation time,and the biofilm biomass was higher on aging microplastics.Therefore,there was a positive correlation between the incubation time and the growth time of biofilm.Compared with the marine and estuarine stations,the river station is the most suitable growth environment for microorganisms in spring and summer.It is speculated that it may be related to the high ambient temperature,nutrient content and low ionic strength of fresh water in this season.Compared with spring,summer has higher temperature,longer illumination time,lower dissolved oxygen content and faster cell metabolism.Therefore,the content of biofilm on the surface of microplastics was significantly higher in summer(1.004 ± 0.54 Abs)than in spring(0.103 ± 0.08 Abs).(3)The results showed that the main factors affecting the growth of microplastic biofilm in spring were the salinity and temperature.The temperature mainly distinguishes the difference between the early and late stages of biofilm development.Salinity mainly distinguishes the difference of biofilm content between different habitats.The ionic strength of river habitat is significantly lower than that of estuary and marine so it is more suitable for microbial colonization.Nutrient content is the main environmental factor limiting the growth of microplastic biofilm in summer.In the early stage of biofilm development,it is directly proportional to the nutrient content,but in the later stage of development,the biofilm content does not increase with the increase of nutrient content.(4)The incubation habitants have significant effect on the biofilm α diversity,and the α diversity of microplastic biofilm incubated in the river habitat was significantly higher than that of marine and estuary,but the aging state of microplastics had a weak impact on it.The analysis of microbial community composition showed that Proteobacteria was the main dominant bacteria in the microbial community of biofilm in spring and summer.In addition,the bacteria with high relative abundance in spring also include Cyanobacteria and Bacteroidea,while the bacteria with high relative abundance in summer include Bacteroidea,Firmicutes,Acidobacteria and Planctomycetes.A certain abundance of potential pathogenic bacteria was also detected in the microplastic biofilm.The main potential pathogenic bacteria in the spring samples were Flavobacterium and Moraxella,while the potential pathogenic bacteria in the summer samples were Burkholderia,Pseudomonas,Aeromonas and Flavobacterium.(5)The incubation habitants and aging states of microplastics can affect the microbial community distribution of microplastics biofilm in spring,but have little effect in summer.The functional prediction results showed that genes related to human pathogens,hydroxide oxidation,sulfate reduction and photosynthetic autotrophy were abundant in biofilm community,and some metabolic functions were closely related to biogeochemical cycle.It is worth noting that the expression of human pathogen related genes in aging microplastics biofilm is significantly higher than that in virgin microplastics.This result provides a basis for aging microplastic biofilm to carry more human pathogens,indicating that aging microplastic may pose a greater potential risk to human health.(6)The analysis of carbon utilization capacity of microplastic biofilm community in summer showed that there were differences in carbon utilization capacity at different incubation sites,which the biofilm community in the river habitant had higher carbon metabolism capacity.The aging treatment of the microplastic has no significant effect on the biofilm carbon metabolism capacity.The microbial community on the microplastic biofilm has different utilization efficiency of 31 carbon sources,indicating that microplastic can significantly affect the microbial mediated carbon cycle process in aquatic ecosystem,and has important ecological significance for the geochemical cycle of carbon.(7)The relative abundance of resistance genes in microplastic biofilm in summer was analyzed,and the relative abundance of resistance genes in estuary station was the highest.It is speculated that it may be due to the frequent human fishing or production activities in estuarine area.The content of resistance genes on aged microplastics is higher than that of virgin microplastics,which may be that aging treatment changes the surface charge and hydrophobicity of microplastics and enhances its adsorption capacity for pollutants.On the other hand,aged microplastics may form more stable hydrogen bonds with resistance gene.Among all the detected resistance gene types,aminoglycoside and fluoroquinolones genes are the main antibiotic resistance gene types.Therefore,aging microplastics may increase the risk of long-distance transmission of antibiotic resistance genes in the aquatic environment. |
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