| Mangrove wetlands are one of the important coastal ecosystems.Mangroves are very prone to intercept plastic waste from continental runoff and tides due to their well-developed root and branches.The plastic can be broken into plastic fragments and microplastics(MPs)by light,weathering,and biological effects.Due to the hydrophobic and adsorptive properties of plastics,plastic surfaces are the ecological niche for microbial colonization and growth.Microorganisms are subjected to selective pressure from antibiotic drugs in the environment,produce antibiotic resistance genes(ARGs)and thus acquire resistance.ARGs can enable pathogenic bacteria to acquire drug resistance through horizontal gene transfer(HGT),posing a significant threat to human health and ecosystems.However,the contamination status of ARGs carried by plastic from mangroves has rarely been systematically reported.In this thesis,the distribution characteristics of MPs,plastic wastes,and ARGs in mangroves from the South China Sea were firstly investigated.The environmental factors affecting the abundance of(micro-)plastic and ARGs were analyzed.In addition,the effects of mangrove surroundings and MPs species on the distribution of ARGs on the surface of MPs were investigated by in situ experiments,and the key factors affecting the transmission of ARGs on the surface of MPs were revealed.Moreover,mangrove ecosystem was constructed through laboratory experiments to study the distribution of MPs in mangrove benthic organisms at different trophic levels,analyze the distribution characteristics of ARGs in the intestinal tract of different organisms,and explore the influence of MPs carrying ARGs on the distribution of intestinal ARGs in mangrove benthic organisms.The main research contents are as follows:(1)In this study,a total of 311 MPs from mangroves in the South China Sea were identified,with the abundance ranging from 0.00 to 579.55 items/kg(dry weight).The main size of MPs was 0.30～1.00 mm(53.38%).The main shape of MPs was fragment(42.77%),followed by pellets(29.26%).The predominant color of MPs was white(50.16%).The primary polymer types of MPs were polystyrene(PS,39.23%),polypropylene(PP,22.51%),and polyethylene(PE,15.76%).The polymer hazard index(PHI)in the mangroves of Shenzhen(SZ)and Haikou(HK)were all level 3.The abundance of MPs in mangrove sediments was significantly negatively correlated with total phosphorus(TP)(p<0.05),suggesting that the presence of MPs in sediments can increase phosphorus consumption by altering the microbial community.In addition,a total of 1047 plastic waste samples from mangroves were identified,and the abundance of plastic waste was 1.30 to 48.00 samples/m~2.The shape of plastic waste was dominated by pellets(42.31%).Transparent was the main color of MPs(36.10%),followed by white(33.24%),due to the fading of(micro-)plastic by sunlight aging.The primary polymer types of plastic waste were PE(30.18%),PP(27.51%),and PS(23.59%).The PHI of plastic waste in mangroves of Shenzhen(SZ),Haikou(HK),Zhuhai(ZH),Zhanjiang(ZJ),Qinzhou(QZ),and Fangchenggang(FCG)were all at level 3,indicating that the serious pollution situation of plastic waste in mangroves.The absolute abundance of ARGs and intl1 on the surface of plastic waste ranged from 1.92×10~6 to 2.73×10~9 copies/g.The detection rates of tet A,sul2,and intl1 were 100%for all sampling sites.Among them,sul2 was dominant in most mangroves with an abundance of 1.25×10~6～1.34×10~9 copies/g.In addition,msb A was dominant at S19with 1.31×10~7 copies/g.Erm F was dominant at S11～S13 with an abundance of 1.31×10~7～1.64×10~9 copies/g.The differences in the abundance of ARGs at different sampling sites may be related to the mangrove surroundings,MGEs,and the intensity of human activities.The abundance of ARGs on the plastic surface of living area(LA)mangroves was significantly higher than that of other mangroves.Different types of ARGs on the plastic waste surface of mangroves showed a significant correlation with intl1(p<0.01),implying that ARGs in mangroves had a higher risk of HGT.Correlation network analysis results showed that regional socioeconomic factors were significantly correlated with all ARGs,indicating that human activities can lead to changes in the number of ARGs carried on plastic surfaces.Furthermore,compared with mangroves in aquaculture pond areas(APA),the protected areas(PA)and LA mangroves had higher ARG storage,which greatly increased the ecological risk.(2)The effects of mangrove surroundings and MPs species on the distribution of ARGs on the MPs surface were investigated by in situ experiments.Results showed that the average abundance of ARGs(3.18×10~8 copies/g)in the Guanhai corridor(GH)mangrove was higher than that in other mangroves.In GH mangroves,the abundance of ARGs on the PP,PS,and PET surfaces was higher than that of other MPs.The average abundance of ARGs on the MPs from GH mangrove and Gaoqiao(GQ)mangrove was higher than that in the sediment,indicating that MPs in mangrove could enrich ARGs from the sediment.Notably,PET enriched the most ARG species in mangroves.In BY mangroves,PCL showed the highest enrichment coefficient of 59.24for intl2.Correlation analysis showed that the number of ARGs significantly correlated with MGEs(intl1 and intl2)in GH mangroves was the largest,indicating a high potential risk of ARG transmission in this type of mangroves.Network correlation analysis showed that the number of microorganisms carrying multiple ARGs in GH mangroves was the largest.Pathogenic bacteria such as Vibrio,Acinetobacter,and Bacillus might be potential hosts of ARGs.These results suggest that GH mangrove MPs may be a major site for the aggregation of multi-drug resistant microorganisms and the pathogenic bacteria carrying ARGs pose a great threat to ecological security and human health.Socioeconomic and environmental factors were correlated to varying degrees with the surface ARGs of MPs.The ARGs of PP and PE surfaces were most influenced by environmental factors.These results suggest that socioeconomic and environmental factors are the main drivers that shape the distribution characteristics of ARGs and MGEs on PP and PE surfaces.Therefore,the recycling rate of plastic products should be increased to mitigate the contamination of ARGs and MPs in mangroves.(3)In this study,the distribution characteristics of different concentrations of MPs in mangrove benthos were investigated by constructing an indoor mangrove ecosystem.The trophic levels of mangrove benthos were studied.The results showed that the Ocypoda stimpsoni Ortmann belonged to the third trophic level.The abundance of MPs in the Ocypoda stimpsoni Ortmann group was higher than that in other benthic organisms,and the actual concentration group and the 10x concentration group were35.00 and 149.50 microplastics/individual,respectively.The correlation analysis showed that the abundance of MPs in the actual concentration group and the 10x concentration group was a significant positive correlation with trophic level(p<0.05),suggesting that MPs accumulate in organisms along the food chain in the environment.The ingestion of MPs in Phascolosoma esculenta and Ocypoda stimpsoni Ortmann decreased the activity of intestinal digestive enzymes and increased the activity of intestinal oxidase.MPs can also cause severe intestinal damage with enteritis cell infiltration,vacuolation,and severe villus defects in Phascolosoma esculenta and Ocypoda stimpsoni Ortmann.The increase of MPs altered the intestinal ARGs and microbial community.As the concentration of MPs increased,the relative abundance of msb A,tet T,and tet B in the intestine of Ocypoda stimpsoni Ortmann gradually increased and the relative abundance of efr B in the intestine of Phascolosoma esculenta gradually increased.The proportion of multi-drug resistant ARGs in the intestinal tracts of Phascolosoma esculenta and Ocypoda stimpsoni Ortmann was the highest.Regression analysis showed a significant positive correlation between the abundance of ARGs and MGEs in the intestine(p<0.05).The results of network analysis revealed that the number of correlations between ARGs and MGEs in the Phascolosoma esculenta intestine was greater than that of the Ocypoda stimpsoni Ortmann group.This indicated that ARGs in the intestine of Phascolosoma esculenta had a higher risk of transmission.The results of Variance Partitioning Analysis revealed that MGEs was the main driving factor affecting the distribution of ARGs in the gut of Phascolosoma esculenta.Microbial community was the main driving factor affecting the distribution of ARGs in the gut of Phascolosoma esculenta. |
