Study Of Microbial Communities In The Intertidal Sediments And Their Responses To Pollutants

Tidal wetland in the estuary is an active zone in which various environmental processes interact between land,ocean and atmosphere.The microbial community plays an important role in regulating the biogeochemical processes in estuarine ecosystem.The tidal wetland sediments are susceptible to interference from dynamic natural environmental factors and anthropogenic impacts.The surface sediments of tidal wetlands are periodically exposed to the air and immersed in the tidal water.The physical,chemical,biological and other environmental elements in the sediments change drastically,and these changes will further affect the associated microbial communities living inside.The microbial community is sensitive to changes in the tidal wetland environment and can be used as an indicator for the tidal wetland ecosystem.At present,we have relatively limited understanding of the variation of microbial communities in tidal wetland sediments under large-scale spatial and temporal changes.It is important to study the structure and ecological functions of the microbial community in the tidal wetland and their changes under environmental disturbances.In this study,seven typical intertidal sediments from different seasons and salinity gradients at Yangtze River estuary were collected.The method of 16 S rRNA highthroughput gene sequencing was used to characterize the microbial communities in the sediments.The dynamics of bacterial structure and ecological functions of microbial communities under seasonal and spatial changes were investigated.The results showed that overall structure of the microbial communities within the intertidal sediments has an important role in indicating the ecological environment of the intertidal wetlands.There was a significant positive correlation between microbial communities' diversity and abundance in the tidal wetlands near the Yangtze River estuary.The overall richness of microbial communities was significantly negatively correlated with the total organic carbon concentration in the sediments.The overall level of microbial community diversity was significantly negatively correlated with the total phosphorus concentration in the sediment.The composition of dominant bacterial communities remained stable in the sediment at phylum level.The top ten dominant bacterial phyla were Proteobacteria(45.35 %),Acidobacteria(9.96 %),Chloroflexi(9.38 %),Verrucomicrobia(6.59 %),Firmicutes(6.54 %),Planctomycetes(5.60 %),Bacteroides(5.44 %),Actinobacteria(2.62 %),Nitrospirae(0.89 %)and Cyanobacteria(0.22 %),respectively.The overall structure(? diversity)of the microbial communities in the tidal wetland of the Yangtze River estuary showed obvious seasonal changes with seasonal patterns.The microbial communities structure in tidal sediments was significantly different in the rainy and dry seasons.There was no significant change in the composition of dominant bacteria in the microbial community in the studied tidal sediments during seasonal changes.However,the relative abundance of dominant bacteria in the studied tidal sediments was significantly different in seasonal variations.The relative abundance of phylum Verrucomicrobia and its lineage(class Verrucomicrobiae,order Verrucomicrobiales,family Verrucomicrobiaceae and genus Luteolibacter)showed significant seasonal changes.These results suggested that the vertical bacterial diversity might affect its abundance under seasonal changes.Redundancy analysis demonstrated that total organic carbon,total phosphrus,and pH had significant impacts on the composition and abundance of dominant bacteria in the microbial community within the tidal sediments.And the total organic carbon had the greatest impact among these three environmental factors.There were no significant changes in the seasonal turnover of the component of ecological functions controlled by microbial communities.The microbial community in the tidal sediments of the Yangtze River estuary regulated nitrification and denitrification processes in the nitrogen cycle.There was obvious coupling relationship between the nitrification and denitrification processes of microbial communities in the studied tidal sediments.Nitrification and denitrification processes in the studied tidal sediments of the Yangtze River estuary were affected by total nitrogen concentration.As the total nitrogen concentration in the sediment increased,the contribution of the denitrification of the microbial community gradually strengthened and dominated.The overall structure of the microbial communities showed significant difference between brackish and freshwater sediments.The concentrations of total sulfur,total nitrogen and total phosphrus had significant differences along the salinity gradients in the collected sediments.Redundancy analysis showed that total sulfur and total nitrogen had great impact on the composition and abundance of dominant bacteria in the microbial community.The variation of microbial community was shaped by comprehensive environmental conditions.Deltaproteobacteria(relative abundance above 10 %)was responsible for respiration of sulfur compounds,which was an important biogeochemical cycle in the intertidal sediments at the Yangtze River estuary.There was a significantly positive relationship between the contribution of respiration of sulfur compounds and the salinity of the associated sediments.The total organic carbon concentration and total nitrogen concentration in the studied tidal sediments showed a significant negative correlation with the relative abundance of Deltapotoobacteria and its contribution of the regulated respiration of sulfur compounds.The results demonstrated that the ecological function of the microbial community is regulated by functional bacteria,and differences in the abundance of functional bacteria under environmental changes can cause changes in the contribution of the ecological functions of the microbial community.The environmental conditions in the tidal sediments at Yangtze River estuary changed drastically.The major composition of bacterial members and ecological functions of microbial communities had no significant differences among seasonal or spatial changes,indicating the resilience of microbial communities under dynamic estuarine ecosystem.The characterization of microbial communities at Yangtze River estuary provided an important understanding for the future study on the biogeochemical cycles in the tidal wetland.Tidal sediments in the coastal zone could be considered as an important sink of regional pollutants.The occurrence,fate and environmental behavior of heavy metals in the surface tidal sediments were complex.Heavy metals in the surface tidal sediments could easily interact with microbial in the surface sediments.This study conducted an environmental risk assessment of heavy metal pollutants in the sediments of the Yangtze River estuary tidal flat and east Chongming Island.The effect of heavy metals on the structure and ecological functions of microbial communities were also investigated.The geo-accumulation index was used to evaluate environmental risk of heavy metal pollution in the studied tidal sediments.The results showed that microbial communities played an important role in indicating the heavy metal pollution.The sediments from Yangtze River estuary tidal flat and east Chongming Island were polluted with moderate level and low level of pollutants,respectively.Redundancy analysis demonstrated that Zn had a significant impact on the compositions and abundances of dominant bacterial phyla,classes,orders and families in the tidal sediments.There was a positive relationship between geo-accumulation index and threshold effect concentration level for Zn,which made it as an indicator of heavy metal pollution in the tidal sediments.Different dominant bacterial groups showed different sensitivity and tolerance to heavy metals.The lineage(class,order,family and genus)of Chloroflexi were tolerant to both Zn and Cr,and their relative abundances had significantly positive correlations with the concentrations of Zn and Cr in the tidal sediments.And Acidobacteria,Nitrosomonadales and Burkholderiales showed adaptability to heavy metal pollution.However,the relative abundance of Desulfobacterales and Desulfobulbaceae had significantly negative correlations with the concentrations of Zn and Cu,indicating that both Desulfobacterales and Desulfobulbaceae were sensitive to Zn and Cu.And Desulfobacterales,Rhodobacterales and Rhodobacteraceae also showed sensitivity to overall pollution level of heavy metals in the tidal sediments.The respiration of sulfur compounds was relatively sensitive to both the concentration of Zn and Cu,and the overall pollution level of heavy metals in the studied tidal sediments.The concentration of Zn and Cu in the studied seven tidal sediments was significantly negatively correlated with the contribution rate of the respiration of sulfur compounds regulated by the microbial communities.Zn played a key role in influencing the structure of twelve families(lineage of Desulfobacterales)that were affiliated with the respiration of sulfur compounds.The above results indicate that heavy metals in the studied tidal sediments can affect the ecological functions of microbial communities by affecting the composition and relative abundance of functional microbial groups.Silver nanoparticle(AgNP)is an emerging antibacterial pollutant with unique antibacterial effect,high usage and wide applications.The surface tidal sediments were sinks for discharged AgNPs.In this study,Bacillus was selected from the sediment samples at Yangtze River estuary as the representative natural bacterial genus.The toxicity,mechanism and effects of AgNPs to Bacillus subtilis were carefully investigated.The result indicated that precipitation of AgNPs will increase the probability of contact with microorganisms in the surface sediment,which might induce potential toxicity to the bacterial community in the sediment.Exposure to AgNPs(900 ?g/L)would reduce the growth rate and cell viability of Bacillus subtilis.The toxicity of AgNPs to Bacillus subtilis was significantly reduced when their surface contact was blocked by a dialysis membrane.Scanning electron microscope images showed that exposure contact to AgNPs resulted in damage of the membrane structure of Bacillus subtilis.These results suggest that particle-cell contact is largely responsible for the observed toxicity of AgNPs.AgNPs had lower toxicity to B.subtilis in estuarine water,which might be due to the observed enhanced aggregation of AgNPs caused by dissolved organic carbon in estuarine water.This study selected environmental bacterial genus Bacillus from environmental samples to evaluate the toxicity of AgNPs.The understandings of behavior of AgNPs in the coastal environment and toxicity of AgNPs to the coastal bacteria can help us to understand the ecotoxicology and environmental risks of emerging pollutants in the coastal ecosystem.The microbial community structure and their ecological functions in the surface sediments at Yangtze River estuary showed significant seasonal and spatial changes.The bacterial diversity and abundance of the microbial community responded to the environmental disturbances,and these responses would further affect their ecological functions in the tidal sediments.The correlation between heavy metals concentrations and bacterial abundance indicated the bacterial sensitivity or adaptability to heavy metal pollution.The eco-toxicological study showed that the surface contact between antibacterial pollutants and bacteria played an important role in determining its toxicity in the estuarine environment.This study characterized the dynamics of microbial communities in the surface sediments at Yangtze River estuary and studied their responses to various pollutants,and it can be used as an important reference for the protection of tidal wetlands and coastal area.