Environmental Behaviors Of Nanoparticles In Intertidal Zone Mediated By Sulfate-reducing Bacteria:Occurrence And Bioavailability

Nanoparticles(NPs),ubiquitous in natural environment,exhibit unique physicochemical properties due to their tiny particle sizes.With the development of nanotechnology,a large number of engineering NPs have been applied to various consumer goods.Because of the widely use of NPs and the accidental NPs produced by human activities,the behaviors and the ecological risks of NPs in the environment are attracting researchers'attention.Once released into the environment,almost all NPs undergo series of translocations and transformations,in which microbes play important roles.Therein,sulfate-reducing bacteria(SRB)can reduced sulfate into sulfide by the dissimilation under anaerobic conditions.Many metal sulfide NPs in the environment could be synthesized by SRB via the precipitation of metal ions or the transformation of NPs.With important ecological functions,intertidal zone is a natural reservoir of many pollutants,such as metals and NPs.However,there is still a lack of the study about the occurrence and behavior of NPs in intertidal zone in the presence/absence of SRB.This study therefore investigated the occurrence and distribution of SRB community and metal NPs in Chinese intertidal sediments,and further explored their related environmental factors.Moreover,taking Ag⁰-NPs as an example,we explored the effects of SRB community on the occurrence,transformation and bioavailability of NPs in the intertidal zone through simulation experiments.The SRB community structure and composition in Chinese intertidal sediment were characterized in this study.The results showed that dsr B gene abundances varied among different sampling sites,with the highest average abundance of SRB at XHR(near the Bohai Sea).The SRB community structures showed obvious spatial distribution pattern with latitude along the coastal area of China,with Desulfobulbus as the dominated genus generally.Correlation analysis and redundancy discriminant analysis(RDA)revealed that TOC and p H showed the significantly correlation with the richness of SRB community,and salinity,p H,sulfate and climatic parameters were the important natural factors influencing the composition of the SRB community.Structural equation model(SEM)analysis indicated that the differences of the SRB community structures in Chinese intertidal sediments could be attributed to the combined impacts of human activities(e.g.,population,economy and industry and agricultural structure and activities)and natural factors(e.g.,salinity,p H and climatic parameters).Moreover,anthropic impacts could drive the SRB community structures directly or via affecting the distribution of metals significantly.In addition,Desulfobulbus,the dominated SRB in Chinese intertidal sediments,was positively correlated with primary industry and its related activities,such as the animal husbandry and the consumption of chemical fertilizer.Furthermore,most metals related to the urbanization rate were the vital factors influencing the relative abundance of Desulfatiglans.Thus,SRB could be suggested as the potential bio-indicator species to indicate the anthropogenic activities and pollutions(especially for metals)in the sediments along the intertidal zones of China.Based on electron microscopy technology,a large number of metal NPs with different shapes and elemental compositions were found in Chinese intertidal sediments,including goethite,hematite,magnetite,pyrrhotite,mackinawite,pyrite,rutile,anatase,Ti_nO_2n-1,wurtzite and other NPs containing Zn,Pb,As,Ce and Zr.Moreover,Single particle(SP)-ICP-MS analysis found that the average size of Fe-containing nanoparticles(Fe-NPs),Ti-containing nanoparticles(Ti-NPs)and Zn-containing nanoparticles(Zn-NPs)were in the ranges of 35-66 nm,50-93 nm and 28-51 nm,respectively.The particle concentrations in number were 5.30×10⁸-6.89×10¹⁰particles/mg for Fe-NPs,8.10×10⁶-3.67×10⁸particles/mg for Ti-NPs and 4.32×10⁶-1.66×10⁹particles/mg for Zn-NPs.RDA and correlation analysis found that Ti-NPs could be regulated by the total concentrations of corresponding metals.TOC,salinity and p H were important environmental factors affecting the occurrence and behavior of Fe-NPs and Ti-NPs in Chinese intertidal sediments.In addition,the occurance of NPs in intertidal sediments could be affected by SRB,and in turn,the SRB community structure could be regulated by NPs.To explore the environmental behavior of Ag⁰-NPs in intertidal environment,we simulated the tidal process in the Yangtze Estuary,and selected Scirpus triqueter to investigate the bioavailability of Ag⁰-NPs.Results showed that the total concentrations of Ag and the particle concentrations of Ag-NPs in intertidal sediments and plants increased with time after being exposed to Ag⁰-NPs/Ag⁺.The concentrations of total silver and Ag-NPs were significantly higher in low tidal flat sediment than those in middle and high tidal flat sediments.Compared to root,the stem had a higher bioconcentration factors for total Ag,and showed a stronger biomagnification effect.The average particle sizes of Ag-NPs in intertidal sediments showed an overall trend of high tidal flat>middle tidal flat>low tidal flat.Moreover,the average size of Ag-NPs in rhizosphere sediments was significantly lower than that in non-rhizosphere sediments(P<0.05).The concentrations of Ag and Ag-NPs in root were lower than those in stem,suggesting that Ag in water is more easily accumulated by plants compared to the Ag in sediment.In addition,the average particle size of Ag-NPs in the root was significantly smaller than that in the stem tissue(P<0.05).To investagete the factors affecting the accumulation of Ag-NPs in the intertidal environment,we analyzed the SRB community and physicochemical properties in the intertidal sediments.Results showed that,the dsr B abundance and the diversity and richness of microbial community decreased in Ag⁰-NPs and Ag⁺treatment group.On the contrary,Ag⁰-NPs/Ag⁺can enhance the increase of some silver resistance gene(SRGs),especially for Sil E gene.RDA and correlation analysis found that TOC,p H and chlorophyll a were the vital factors affecting the occurrence of total Ag and Ag-NPs.Moreover,the average particle size of Ag-NPs in intertidal sediment was significantly negatively correlated with the abundance of dsr B(P<0.05).This suggests that SRB may affect the accumulation of Ag-NPs in plants by affecting the occurrence characteristics of Ag-NPs in the intertidal sediments.Correlation analysis also found that SRB could enhance the uptake of Ag-NPs by plants during the exposure of the plant to Ag⁰-NPs.The particle concentration of Ag-NPs in the sediments was significantly positively correlated with the Sil E gene,indicating that bacteria with the Sil E gene could secrete binding proteins to enhance the accumulation of Ag-NPs in the sediments.Network analysis and correclation analysis found that there was a significantly positive correlation between the particle concentration of Ag-NPs and the abundances of many microorganisms,including the traditional SRB(e.g.,desulfosporosinus)and non-traditional SRB(e.g.,acidaminobacter),which indicates that many non-traditional SRB may host the dsr B gene and are also likely to be the protagonists involved in the global sulfate reduction process.In order to further explore the influence of SRB on the bioavailability of Ag⁰-NPs,we isolated an SRB(Enterobacter sp.)from Scirpus triqueter rhizosphere sediments collected at the Yangtze Estuary,China and applied it to the laboratory simulation experiment on the uptake of Ag⁰-NPs/Ag⁺by Scirpus triqueter.The results showed that although the presence of SRB could reduce the uptake of total Ag by 37%during the exposure of the plant to Ag ions,it can significantly enhance the uptake of total Ag during exposure of the plant to Ag⁰-NPs,likely by transforming Ag⁰-NPs into Ag-sulfide NPs with smaller particle sizes.Transmission electron microscopy data revealed that biogenic secondary Ag-sulfide particles smaller than 10 nm in size form in the vicinity of pristine Ag⁰-NPs.These NPs are likely generated from the parent Ag⁰-NPs via a dissolution-diffusion-sulfidation process.Moreover,the phyto-uptake of Ag⁰-NPs of various sizes(i.e.,20,40 and 80 nm)in the presence/absence of SRB also confirmed a size dependent pattern,with more silver identified in the plant exposed to smaller Ag-NPs.The combined results suggest that the enhanced bioavailability of Ag-NPs to Scirpus triqueter in the presence of SRB is mainly attributed to the formation of secondary biogenic NPs with minute size.This result points to the importance of complex,coupled interactions between aqueous solutions,bacteria,plants,and labile NPs.