Response Of Soil Microbial Diversity And Resistance To Heavy Metal Pollution

Heavy metal pollution imposed health risks on organisms within the ecosystem.Due to the direct contact with the heavy metal,the structures and functions of microbial community were seriously affected.Meantime,microorganisms developed a series of resistance systems under the selection pressure of heavy metal pollution.The genetic basis of these resistance systems are heavy metal resistance genes(MRGs)and antibiotic resistance genes(ARGs).Changes in microbial community structure,function and resistance system,which could indicate the heavy metal pollution in environmental compartments,were important indicators of environmental health risk in heavy metal pollution areas.In this paper,field investigation were conducted firstly in a copper tailing dam,which was built up since 1969,and its surrounding area.Four sampling sites,TD0,TD1,TD2 and TD3,were selected to analyze the soil heavy metal pollution characteristics,potential ecological risks,soil microbial structure,function and resistance.TD0 site is on the top of tailing dam and covered with herbs.TD1 site is at the bottom of the dam and has been associated with poplar plantation of 15 years.TD2 site,adjacent to TD1 site,is farmland.TD3 site is the most far away from the dam and has been associated with poplar plantation of 10 years.Based on the field research,the method of potential ecological risk(RI)assessment were used to establish soil microcosm.Cu with a low toxic factor value(TF)of 5 and Cd with a high TF of 30 were manipulated to achieve three RI levels,low(L),moderate(M)and high risk level(H),in soil microcosm.The soil microbial structure,function and resistance under the three risk levels were analyzed after the incubation of soil microcosm.The main conclusions are as follows:(1)There were significant differences in the potential ecological risks among four sampling sites.The highest RI was found in TD1 site,while the lowest one was at TD0 site.Such differences might be the results of migration of heavy metals caused by seepage water and the fixation of heavy metals with vegetation reconstruction.(2)It was found that 16 S r DNA and MRGs in TD1 site was significant higher than the other three sites,and in TD0 site was the lowest.The abundance of MRGs is positively correlated with soil Cd concentration,such relationships indicated the importance of Cd in the selection of MRGs.The co-occurrence networks of MRGs and ARGs shows that there might be the co-selection of MRGs and ARGs in study sites.The risk of the proliferation of ARGs should be highlighted in such area.(3)The dominant bacteria taxa were Proteobacteria,Actinobacteria,Bacteroidetes,Gemmatimonadetes,Acidobacteria and Chloroflexi in study sites.The non-random cooccurrence between the dominant bacteria taxa and resistant genes showed that Nitrospira and MND1 might be the potential hosts of pco A,czcd and a variety of tetracycline ARGs.There were significant differences in the bacterial community functional diversity of carbon source utilization among the four sampling sites,which indicated that no functional redundancy might be for bacterial community in our study area.(4)There were significant differences in the abundance,structure and functional diversity of fungal community among four sampling sites.The abundance and the species diversity of the fungal community were the highest in TD1 site and the lowest in TD0 sites.However,the fungal community functional diversity of carbon source utilization in TD0 sites was significantly higher than those in TD2 and TD3 sites.The utilization intensity of glycoside carbon sources of the fungal community in TD0 site was significantly higher than the other three sites.We also found that the relative abundance of Ascomycota,one of the dominant fungal taxa,was up to a percentage of 92.92%.The differences in the structure and the carbon sources selection of fungal communities might be the key factors to the differences in the fungal community functional diversity of carbon utilization among the four sites.The network analysis showed that it was easy to form a stable network in their trophic level for bacterial and fungal communities,respectively.(5)As shown in soil microcosm under different RI levels,the abundance of bacterial community decreased and the MRGs increased with the increase of potential ecological risk levels.The main components of MRGs were cop A,czc A and czc N genes in soil microcosm.The relative abundance of cop A genes resistant to Cu would decrease while czcs genes resistant to multi-metals would increase under the condition of Cu and CD combined pollution with in M and H risk level,respectively.The ratio of Cu and Cd might be the key factor in the composition of MRGs within each risk level.The relative abundance of FCA genes resistant to multidrug were the main ARGs in L and M risk levels,while ARGs resistance to ? – lactam was most abundant in H risk level.The relative abundance of class I integron,int I1,were the main mobile genes elements(MGEs)in L and M risk level,respectively,and TP614 in H level,which indicated the different mechanism in horizontal migration of resistant genes under different risk levels(6)Among all the bacterial taxa,Actinobacteria,Proteobacteria and Acidobacteria were abundant in L and M risk levels,while Firmicutes and Bacteroidetes were dominant in H risk level.The main carbon sources used by bacterial community were amines,esters and alcohols.However,the utilization intensity of carbohydrate was higher in treatments with combined pollution by Cu and Cd than in the treatments polluted by single Cu or Single Cd under each ecological risk level.The significant correlations between the carbon utilization patterns and MRGs indicated that the bacterial resistant system in soil bacterial community might play a key role in bacterial functional diversity of carbon source utilization.(7)Soil bacterial community structure,function and resistance were significantly different among different treatments under the same ecological risk level.Compared to Cd,bacterial community structure,function and resistance were significantly correlated with the absolute content of Cu.Thus,we speculated that the TF of Cu was underestimated.We adjusted this value by mediating the correlations between MRGs and RI level by slope single change point analysis,and suggested that TF value of Cu should be adjusted to 6 during the process of potential ecological risk assessment.In general,both heavy metal pollution and land use type affect the structure,function and resistance of soil microbial community.There were different characteristics in microbial community under the single and combined pollution from Cu and Cd within the same RI level.This study improves the understanding of the relationship between heavy metals and soil microbial community,and underlies the exploration of the ecological restoration of heavy metal contamination areas.