Microbial Community Succession In Responses To A Heavy Metal Gradient And Along A Chronosequence In Slags And Soils Near A Smelting Plant

Release of heavy metals(HMs)naturally occurs through weathering,geothermal activities,forest fires,and microbial activities.However,this process has been accelerated rapidly by anthropogenic activities,especially industrial production,causing the increased global dispersion of HMs thereby constituting a great threat for human and ecosystem health.Nowadays,anthropogenic emissions of heavy metals into the environment have exceeded the tolerable baseline of natural cycling in many ecosystems.Urbanized areas with intensive mining and industrial activities are typical hot spots of heavy metal contamination.Long-term exposure and absorption of high-concentration HMs are harmful to microbes,plants,and animals and potentially damage ecosystems.Microbial communities are highly sensitive to the physical and chemical changes in environments.These factors including HMs,electrical conductivity(EC),pH value,nutrient availability and moisture content are environmental driving forces which directly or indirectly control the changes of microbial communities.Despite increasing knowledge concerning the impact of metal pollution on microbial communities and ecological functions,it is still a valuable problem to identify the driving patterns of succession of microbial communities along gradients of elevated metals and other environment factors in natural environments.In the present study,the bacterial and fungal communities in the uncultured soils of 7500-m distance from a smelter and in the slags of non-ferrous metals smelted along a stacking chronosequence were investigated using Illumina MiSeq of 16S rRNA genes and 18S rRNA ITS sequences in arid region loess soil in Northwest China.Meanwhile,the concentration varies of multiple heavy metals,electrical conductivity(EC),pH,total nitrogen(TN),total carbon(TC),water content(GWC)of the soils and slags were measured.Based on the analysis of the bacterial and fungal communities along environmental factors,we obtained the main results are as follows.1.The concentrations of multiple HMs and the levels of EC value,pH value,TC,TN,GWC in the natural loess along a 7500-m distance from a smelter were measured.The results showed that a soil HMs gradient was detected over a distance scale.Decreasing gradients of Pb,Zn,Cd,Cu,Ni,Cr,and total metals(TMs)levels with the increase of the distance from the smelter,which provides an ideal soil pattern for unveiling soil microbes in response to multiple HMs exposure.Moreover,the differences in soil pH and EC are helpful for exploring the response of microbes to HMs exposure in combination with soil salinization.2.The results from high-throughput 16S rRNA gene sequencing suggested the differential responses of soil bacterial and fungal community structures to HMs,EC,and pH.High HMs increased the abundances of the bacterial phyla Actinobacteria,Bacteroidetes,Deinococcus-Thermus,and Chloroflexi,and the genera Blastococcus,Rubrobacter,Quadrisphaera,and Tunicatimonas,whereas they decreased the abundances of the phyla Proteobacteria and Acidobacteria and the genera Streptomyces and Nocardioides.High EC and low pH decreased the abundance of most of the dominant bacterial phyla but increased the abundances of Firmicutes,Deinococcus-Thermus,and Nitrospirae.Furthermore,high HMs and EC reduced the numbers of soil-specific bacterial and fungal groups and drove the succession of certain groups that were highly resistant to increased HMs and EC.In addition,many bacterial and fungal groups exhibited different response patterns to each HMs,implying that,in multiple HMs-contaminated soils,HMs jointly shaped the microbial communities.RDA/PCA analyses revealed that soil pH,HMs,and EC jointly affected the bacterial communities in the soils.PICRUSt analysis suggested that high HMs significantly decreased the total gene abundance and most KEGG modules in the soils.High EC and low pH significantly enhanced the abundances of several two-component system-,electron transfer-,and methanogenesis-related modules.We conclude that excessive multiple HMs and EC principally repressed the microbial activity and severely drove the gradient succession of bacterial and fungal communities.3.The concentrations of multiple HMs and levels of EC,pH,TC,TN,GWC in the slags sampled along a stacking chronosequence were investigated.The results indicated that Long-term high HMs levels caused an insignificant pH gradient decrease in the slags along the chronoseqence.The results from high-throuput sequencing indicated that high HMs level significantly reduced the diversity and richness of bacterial and fungal communities in the slags.The diversity,richness and complexity of bacterial and fungal communities were highest in the middle stage of stacking chronosequence series sampled with significant variation.Long-term high HMs exposure significantly reduced the diversity and abundance of bacterial and fungal communities and the OUT number in the slags,and enriched some bacterial and fungal groups in the slags,including the dominant bacterial phyla Chloroflexi(26.26%),Actinobacteria(25.63%),Proteobacteria(14.77%),the bacterial genera unclassified_f_Gemmatimonadaceae(6.98%),norank_p_Saccharibacteria(6.07%),Thermobaculum(5.51%),the dominant fungal phyla Ascomycota(80.74%),unclassified-k-Fungi(8.67%),Basidiomycota(6.86%),and the fungal genera Aspergillus,(20.55%),unclassified_k_Fungi(8.67%),Chaetomium,(6.83%),Phoma(4.67%),Fusarium,(4.47%),and Alternaria(3.62%).The bacterial phyla Chloroflexi and Actinobacteria,the bacterial genera Thermobaculum,and the fungal phyla Ascomycota,were the screened groups of long-time exposure of high multiple HMs.These taxa have the potential of bioremediation for multiple HMs contamination.4.RDA/CCA analyses revealed that Ni significantly correlated with the variations of bacterial communities,whereas,multiple factors(Ni,Cu,TN,TC,and HMs)jointly affected the fungal communities.Two-factor correlation network analysis implied the simple correlation between bacterial groups and environmental factors(mainly Cr,Cd,GWC,Cu,Cr,and Ni),but the complex correlation between fungal groups and environmental factors(mainly TN,TC,Cu,Ni,Zn,Pb,Cd,GWC,pH and HMs).Species correlation network analyses showed that the fungal phyla Ascomycota were negatively related to other groups,and the fungal genera groups belong to phyla Ascomycota were mainly positively related to the other fungal groups,suggesting co-evolution in fungal genera groups and competition strategy in Ascomycota.Most bacterial phyla groups were positively correlated with the other groups,but the bacterial phyla Chloroflexi,Proteobacteria,and Actinobacteria were both positively and negatively correlated with the others.We concluded that excessive HMs concentrations seriously inhibited microbial activities in the slags and drovn the gradual successions of bacterial and fungal communities in structure and function to HMs-tolerant groups.5.PICRUSt analysis suggested that high HMs significantly decreased the total abundance of bacterial KEGG modules,enzymes,KOs,and pathways in the slags,but obviously enhanced the abundances of nitrogen metabolism-,two-component system-,methanogenesis-,multidrug resistance-,secretion systems-,electron transfers-,and transporters-related KEGG modules.The high HMs also decreased the total abundance of fungal KEGG enzymes in the slags,but the abundances of ATP generation pathways-and gluconeogenesis pathway-related enzymes were the tops.To sum up,the structure and succession of bacterial and fungal communities in the slags and soils around smelting plant maily affected by the chronosequence and heavy metals.Heavy metals inhibited the activity of most microorganisms,and gradually promoted the community structure and function succession of bacterial and fungal to the heavy metals resistant groups.These results help the environmental governance and recovery of heavy metal pollution mining area.