Diversity Of Iron-and Sulfur-cycling Microorganism And The Role Of Microbial Activity In Fe(?) Hydroxysulfate Mineral Transformations In A River Affected By Acid Mine Drainage

Acid mine drainage?AMD?is characterized by a relatively low pH,high concentrations of sulfate and a number of heavy metals.AMD has been a significant environmental challenge for AMD-receiving aquatic systems.Previous studies showed that Fe-and S-cycling microorganisms might play important roles both in the formation and the natural attenuation of AMD.Therefore,a better understanding of the diversity of Fe-and S-cycling microorganisms in AMD sites and microbial processes of the Fe and S cycling,will enable the deciphering of the formation mechanism of AMD,and improve the development of techniques to control AMD formation and enhance bioremediation in AMD sites.In addition,a large amount of Fe?III?hydroxysulfate minerals were found in the sediments in the upstream of the Hengshi River in the Dabaoshan mine.The formation and transformation of Fe?III?hydroxysulfate minerals exert significant control on the migration and transformation of toxic elements in mining area.While there are many studies conducted on the influences of chemico-physical factors on the Fe?III?hydroxysulfate mineral transformation,the role of indigenous microbial activity in Fe?III?hydroxysulfate mineral transformation in AMD sites has not received sufficient attenation.In the present work,we surveyed the Hengshi River watershed which was continually contaminated by AMD produced from upstream Dabaoshan Mine,a multi-metallic sulphide mine in Guangdong Province.Characterization of AMD pollution in the Hengshi River watershed was investigated.Then four sampling campaigns were carried out in Jule 2015,January 2016,August 2016,and November 2016.Spatiotemporal dynamics of microbial communities in the Hengshi River watershed were comprehensively investigated by Illumina high-throughput sequencing.The interrelationships between microbial communities and the environmental variables were analyzed.Meanwhile,the diversity and spatial distribution of Fe-and S-cycling microorganisms were combined with Fe and S species distribution to explore the biogeochemical processes regulating the Fe and S cycling within the Dabaoshan Mine.Finally,a microcosm-based study was conducted to investigate the role of microbial activity in Fe?III?hydroxysulfate mineral transformations in AMD sites.?1?Characterization of AMD pollution in the Hengshi River watershed.AMD contamination gradually decreased throughout the Hengshi River,creating an AMD pollution gradient.The two mud impoundments were the most heavily contaminated areas.The water pH of the two mud impoundments was no more than 3.0,the SO₄^2-concentrations was above2 000 mg/L,and the dissolved total Fe was up to 570 mg/L.The concentrations of dissolved Mn,Zn,Cu,Cd,and Pb in the two impoundments were beyond the levels of the Environmental Quality Standards for Surface Water seriously.In the midstream of Hengshi River,the large treatment plant had a calming effect on dealing of AMD,which turned its downstream water form acidity to alkalinity,and made Fe²⁺,Fe³⁺,and SO₄^2-concentrations decreased obviously.While the river water in the midstream were moderately acidic?pH3.0⁵.0?before the operation of the large treatment plant.The physicochemical characteristics of the downstream river waters were similar to the control site,indicating only light AMD contamination in this area.?2?Spatiotemporal dynamics of microbial communities in Hengshi River watershed.Spatiotemporal dynamics of microbial communities in Hengshi River watershed were significant.Microbial communities were patterned along the AMD pollution gradients.The two mud impoundments were the most heavily contaminated areas?pH<3.0?,which had the lowest microbial abundance and diversity,and genera detected in these areas were predominantly acidophiles and acid-tolerant microorganisms,such as Acidithiobacillus,Acidiphilium,Leptospirillum,Citrobacter,Thermogymnomonas,Acinetobacter,and Pseudomonas.In the midstream river,the large treatment plant had an important influcence on the microbial communities.Before the operation of the large treatment plant,its downstream water was acidic?pH 3.0⁵.0?,the dominant genera detected in water samples were acidophiles and acid-tolerant microorganisms,mainly including Acidithiobacillus,Acidiphilium,Leptospirillum,Thermoplasma,Thermogymnomonas,Ferroplasma,and Acinetobacter.While after the operation of the large treatment plant,its downstream water turned to alkalinity,and the relative abundance of acidophiles and acid-tolerant bacteria decreased obviously,and Sediminibacterium,Flavobacterium,and Novosphingobium,which are commonly found in freshwater,become the dominant genus.Before the operation of the large treatment plant,the sediment samples from Liangqiao?S6?were acidic?pH³.2?and the dominant genera in the samples were acidophiles and acid-tolerant microorganisms.While the relative abundance of acidophiles in the sediment samples from Liangqiao?S6?decreased significantly after the operation of the large treatment plant along with the increase of the pH?⁴.6?,and the relative abundance of acid-tolerant genera,Gallionella and Geothrix,increased obviously.The relative abundance of acid-tolerant genera,Geobacter,Geothrix,Gallionella,and Desulfosporosinus,in sediments from Shangba?S8?increased significantly after the operation of the large treatment plant,due to the increase of sediment pH.In the downstream of the Hengshi Rvier where the AMD contamination was light,the dominant genera were very similar to those found in the control stream.The dominant genera detected in samples from the downstream were Flavobacterium,Novosphingobium,Acinetobacter,Emticicia,etc.,which are commonly found in freshwater.CCA analysis indicated that pH,TS and TFe concentrations were significant factors in structuring the microbial community in the Hengshi River.?3?Diversity of Fe-and S-cycling microorganisms and pathways of microbially mediated Fe and S cycling in the Hengshi River watershed.Many Fe-and S-cycling microorganisms were found in the mining area and the Hengshi River in the Dabaoshan mine.The dominant Fe-and S-cycling bacteria were Acidiphilium,Acidithiobacillus,Acidocella,Leptospirillum,Acinetobacter,Aquabacterium,Acidovorax,Bacteroides,Thiomonas,Halothiobacillus,Citrobacter,Microbacterium,Gallionella,Geothrix,Geobacter,Anaeromyxobacter,Lactobacillus,Thiobacillus,Sphingomonas,Desulfosporosinus,and Desulfobulbus.The dominant Fe-and S-cycling archaea were Thermoplasma,Thermogymnomonas,and Ferroplasma.PICRUSt analysis showed that there were many iron and sulfur metabolism-related genes in the Hengshi River watershed,which further demonstrated the important role of microorganisms in the cycling of iron and sulfur.Proposed pathways of microbial cycling of iron and sulfur in the Hengshi River watershed:?1?AMD is produced from oxidation of sulfide-rich mineral waste by iron-oxidizing bacteria or sulfur-oxidizing bacteria.When AMD enters into the Hengshi River,migration and transformation of iron and sulfur occur.?2?In the river water,Fe²⁺is oxidized to Fe³⁺by iron-oxidizing bacteria,while Fe³⁺is reduced to Fe²⁺by iron-reducing bacteria.Fe³⁺is unstable in river water,which has the potential to precipitate as secondary iron minerals.?3?The major pathway for the transfromation of sulfur in the river water is oxidation.The activity of sufate reducing bacteria is inhibited by the high concentration of dissolved oxygen in river water.Reduced sulfur compounds in river water may mainly due to the release of hydrogen sulfide from the sediment.?4?In the sediment,Fe²⁺and Fe³⁺can be convent to one another by iron-oxidizing bacteria or iron-reducing bacteria.The precipitation of Fe³⁺result in the high content of Fe³⁺in the sedimnt in the upstream river.As the AMD contamination gradually decreased along the Hengshi River,the activity of iron-reducing bacteria is stimulated resulting in the high content of Fe²⁺in sediments in the middle and downstream river.?5?In the sediment,sulfate and reduced sulfur compounds can be convent to one another by sulfur-oxidizing bacteria or sulfate-reducing bacteria.?4?Role of microbial activity in Fe?III?hydroxysulfate mineral transformations in AMD sites.Microbial activity may play a decisive role in the mineralogical transformation of schwertmannite/jarosite in the AMD-contaminated site when organic carbon is available.In the AMD-contaminated river at Dabaoshan Mining area,indigenous bacteria that promoted the mineralogical transformation of schwertmannite/jarosite were iron-reducing bacteria and sulfate-reducing bacteria,including Geobacter,Desulfosporosinus,Geothrix,Desulfovibrio,Anaeromyxobacter,and Desulfurispora.Three possible pathways for the microbial transformation of Fe?III?hydroxysulfate minerals in AMD sites:?1?Fe?III?in Fe?III?hydroxysulfate mineral is reduced by FeRB via the direct,enzymatic reaction,resulted in the release of Fe?II?and sulfate to the aqueous solution,then induce the transformation of Fe?III?hydroxysulfate mineral to goethite.?2?Fe?III?in Fe?III?hydroxysulfate mineral is reduced by sulfide produced by sulfate reducing bacteria,then induce the transformation of Fe?III?hydroxysulfate mineral.?3?When pH is>5,the sorption of Fe?II?onto schwertmannite is significant and the sorption of Fe?II?facilitates the Fe?II?-catalyzed transformation of Fe?III?hydroxysulfate mineral to goethite in anoxic environments.