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Microbes involved in arsenic removal in passive treatment systems Khoshnoodi, Maryam


Biogeochemical cycling of arsenic and speculation on mechanisms of arsenic removal are interest in the environmental remediation of contaminated sites. In the present study, combination of metagenomic molecular biology techniques with mineralogical analyses were used to study a biochemical reactor (BCR) that was successfully removing arsenic, zinc, copper and cadmium. First the metal and mineralogical content of the BCR solids was investigated. X-ray diffraction (XRD) and automated quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) were used for mineralogical characterization. Analysis indicated that sulfates and sulfides were the predominant types of Zn and As minerals formed in the BCR. Arsenic minerals were detected as sulfides (arsenopyrite, tennantite), arsenates(wihelmkleinite), oxides (unknown zinc arsenic oxides) and zincarsenic sulfides, which showed evidence of metal adsorption on the surfaces of other solids such as silicates. Energy-dispersive X-ray spectroscopy verified that arsenic was associated with iron, zinc and sometimes cadmium as arsenopyrite-type minerals. Using a SSU rRNA survey of the site, the following taxa were correlated with high metal content: Bacteroidetes, Synergistaceae, Victivallales, methanogens (Methanocorpusculum, Methanospirillum, Methanosarcina) and new phyla such as VadinHA17, M2PB4-65, candidate division WS6, RF3 and TM6. Next, enrichment culturing and arsenic chemical speciation monitoring were performed to assess potential for arsenic species transformations in the BCR. Most predominant groups in the As(III) and As(V) media, were Simplicispira (β-proteobacterium) and Sedimentibacter (Firmicutes), respectively. Chemical arsenic speciation monitoring of the enrichments suggested that arsenite oxidation and arsenate reduction occurred. These genera were not previously reported for arsenic transformation. Finally, functional metagenomic workflow was applied to study arsenic resistance genes. Functional screening and end-sequencing of large insert fosmid libraries demonstrated that arsenic(V) resistance genes were taxonomically widespread and different class of arsenic resistance genes related to periplasmic arsenate reduction, arsenite efflux, bioaccumulation (phosphate, metal transporters) and arsenite oxidation were present. Fewer genes were associated with dissimilatory arsenate reduction and arsenic volatilization mechanisms. Methanomicrobia were predominant in the BCR and identification of methanogen-related arsenic resistance genes indicated that methanogens potentially played a role in arsenic removal inside the BCR.

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