Study On The Mechanism Of Iron And Sulfur Redox Coupled Mercury Transformation Of Acidithiobacillus Ferrooxidans

Bioleaching or microbially mediated iron/sulfur redox process in acid mine drainage（AMD）results in mineral dissolution and transformation,release of mercury and other heavy metal ions,and changes in the occurrence forms and concentration of mercury.The study of microbial iron/sulfur redox mediated mercury transformation is of great significance to the treatment and remediation of mercury pollution in heavy metal polluted areas.Therefore,aiming at the typical acidophilic microorganism Acidithiobacillus ferrooxidans,this paper studies and reveals its mercury conversion mechanism coupled with Fe/S redox under aerobic,anaerobic and aerobic anaerobic coupling conditions,respectively.Based on the determination of cell density,p H,ORP changes and concentration of Fe,S and Hg,the mineral morphology,element distribution,phase evolution,and the speciation transformation of Fe/S/Hg were simultaneously characterized by integrating use of combination of analytical techniques including X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FT-IR）,Raman spectroscopy（Raman）,scanning electron microscope-energy dispersive spectroscopy（SEM-EDS）,transmission electron microscopy（TEM）-EDS,X-ray photoelectron spectroscopy（XPS）and X-ray absorption near edge structure（XANES）spectroscopy.Differential expression analysis of genes related to iron,sulfur and mercury metabolism was performed based on transcriptome sequencing（RNA-seq）and comparative transcriptomics.The details are summarized as follows:（1）Under aerobic conditions,the results showed that Hg²⁺significantly inhibited the Fe/S oxidation process of A.ferrooxidans under aerobic conditions,and significantly changed the composition of polymers on the cell surface and the element distribution of C、N、Fe and S in the cell;Hg²⁺increased the gene expression of cytochrome c,isodisulfide reductase and other genes related to Fe/S metabolism of A.ferrooxidans.The expression of mercury resistant genes dominated by glutathione,cytochrome c oxidase and Hg²⁺reductase in S⁰ oxidation system increased,and the expression of growth and reproduction related genes in Fe²⁺oxidation system was significantly inhibited.The process of S⁰/Fe²⁺oxidation by A.ferrooxidans changes the forms of S/Fe and Hg in the solution,so that Hg²⁺in the solution exists on the surface of substrate residue in the form of Hg⁰,Hg SO₄ and Hg S.（2）Under anaerobic conditions,the results showed that the presence of Hg²⁺significantly inhibited the iron reduction and sulfur oxidation of A.ferrooxidans.The contents of sugars and phospholipids in the surface polymer of A.ferrooxidans cells increased,and structural information changes occurred significantly due to the bonding with Hg;In the presence of Hg²⁺,the expression of iron and sulfur metabolism related genes of A.ferrooxidans was inhibited,the expression of mercury reductase related genes was promoted,and the expression in the early stage of exponential growth was higher than that in the middle and late stage.Under anaerobic conditions,the iron sulfur metabolism and mercury conversion of A.ferrooxidans lead to the deposition of Hg²⁺on the surface of substrate residue in the form of Hg SO₄,Hg S and Hg⁰,which reduces the concentration of Hg²⁺in the solution.（3）Under the condition of aerobic anaerobic coupling,the mercury conversion in the iron/sulfur redox process of A.ferrooxidans was systematically studied.Pyrite has a strong affinity for Hg²⁺in solution,resulting in a small impact of Hg²⁺on the growth,metabolism and other physiological processes of A.ferrooxidans.With the utilization of substrate by A.ferrooxidans,Hg elements on the surface of pyrite are released,resulting in continuous stress on A.ferrooxidans.The presence of Hg²⁺significantly inhibited the expression of functional genes related to iron and sulfur metabolism of A.ferrooxidans,increased the expression level of mercury reduction genes,and weakened the stress effect after entering the anaerobic stage.Under abiotic factors,Hg²⁺in the solution is combined with pyrite surface in the form of Hg S;Under the action of A.ferrooxidans,part of Hg²⁺is reduced to Hg⁰,and the rest is fixed on the surface of substrate residue in the form of Hg SO₄ and Hg S.The above research results reveal the correlation between Fe/S redox and mercury transformation of typical acidophilic bacteria A.ferrooxidans under aerobic,anaerobic and aerobic anaerobic coupling conditions,and clarify the mercury transformation mechanism mediated by Fe/S redox of A.ferrooxidans,which provides an important reference for exploring the role and regulation mechanism of extreme acidophilic microorganisms in the geochemical cycle of mercury in AMD environment.