Microbial Reduction Of Schwertmannite By Iron-and Sulfate-reducing Bacteria

Schwertmannite is an iron-hydroxysulfate mineral commonly found in acid mine drainage contaminated environment,which involves in numerous biogeochemical processes such as iron-sulfur cycling and heavy metal migration and transformation.Microorganisms including iron reducing bacteria（FeRB）and sulfate(SO₄^2-)reducing bacteria（SRB）,often inhabit the environment where schwertmannite occur.They can use Fe（Ⅲ）and SO₄^2-derived from minerals as terminal electron receptors in the respiratory process,triggering reductive dissolution,transformation,and recrystallization of minerals to generate new secondary minerals.There have been reports of pure bacteria（FeRB or SRB）mediated reduction and transformation of schwertmannite,providing some insights into understanding microbe-mineral interactions.Microorganisms in the environment live in mixed communities,which work together to cause the dissolution,transformation,and formation of minerals,however,there are still some limitations for understanding microbe-mineral interactions due to the complexity of microbial communities.For example,it is unclear how dominant bacteria（e.g.,FeRB and SRB）and their interactions affect the mineral transformation,and how the microbial metabolites and mineral transformation products affect the microbial reduction of minerals.Therefore,this study investigated the schwertmannite reduction by Shewanella oneidensis（FeRB）and Desulfosporosinus meridiei（SRB）,and on this basis,the effects of metabolites and secondary minerals during the bioreduction process on the microbial reduction of schwertmannite were further carried out,with the hope of further exploring the microbial reduction mechanism of schwertmannite.The main results obtained are as follows:（1）Schwertmannite reduction transformation pathway mediated by FeRB and SRB and their interactions.Two key stages were supposed to be responsible for the reduction of schwertmannite by FeRB and SRB:an initial phase in which FeRB predominated and Fe（Ⅲ）in schwertmannite was reduced,accompanied with the release of SO₄^2-,and the detected secondary minerals were mainly vivianite（Fe₃（PO₄）₂·8H₂O）;the second phase in which SRB predominated and mediated the reduction of SO₄^2-,producing minerals including mackinawite（FeS）and siderite（FeCO₃）.The addition of SO₄^2-altered the bioreduction order and phase transition path of schwertmannite with 10 mM SO₄^2-.The intermediate product formed by schwertmannite transformation was transformed from vivianite to lepidocrocite（β-FeOOH）or goethite（α-FeOOH）.Fluorescence quantitative PCR（qPCR）and transmission electron microscopy were used to investigate microbial abundance,functional gene expression,and the structure and composition of the cell/mineral interface.The relative abundance of FeRB and SRB in the consortia decreased,and more minerals aggregated inside and outside the cell;correspondingly,the transcription levels of genes（cymA,omcA,and mtrCBA）related to Fe（Ⅲ）reduction were down-regulated,while the transcription levels of SO₄^2--reducing genes（sat,aprAB,dsr（C））were generally up-regulated.Adopting low-intensity ultrasound treatment effectively restored the SRB cell activity for reducing SO₄^2-by disintegrating the cell-mineral aggregation.These phenomena suggested that secondary minerals produced during the microbial reduction of schwertmannite deposited on the cell surface,limiting bacterial growth,and the presence of SRB was detrimental to dissimilatory Fe（Ⅲ）reduction,while existed FeRB was in favor of dissimilatory SO₄^2-reduction.（2）The effect of metabolites on the microbial reduction of schwertmannite.Through cell-free supernatant exchange experiments,it was found that the metabolites secreted by microorganisms slowed down the inhibition of FeRB and SRB by complexing Fe²⁺,thus promoting the reduction of schwertmannite by FeRB or SRB.qPCR results showed that under the mediation of metabolites,FeRB mediated the Fe（Ⅲ）reduction by up-regulating the expression of genes（mtrC and omcA）rather than increasing the cell abundance,while FeRB incubated in medium drove Fe（Ⅲ）reduction by increasing cell abundance,and the mineral transformation path was schwertmannite→goethite→siderite;for SRB,despite metabolites had no effect on its abundance,they up-regulated the expression of dsr gene to accelerate the process of SRB reducing SO₄^2-,and the mineral transformation path was schwertmannite→goethite and FeS→siderite,FeS and pyrite（FeS₂）.Combined with metabolomics,high-performance liquid chromatography,UV-visible absorption spectroscopy and three-dimensional fluorescence,a large amount of phenylalanine（～1.5 mM）and citric acid（～0.2 mM）were present in the metabolites secreted by FeRB and SRB,respectively,which complexed Fe²⁺,and citric acid complexed Fe（Ⅲ）to increase the bioavailability of Fe（Ⅲ）minerals,so as to promote the bioreduction of schwertmannite.（3）The effect of secondary mineral on the microbial reduction of schwertmannite.The secondary mineral biogenic FeS（Bio-FeS）content mediated the extracellular electron transfer（EET）of microorganisms,which strongly affected the Fe（Ⅲ）reduction rate of schwertmannite,thereby altering the mineral transformation pathway.High content of Bio-FeS resulted in a lower Fe（Ⅲ）reduction rate,causing schwertmannite to be converted into lepidocrocite,while schwertmannite was converted into goethite.Moderate amounts of Bio-FeS（FeRB:FeS≤2 mM;SRB:FeS≤6 mM）could effectively promote EET-mediated microbial reduction of schwertmannite,while excessive Bio-FeS（FeRB:FeS>2 mM;SRB:FeS>6 mM）would attach to the cell surface and form a thick"shell",thereby slowing down or even inhibiting microbial reduction of schwertmannite.qPCR results showed that Bio-FeS simultaneously reduced FeRB abundance and gene expression,while for SRB,Bio-FeS decreased SRB abundance and instead increased dsr gene expression.These results indicated that FeRB and SRB exhibited a hormesis-like effect on Bio-FeS and its mediated EET processes.Collectively,this study explored the reduction mechanism of FeRB and SRB on schwertmannite from the perspective of the interaction of microbe-mineral,metabolites-microbe-mineral and secondary mineral-microbe-mineral.Briefly,FeRB and SRB played a dominant role in schwertmannite reduction,which would affect each other,leading to different gene expression patterns and mineral transformation pathways.Microbial metabolites and mineral transformation products played an important role in microbe-schwertmannite interactions.Metabolites complexed with Fe²⁺for detoxification,and complexed with Fe（Ⅲ）to increase the bioavailability of Fe（Ⅲ）minerals,thereby altering the metabolic pathways of microorganisms and mineral phase transition pathways;the secondary mineral Bio-FeS（<2 mM）promoted the microbial EET process to mediate the microbial reduction of schwertmannite,but with the progress of the reaction,Bio-FeS,together with other secondary minerals（e.g.,vivianite and siderite）,limited the metabolic activity of microorganisms.This work is of great significance for further understanding the microbial reduction mechanism of schwertmannite and the microbe-mineral interactions.