Schwertmannite Transformation Via Direct Or Indirect Electron Transfer By Sulfate Reducing Bacterial Enrichment

Mining pollution is an inevitable environmental problem during the development of national industrialization.Given this background,acid mine drainage(AMD)has become the most significant and durable pollution,posing a great threat to the local ecology and agricultural safety.Therefore,understanding the mechanism of microbial transformation of Fe(III)-oxyhydroxysulfate minerals forming under the specifically AMD-inducing geochemical condition,for instance,schwertmannite,is of great interest for revealing the cryptic Fe-S cycle in the mining area.Here,we examined the dissolution and transformation mechanism of schwertmannite incubated with a sulfate reducing enrichment culture,through the direct and indirect routes of electron transfer,respectively,and offered the insight of the structure variation and functional prediction within microbial community.The study found that in the direct treatments of extracellular electron transfer,which means bacteria can attach mineral freely,more reductive dissolution of schwertmannite was observed,and the addition of AQDS as exogenous electron shuttle could speed up this process.While in the indirect treatment where microorganism and were separated from mineral by dialyze bags,although the same solid product,vivianite,was formed as the result of direct treatments,the amount sulfate reduction was lower than that of direct treatments.Large amout of sulfur was observed in all treatment due to the re-oxidation of ferric ions,along with the minor products including magnetite and hematite.The 16 s rRNA high-throughput sequencing results demonstrated that different functional bacteria dominating in the different treatments of extracellular electron transfer.The initial sulfate reducing bacterial enrichment was dominated by the genus Citrobacter.After incubation for 20 days,Desulfovibrio replaced Citrobacter as the dominant bacteria in the direct treatments,while an unknown genus most closely related to Citrobacter within Enterobacteriaceae family highly dominated(over 90% of relative abundance)in indirect treatments.This monodominant community in indirect treatments was able to reduce schwertmanntie indirectly through the endogenous redox organic matters while the exogenous addition of AQDS insignificantly shaped the microbial structure.The PICRUSt prediction implied that there are functional adaptions for different electron transfer routes.The bacteria in direct treatments more relied on the pathways of membrane-bound structure protein,in contrast that the bacteria in indirect treatment presented the potential to generate more dissolved redox organic matters.Simultaneously the result of UV-spectrum demonstrated the existence of these matters in indirect treatment.Furthermore,the KOs genes within sulfur metabolism diverged into definite pattern between each treatment.In indirect treatment,the absence of pivotal Dsr genes during dissimilatory sulfate reduction and significant increase of dimethyl sulfoxide reductase which acted as electron carriers were expected to indicate a new form of functional marker.Based on the calculation of electron transfer,it is possible that the indirect electron transfer were contributed by fermenting characteristic within Enterobacteriaceae family and their endproducts.