Mechanism Of Enhanced Extracellular Electrons Transfer For Anaerobic Cr(?) Reduction By Shewanella In The Presence Of Graphene Film

Extracellular electron transfer(EET)was a specific microbial anaerobic respiration,which had potential for bioremediation of soil and water contaminated.Shewanella was a typical EET strain,which has been reported that the efficiency of EET would be greatly improved by the addition of conductive nano-carbon materials such as graphene oxide(GO)and modified GO.However,the mechanism was still needed to be clarified.In this study,the effect of GO and graphene oxide/polyvinyl alcohol(GO/PVA)films on anaerobic reduction of Cr(?)by Shewanella xiamenensis BC01(S.xiamenensis)were investigated.The main research results are as following:(1)Initially,GO and GO/PVA films were prepared by vacuum filtration.Then,it was indicated that Cr(?)removal efficiencies of GO and GO/PVA films groups were 2.67-and 3.13-fold higher than that of control group after the addition of GO and GO/PVA films at initial concentration of Cr(?)(50 mg/L).Moreover,it was found that GO and GO/PVA films were gradually evolved to rGO and rGO/PVA films by the detection of FTIR,Raman and XPS.Therefore,the whole reduction process could be divided into two stages,including the microbial Cr(?)reduction stage mediated by GO(stage ?)and the microbial Cr(?)reduction stage mediated by reduced GO(rGO)(stage ?).In addition,the results lactate(electron donor)consumption of GO and GO/PVA films groups were higher than that of control assay.(2)Further,the expression levels of genes of c-type cytochromes(c-Cyts)revealed that functional genes of mtrA,mtrB,mtrC,mtrD,mtrF,petC,SO.4047 and omcA were actively up-regulated by the induction of GO and GO/PVA films.In addition,the results of ectopic expression of c-Cyts genes of S.xiamenensis strain confirmed that the up-regulated expression levels of c-Cyts genes were responsible for the enhancement of efficiency of Cr(?)reduction.In addition,there were significant differences in the expression levels of c-Cyts genes during the two stages of reduction after the addition of GO and GO/PVA films.Also the activities of c-Cyts are gradually boosted from outer membrane to inter membrane.The expression levels of c-Cyts genes located in outermembrane would be boost firstly,in resulting the active expression of c-Cyts genes at both two stages.The improvement of efficiency of Cr(?)reduction at stage ?was due to the activation of mtrD-mtrF and mtrD-mtrC while the efficiency of Cr(?)reduction was improved by the enhancement of activity of mtrA-omcA at stage ?.Therefore,the expression levels of different c-Cyts genes were up-regulated for accelerating the consumption of lactate to produce more electrons during the Cr(?)reduction after the addition of GO and GO/PVA films.Simultaneously,GO and GO/PVA films were reduced to be the excellent redox mediators.Based on these two factors,the efficiency of Cr(?)reduction was improved.