The Adsorption And Reduction Mechanism Of Shewanella On The Surface Of Hematite:the Role Of BpfA Protein

Iron oxides are widespread reactive substances in the soil environment and serve as effective adsorbents for pollutants and as energy sources for heterogeneous iron-reducing bacteria.In soil environments lacking oxygen and other electron acceptors,heterogeneous iron-reducing bacteria can reduce iron oxides as electron acceptors.The electron transfer process at the mineral-microorganism interface can not only drive minerals to dissolve,precipitate,and transform,but also have an important impact on the growth,metabolism,and community succession of microorganisms.Electron transfer between bacteria and minerals is mainly through direct contact,secretion of chelating agents,the use of electronic shuttles and conductive fimbriae.Among them,in the process of direct contact,the electron transfer efficiency is the highest.Thus,contact adsorption of bacteria to iron oxides is a key step in extracellular electron transfer and plays an important role in the iron reduction process and the elemental cycle it drives.Shewanella is one of the most common bacteria with iron oxide reducing ability in the environment,and it is also a typical representative in extracellular electron transport research.In this study,Shewanella oneidensis MR-1 and a mutant strain lacking Bpf A adhesion protein are selected as experimental materials for adsorption kinetics and electrochemical experiments.The main results of the study are as follows:This study uses a quartz crystal microbalance(QCM-D)to perform adsorption kinetics experiments and fits the experimental data to an equivalent circuit(EC)in the model.We found that bacteria exhibit different adsorption states on the hematite surface after Bpf A adhesion protein deletion:the?bpf A mutant strain of adsorption is reduced by about 10%compared to the wild type,and the equivalent circuit(EC)model fit exhibits contact with the?bpf A mutant strain elasticity(?_c),adherent bacterial density(N_p)and contact zone radius(r_c)were smaller than those of the wild type.This indicates that due to the lack of adhesion factors,the contact area between the cell and hematite surface is reduced,bacteria easily move with the solution medium and are difficult to firmly adsorb on the hematite surface,and the?bpf A mutant adsorbs loosely on the hematite surface,forming an adsorption layer that exhibits stronger viscoelastic characteristics,and the overall adsorption process exhibits a hysteresis compared with the wild type.In addition,by ATR-FTIR and two-dimensional correlation analysis,we find that the sequence of interactions between various functional groups contained in biomolecules on the surface of the two cell types and the covalent bonds formed on the hematite surface also change.Quasi-first-degree kinetic equation fitting yielded slightly smaller rate constant k-values for the?bpf A mutant than for wild-type,suggesting that hematite has a greater affinity for the adsorption of proteins on the cell surface of wild-type.During short-term adsorption,the phosphate groups on the cell surface of the?bpf A mutant failed to interact with the hematite surface to form the key mono-and double-toothed inner ring complexes,and during long-term adsorption,the cell surface proteins failed to form a randomly coiled structure more conducive to intercellular connections.At the same time,the different adsorption states of Shewanella wild-type and?bpf A mutants during hematite surface adsorption affect the extracellular electron transfer process between bacteria and hematite.We examine the electrochemical properties of the wild-type and?bpf A mutant strains by assembling a three-electrode system and using electrochemical and other instruments,and find significant differences in the extracellular electron transfer carried out between the wild-type and?bpf A mutant strains and hematite.The performance of the wild-type electrochemical working system is much higher than that of the?bpf A mutant system.The affinity of wild-type to the hematite anode surface was higher,the output current density was greater,and the interfacial electron transfer rate between wild-type and the hematite-modified anode is significantly higher than that of the?bpf A mutant strain.However,there is no significant difference in the amount of adsorption between wild-type and?bpf A mutant on the hematite anode surface,indicating that the differences in output current density and interfacial electron transfer rate were independent of cellular adsorption.The above results indicate that the different adsorption states between heterogeneous iron-reducing bacteria and iron oxides widely present in the environment have a crucial influence on the extracellular electron transfer process,which may further affect the growth and metabolism of microorganisms as well as the solubilization,precipitation and transformation of iron oxides,thus affecting the biogeochemical cycle of iron.