Characteristics And Mechanism Of Extracellular Polymers During Iron Reduction By Salt Tolerant Bacteria

Saline wastewater poses a challenge in biological treatment because salinity affects the physiological activity of microorganisms and inhibits their growth and metabolism.Therefore,screening strains with stronger salt tolerance is an urgent research bottleneck in the process of salt wastewater treatment.Based on a salt tolerant dissimilatory metal reducing strain Shewanella aquimarina XMS-1 screened from marine sediments,the growth curve of the strain and the effects of different exogenous electron transporters on the iron reduction performance of the strain were measured to investigate its iron reduction characteristics,and the characteristics of Fe³⁺reduction ability,EPS content and composition of the strain under different salinity conditions were studied.Meanwhile,further exploring the effect and mechanism of shiwanella EPS on different Fe³⁺reduction process combined with the spectral analysis method.Finally,the effect of Shewanella XMS-1 EPS on Fe³⁺reduction under different redox states was studied.The specific research is as follows:（1）The maximum tolerance concentration of Shewanella XMS-1 to Na Cl can reach 5 g/L.the reduction process of Fe³⁺is promoted when the salinity is 1%～4%,but inhibited when the salinity is higher than 5%.Too high salinity will inhibit the growth of XMS-1,resulting in the decrease of Fe³⁺reduction rate.The reduction rate of Fe³⁺at 3%salinity can reach 44.1%,which is 2.18 times higher than that of the control group;The EPS production of XMS-1 increased under 3%salinity,indicating that XMS-1 will secrete more EPS to protect cells from normal physiological activities in high salt environment.The addition of exogenous electron shuttles can significantly improve the reduction ability of shiwanella,in which AQDS can promote the iron reduction of shiwanella to the greatest extent.The higher the concentration of AQDS,the higher the reduction rate of Fe³⁺.（2）Various spectral analysis showed that the peak intensity of tryptophan and tyrosine proteins in EPS decreased during Fe³⁺reduction,which was the same as that of protein content in EPS,indicating that the two proteins were involved in Fe³⁺reduction.Combined with 2D-COS,it was found that tryptophan proteins changed first and significantly,and played a stronger role in the process of Fe³⁺reduction.EPS contains metal ion redox functional groups such as carboxyl,hydroxyl,amino and carbonyl groups.The substances containing O-and N-groups participate in the process of Fe³⁺reduction,in which the amide group of proteins plays a major role.After AQDS was added,the contents of amide III and tryptophan in EPS increased,which confirmed that AQDS could promote the synthesis of XMS-1 protein and promote the reduction of Fe³⁺.（3）EPS can be used as electron donor and electron transfer medium to affect the redox state of iron.The original EPS has only weak Fe³⁺reduction ability,while the Fe³⁺reduction ability of the reduced EPS is 5.2 times that of the original EPS.There is cytochrome c in the EPS of shiwanella XMS-1.After reducing EPS and Fe³⁺under anaerobic conditions,the UV-Vis spectrum shows that the c-type cytochrome returns to the oxidation state,indicating that the electron is transferred from the reduced cytochrome c to Fe³⁺,indicating that the cytochrome c in EPS promotes the reduction of Fe³⁺in the process of the interaction between EPS and Fe³⁺.To sum up,this paper explores the effect of shiwanella EPS on Fe³⁺reduction process and its main mechanism under high salt conditions,which provides a theoretical basis and research basis for the study of biological treatment mechanism of high salt wastewater.