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Summary
The growth and metabolism of microorganisms are often affected by salinity, so screening the salt-tolerant strains is of great significance for the biological treatment of saline wastewater. In this paper, Shewanella aquimarina XMS-1, a marine strain with salt tolerant metal reduction DMRB), was selected as the research object to explore the effects of salinity on the reduction process of Fe3+ and the changes of extracellular polymers. The Fe3+ reduction ability of XMS-1 and the extracellular polymer (EPS) content under different salinity were investigated. Furthermore, Three dimensional fluorescence excitation-emission ( 3D-EEM) Raman spectra ( Raman) Fourier transform infrared spectroscopy ( FTIR) and Twodimensional correlation spectroscopy (2D-COS) were used to analyze the changes in extracellular polymers during the reduction of Fe3+ by XMS-1. The results show that protein is the main substance in XMS-1 EPS, accounting for more than 80% of EPS content, and polysaccharide content is relatively small. 3% salinity conditions promote EPS production. XMS-1 will secrete more EPS in the high salt environment to protect cells from normal physiological activities. The reduction process of Fe3+ is accelerated at the salinity of 1%, but is inhibited when the salinity is higher than 5%0. Too high salinity would inhibit the growth of XMS-1, resulting in the decrease of the Fe3+ reduction rate. The reduction rate of Fe3+ was increased by 2. 18 times and reached 44. 1% at the salinity of 3%. FTIR and Raman spectra showed that XMS-1 EPS contained metal ion redox functional groups such as carboxyl, hydroxyl, amino and carbonyl. The peaks of protein amides and polysaccharides in EPS were enhanced at 3% salinity, and the representative peaks of protein amides changed significantly. The O- and N- groups were effective redox groups in Fe3+ reduction. In addition, the Three-dimensional fluorescence results showed that after the Fe3+ reduction process, the intensities of the two fluorescent components of tryptophan and tyrosine in EPS decreased. Combined with the analysis of 2D-COS spectral results, it was found that tryptophan-like proteins changed significantly during the reduction of Fe3+, indicating that the two fluorescent components were involved in the reduction process of Fe3+, and tryptophan-like proteins played a stronger role in the reduction process. This study enriched the understanding of the extracellular electron transfer process of EPS in halophilic bacteria, and highlighted the significance of EPS in iron redox transformation in natural environment.
