Table_1_Mediation of Extracellular Polymeric Substances in Microbial Reduction of Hematite by Shewanella oneidensis MR-1.docx

Extracellular electron transfer (EET) plays a fundamental role in microbial reduction/oxidation of minerals. Extracellular polymeric substances (EPS) surrounding the cells constitute a matrix that separates the cell’s outer membrane from insoluble minerals and environmental fluid. This study investigated the effects of EPS on EET processes during microbial reduction of hematite by the iron-reducing strain Shewanella oneidensis MR-1 (MR-1). Electrochemical characterization techniques were employed to determine the influence of EPS components on the redox ability of MR-1. Cells with removed EPS exhibited approximately 30% higher hematite reduction than regular MR-1 cells, and produced a current density of 56 μA cm-2, corresponding to 3–4 fold that of regular MR-1. The superior EET of EPS-deprived cells could be attributed to direct contact between outer membrane proteins and hematite surface, as indicated by more redox peaks being detected by cyclic voltammetry and differential pulse voltammetry. The significantly reduced current density of MR-1 cells treated with proteinase K and deoxyribonuclease suggests that the electron transfer capacity across the EPS layer depends mainly on the spatial distribution of specific proteins and electron shuttles. Exopolysaccharides in EPS tend to inhibit electron transfer, however they also favor the attachment of cells onto hematite surfaces. Consistently, the charge transfer resistance of cells lacking EPS was only 116.3 Ω, approximately 44 times lower than that of regular cells (5,139.1 Ω). These findings point to a negative influence of EPS on EET processes for microbial reduction/oxidation of minerals.

胞外电子传递（Extracellular electron transfer, EET）在微生物还原与氧化矿物的过程中发挥着基础性作用。包裹于细胞外周的胞外聚合物（Extracellular polymeric substances, EPS）构成一层基质，将细胞外膜与不溶性矿物及环境流体相分隔。本研究以铁还原菌株奥奈达希瓦氏菌MR-1（Shewanella oneidensis MR-1, MR-1）为研究对象，探究了EPS在其还原赤铁矿过程中对EET过程的影响。研究采用电化学表征技术，解析了EPS组分对MR-1氧化还原能力的调控作用。去除EPS的细胞相较于常规MR-1细胞，其赤铁矿还原效率提升约30%，且产生的电流密度可达56 μA·cm⁻²，约为常规MR-1细胞的3~4倍。缺乏EPS的细胞所展现出的更优异EET能力，可归因于外膜蛋白与赤铁矿表面的直接接触——循环伏安法与差分脉冲伏安法检测到了更多的氧化还原峰，印证了这一机制。经蛋白酶K与脱氧核糖核酸酶处理后的MR-1细胞，其电流密度显著降低，这表明EPS层的电子传递能力主要依赖于特定蛋白与电子穿梭体的空间分布。EPS中的胞外多糖往往会抑制电子传递，但同时也能促进细胞在赤铁矿表面的附着。与之相一致的是，缺乏EPS的细胞其电荷转移电阻仅为116.3 Ω，约为常规细胞（5139.1 Ω）的1/44。上述研究结果表明，EPS对微生物还原/氧化矿物过程中的EET过程具有负面影响。