Abstract
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.
Highlights
Extracellular electron transfer (EET) has been extensively studied in dissimilatory metal-reducing bacteria, and was found to depend on outer membrane protein c-type cytochromes (MtrB, OmcA, MtrC) (Shi et al, 2007) or surface pili (Eggleston et al, 2006; Gorby et al, 2006; Smith et al, 2013)
Approximately 20 extracellular and outer membrane proteins have been identified in extracellular polymeric substances (EPS), but only c-type cytochromes and flavine have been shown to participate in EET processes (Harper et al, 2008; Cao et al, 2011; Harish et al, 2012)
This study investigated the effects of various EPS components on EET between MR-1 cells and hematite using a single threeelectrode chamber of a microbial fuel cell (MFC) with different cells on the anode
Summary
Extracellular electron transfer (EET) has been extensively studied in dissimilatory metal-reducing bacteria, and was found to depend on outer membrane protein c-type cytochromes (MtrB, OmcA, MtrC) (Shi et al, 2007) or surface pili (Eggleston et al, 2006; Gorby et al, 2006; Smith et al, 2013). Polysaccharides, including galactose, glucose, mannose, as well as N-acetyl glucosamine, glucuronic acids, and fucose (Lawrence et al, 2016), unavoidably interfere with EET processes (Kouzuma et al, 2010; Kitayama et al, 2017) This dichotomy makes it difficult to evaluate the overall influence of EPS on EET processes and the roles of different EPS components remain unexplored. Based on experimental results of hematite microbial reduction and electrochemical measurements, the EET capacity of different cells as well as the contribution of EPS components to EET processes are discussed
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