Chemical Characteristics of Electron Shuttles Affect Extracellular Electron Transfer: Shewanella decolorationis NTOU1 Simultaneously Exploiting Acetate and Mediators

Abstract

In the present study, we found that our isolate Shewanella decolorationis NTOU1 is able to degrade acetate under anaerobic condition with concomitant implementation of extracellular electron transfer (EET). With +0.63 V (vs. SHE) poised on the anode, in a 72-h experiment digesting acetate, only 2 mM acetate was consumed, which provides 6% of the electron equivalents derived from the initial substrate mass to support biomass (5%) and current generation (1%). To clarify the effects on EET of the addition of electron-shuttles, riboflavin, anthraquinone-2,6-disulfonate (AQDS), hexaammineruthenium, and hexacyanoferrate were selected to be spiked into the electrochemical cell in four individual experiments. It was found that the mediators with proton-associated characteristics (i.e., riboflavin and AQDS) would not enhance current generation, but the metal-complex mediators (i.e., hexaammineruthenium, and hexacyanoferrate) significantly enhanced current generation as the concentration increased. According to the results of electrochemical analyses, the i-V graphs represent that the catalytic current induced by the primitive electron shuttles started at the onset potential of −0.27 V and continued increasing until +0.73 V. In the riboflavin-addition experiment, the catalytic current initiated at the same potential but rapid saturated beyond −0.07 V; this indicated that the addition of riboflavin affects mediator secretion by S. decolorationis NTOU1. It was also found that the current was eliminated after adding 48 mM N-acetyl-L-methionine (i.e., the cytochrome inhibitor) when using acetate as a substrate, indicating the importance of outer-membrane cytochrome.

Understanding the Properties of Phenazine Mediators that Promote Extracellular Electron Transfer in Escherichia coli

“…[5][6][7] This interaction, most commonly studied on the anode, is driven by the metabolism of abundant organic substances (e. g., sugars and low molecular weight organic acids) via complex oxidation-reduction (redox) reactions during microbial respiration. [8][9][10][11][12][13] The released electrons from these microbial redox processes are transferred from the microbes to the anode through various mechanisms of extracellular electron transfer (EET), reaching the cathode via an external electric circuit. 14 In this context, establishing an efficient EET between the microorganism and the anode surface plays a fundamental role in improving the power generation and performance of microbialbased bioelectrochemical systems.…”

“…17,19 Particularly, non-electrogenic microbes (e.g., those that do not readily carry out EET) can be encouraged to chemically interact with an electrode via the addition of soluble, diffusive electron shuttles, known as mediators. 12,17,20,21 A suitable electron shuttle for bacterial-based bioelectrochemical systems should be (1) soluble, (2) environmentally friendly, (3) biologically compatible, (4) stable and reusable, and (5) should have a suitable redox potential. (3) Redox mediators, such as quinones, 20 flavins, 22 and phenazines, 23,24 have a significant role in bioenergy metabolism and are recognized for their ability to enhance electron transfer to anode surfaces.…”

Marketability Prospects of Microbial Fuel Cells for Sustainable Energy Generation

“…It can transfer electron in two ways, directly through the outer membrane c -type cytochrome (OM c-Cyts) protein complex (omcA) and indirectly through riboflavin (RF), which is an electron transferor for MR-1. The riboflavin is released from MR-1 and facilitates the reduction of insoluble substrates. − Although various electron transferors have been recognized to date, only flavin mononucleotide and RF are identified to function as non-covalently bonded redox cofactors in OM c-Cyts Figure shows the mechanism of electron transport (ET) of S.…”

Environmental predictors of electroactive bacterioplankton in small boreal lakes

“…Electroactive organisms can commonly be found in and on structured environments such as rocks, soils, sediments, and conductive surfaces, using EET to connect internal electrochemistry to the external environment (Coates et al, 2002; Keffer et al, 2021; McAllister et al, 2020; Roden et al, 2010; Rowe et al, 2021; Tang et al, 2019) or to other bacteria (Beyenal et al, 2017; Hegler et al, 2008; Ishii et al, 2018; Shi et al, 2016). However, the ability of bacteria to reduce soluble electroactive substances outside of the cell is well‐known, and this raises the possibility of electroactive solutes being used as substrates for EET rather than as mediators or electron shuttles to a solid substrate (Aeschbacher et al, 2011; Bond & Lovley, 2005; Keller et al, 2009; Li et al, 2019; Lovley et al, 1991). For example, a surprisingly high number of genes encoding multiheme cytochromes (MHCs) and other putative EET protein‐encoding genes (hereinafter referred to as ‘EET genes’ for simplicity) were found in bacteria inhabiting the water column of Trout Bog Lake, a small humic lake in WI, USA, and researchers pointed to the high electron‐accepting capacity of the dissolved organic matter (DOM) as being a possible explanation (He et al, 2019).…”