Handa Notebook Thesis

[Audio] Hello Everyone, I am Jasreen Kaur from the Department of Environment and Energy Engineering. First and foremost, I would like to thank Professor Jung Sokhee for allowing me to present this very interesting paper entitled a kinetic perspective on extracellular electron transfer by anode-respiring bacteria. Without further a due let's get started..

[Audio] This presentation is divided into four section which are the introduction and objectives, anode potential losses, EET mechanisms types used by ARB and last but not least the conclusion. At the end of the slides there will also be a section on appendix.

[Audio] Diving into part 1 which is the Introduction. In microbial fuel cells and electrolysis cells , anode-respiring bacteria oxidize organic substrates to produce electrical current. In order to develop an electrical current, A R B must transfer electrons to a solid anode through extracellular electron transfer. A R B use various E E T mechanisms to transfer electrons to the anode. The first mechanism is the direct electron transfer. Direct Electron Transfer between electron carriers in the bacteria and the solid electron acceptor. This mechanism is supported by the presence of outer-membrane cytochromes that can interact directly with the solid surface to carry out respiration. Bacteria using this mechanism require direct contact with the solid electron acceptor and, thus, cannot form a biofilm. The second E E T mechanism is the soluble electron shuttle. Soluble Electron Shuttle: a compound that carries electrons from the bacteria by diffusive transport to the surface of the metal oxide and is able to react with it, discharging its electrons. Then, this compound, in its oxidized state, diffuses back to the cells, which should be able to use the same compound repeatedly .hence the name shuttle. The third E E T mechanism is the solid component. Solid Component that is part of the extracellular biofilm matrix and is conductive for electron transfer from the bacteria to the solid surface. This mechanism is supported by the recent discovery of the possible role of cellular pili as nanowires, which are being characterized for their capability to conduct electrons. Other components may also be conductive and contribute in EET, such as extracellular cytochromes or bound electron mediators.

[Audio] Thus, coming to the objectives of this review paper. There are two main objective whereby the first one is that ARB must be able to perform a twofold task which are A to produce a high current density that minimizes anode materials and reactor size and B at a low anode potential or can be defined as being as close as possible to the redox potential of the substrate being oxidized, which translates into minimal anode potential losses and high energy output. The second objective is to evaluate how well each EET mechanism can produce a high current density more than 10 Ampere per meter square without a large anode potential loss.

[Audio] Second part is the anode potential losses. In this section is it divided into 4 parts which are the intracellular potential losses, rate of substrate utilization by ARB, rate of electron production by A R B and the extracellular potential losses and its mechanisms..

1. 2. 3. How to characterize anode potential (osses? figure 3 depicts the sequential losses of electrical potential that become conceptually important when addressing the function of an ARB biofilm and contribute to the anode potential loss For our analysis, we consider ARB that are located at a certain distance from the anode surface. The total anode potential loss (nan ode) is defined as the difference between the electron-donor potential and the anode FX)tential (nanode = Edonor — Eanode EET 3. Pot—Gs of the to — EET ts — The can refer to the energy Of electrons. 'Energy' and 'potential' interchangeably throughout the manuscript when referring to electrons After ARB release the electrons. two additional processes can result in losses- The first is the SET mechanism that transports electrons to the anode interface, which Will change the electron energy from to E_interface- The secorri is the reaction occurring at the anode interface, which can decrease the from E_interface to E _anode Because MXCs must be operated to minimize (low ancxie potential losses}, the total energy given to ARB to carry out intracellular and extracellular Frocesses iS minimized. Thus. the ARB community must manage energy efficiently by maximizing energy conversion to ATP (intracellular processes) and minirhi2ing losses due to extracellular processes (EET, anode interface reaction) that do not yÉId energy to the cell.

[Audio] Coming into Part 1 of section 2 the intracellular potential losses Two kinetic processes are involved in intracellular potential losses from E donor minus E O M these processes are common in all respiratory bacteria and are depicted in Figure. 5. From Figure. 5.1 First, bacteria oxidize the electron donor, producing intracellular reducing power in the form of an electron carrier such as N A D H. To generate energy for the cells, the electron carrier is oxidized by transferring its electrons into the membrane associated proteins that are part of the electron transport chain, which ultimately leads to the external electron acceptor. From Figure. 5.2 In the case of A R B, the electrons are transferred to O M proteins that initiate the EET process..

[Audio] Part 2 of section 2 is the rate of substrate utilization by A R B. The rate of substrate utilization in microbial processes are frequently modeled using the Monod relationship. This relationship explicitly describes the rate at which bacteria oxidize the substrate and produce the reduced intracellular electron carrier. When the substrate is rate-limiting for the entire process of metabolism and EET, the current density generated by A R B can be written as equation. 1 for a biofilm setting where j is current density obtained by A R B, j max is maximum current density of the ARB biofilm, S is substrate concentration in the liquid, K s,apparent is the apparent half-saturation substrate concentration in a biofilm..

[Audio] Part 3 is rate of electron production by A R B Once ARB have produced the reduced intracellular carrier, they initiate electron flow through the electron transport chain until the electrons reach membrane-bound cytochromes that, in turn, initiate EET. The rate of reduction of electron shuttles can be modeled by the Monod relationship expressed in terms of the concentration of the soluble shuttle The problem is that Because the electron acceptor for A R B is a solid anode, we cannot use the Monod relationship, because we cannot define an anode concentration. Thus, we transform the concentration of an electron acceptor into the anode potential, which is achieved using the Nernst Monod relationship developed to express the current density in an A R B biofilm. The formula is defined as below where R is the ideal gas constant, F is the Faraday constant, T is the temperature in kelvin and E k a is the potential at which the current density one over two maximum current density.

[Audio] The Nernst– Monod relationship combines the Monod relationship typically used to calculate the rate of electron acceptor utilization with the Nernst equation for describing the anode potential availability as the electron acceptor for ARB Because enzyme electrode and bacteria electrode systems have similar kinetic processes, we can analyze the potential losses in the ARB biofilm depicted in Figure. 6 in a similar manner as is performed in enzyme electrochemistry. Nernst– Monod equation as the baseline to distinguish intracellular potential losses from n anode. Thus, deviations from the Nernst–Monod equation will help us determine extracellular potential losses due to EET and interface electron transfer..

[Audio] The Nernst Monod relationship combines the Monod relationship typically used to calculate the rate of electron acceptor utilization with the Nernst equation for describing the anode potential availability as the electron acceptor for A R B Because enzyme electrode and bacteria electrode systems have similar kinetic processes, we can analyze the potential losses in the A R B biofilm depicted in Figure. 6 in a similar manner as is performed in enzyme electrochemistry. Nernst Monod equation as the baseline to distinguish intracellular potential losses from n anode. Thus, deviations from the Nernst Monod equation will help us determine extracellular potential losses due to EET and interface electron transfer..

[Audio] Moving to Part 4 which is the extracellular potential losses. Two kinetic processes are involved in extracellular potential losses. First, electrons are transported from the A R B's O M proteins example cytochromes to the surface of the anode by either electron shuttles Figure. 6.3a or by a solid conductive matrix Figure 6.3 b This reduces the electron energy from E O M to E interface Then, electrons are transferred to the electrode by interface electrode transfer Figure 6.4,reducing the electron energy from E interface to E anode Given that these potential losses occur on the outside of the cell, it is unlikely that these losses are associated with A R B energy conservation and growth The underlying mechanisms involved in these kinetic processes includes diffusive transport, conduction, and electrochemical reactions are known to dissipate energy as heat or as an increase in entropy, thus strengthening our assumption that A R B cannot recover this energy for growth In order to maximize their growth by capturing as much energy as they can, A R B must minimize extracellular potential losses. Hence, the next section will be talking about the first mechanism which is the direct contact mechanism.

Extracellular potential losses -EET Mechanisms used by ARB.

Extracellular potential losses -EET Mechanisms used by ARB.

Extracellular potential losses -EET Mechanisms used by ARB.

Extracellular potential losses -EET Mechanisms used by ARB.

Extracellular potential losses -EET Mechanisms used by ARB.

Extracellular potential losses -EET Mechanisms used by ARB.

Extracellular potential losses -EET Mechanisms used by ARB.

Extracellular potential losses -EET Mechanisms used by ARB.

70 60 40 S 30 20 10 (c) -0.6 -0.4 -0.2 0.0 E / V vs. Ag/AgCl 0.2 0.4.

Figure 8d : Comparison of LSCV of various ARB communities.

Conclusions: Perspective of EET mechanisms in MXCs.

What are the three underlying mechanisms involved in the kinetic process in EET that are known to dissipate energy as heat? What are the three EET mechanisms discussed in this review article? What are the three differences between MXCs and natural systems that can affect the preferred EET mechanism used by bacteria?.

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Appendix. Box 1 – Table 1: Comparison of biofilm substrate fluxes and electron fluxes (expressed as current density) of various biofilm reactors used in the literature Box 2 – Voltammetric techniques as a tool to evaluate ARB kinetics; Figure 4: LSCV of a young ARB biofilm Figure 7(a) & (b) – Direct Electron Transfer (a) An MXC used for kinetic experiments shows a visible orange biofilm developing on the surface of graphite rods. (b) Scanning electron micrograph of the biofilm shown in (a) shows putative nanowires. Box 3 – EET mechanisms in natural system.

In the case of ARBs, the anode is not the terminal electron acceptor, as it transfers the electrons to the cathode, where a variety of compounds can act as a terminal electron acceptor (e.g. O2, H2O, H1). The terminal electron acceptor in the cathode does not control the rate at which bacteria respire in the anode, because it is remote from the bacteria; instead , the anode potential determines their rate of respiration (Torres et al., 2008b). Therefore, the anode potential is the analog to the concentration of a soluble electron acceptor in conventional respiration processes, and respiration occurs only when the anode accepts the electrons. Definition of ARB: ARB as any bacteria capable of transferring electrons to an anode, regardless of the EET mechanism used.

LSCV is a powerful voltammetric technique, as it measures the steadystate response of ARB as a function of the anode potential. In LSCV, the anode potential is scanned within a potential range at a slow enough rate to allow ARB to reach a steady-state metabolic condition for all potentials Figure 4 shows an example LSCV curve from experiments (Torres et al., 2008a) with a young ARB biofilm producing a maximum current density of 0.15Am^-2 The forward and backward curves are similar to each other, indicating steady-state conditions that are not affected by the scanning direction. At Eanode = Edonor , ARB cannot gain any energy-transferring electrons to the anode; therefore, j~0.

Figure 7(a) & (b) – Direct Electron Transfer. Fig. 7. (a) An MXC used for kinetic ex#riments shows a visible orange biofilm developing on the surface of graphite (b) Scanning electron micrograph of the biofilm shown in (a) shovvs putative nanowires..

Three differences between MXCs and natural systems that can affect the preferred EET mechanism used by bacteria. MXC researchers run their experiments with excess electron donor, nutrients, and carbon source. Bacteria in natural systems often encounter limitations of these building blocks of life, which leads them to minimize biomass production and have a slower metabolism. bacteria-reducing metal oxides probably would prefer an EET mechanism that minimizes nutrient and carbon consumption. Direct electron transfer may meet this goal best, as ARB in this case do not need to produce exogenous shuttles or EPS Solid conduction may imply the highest use of nutrients and carbon, due to the need to produce a continuous solid to transfer the electron most MXCs operate with a continuous feed and a small hydraulic retention time, both typical of biofilm processes ( Rittmann & McCarty, 2001).. leading to increasing energy costs by bacteria to reproduce the lost have minimal water flow, which could allow bacteria to maintain a higher concentration of electron shuttles sediments often shuttles in solution metal oxides in sediments may be present in small precipitates that can be totally reduced by bacteria in a small period ..