Journal of Energy Bioscience 2024, Vol.15, No.2, 85-95 http://bioscipublisher.com/index.php/jeb 88 4 Direct Electron Transfer (DET) Mechanisms 4.1 Mechanisms of direct electron transfer between bacteria and electrodes Direct electron transfer (DET) is a crucial process in microbial fuel cells (MFCs) where electroactive bacteria transfer electrons directly to the electrode without the need for soluble electron shuttles. This process is facilitated by specific structural and functional adaptations in bacteria, such as the presence of outer membrane c-type cytochromes (OM c-Cyts) and conductive pili or nanowires. These components form a conductive pathway that allows electrons generated from cellular metabolism to be transferred across the cell surface to the electrode (Figure 1) (Aiyer, 2019; Li et al., 2021; Paquete et al., 2022).The spatial structure of electroactive biofilms (EABs) plays a significant role in enhancing DET. For instance, modifying the anode with materials like iron phthalocyanine (FePc) can improve the affinity between the anode and OM c-Cyts, leading to a more active EAB and higher power density in MFCs (Li et al., 2021). This modification decreases the charge transfer resistance and accelerates the interfacial reaction rate, thereby promoting DET (Li et al., 2021). Figure 1 Proteins involved in EET processes of S. oneidensis MR-1 (Adopted from Paquete et al., 2022) Image caption: This diagram illustrates the key proteins in the bacterial electron transport chain and their locations within the cell membrane. The image includes various electron transport proteins located in the outer membrane (OM), periplasm (P), and inner membrane (IM). DmsA, DmsB, DmsE, and DmsF are responsible for electron transfer from the outer membrane to the inner membrane. OmcA, MtrC, MtrA, and MtrB form a transmembrane complex that assists in electron transfer across the outer membrane. CymA, TorC, and FccA are located in the inner membrane and play roles in electron transport. SO4360 and SO4359 indicate proteins associated with metal reduction. The coordinated action of these proteins facilitates the electron transfer process within the cell, which is significant for the study of microbial fuel cells and other bioelectrochemical systems (Adopted from Paquete et al., 2022) 4.2 Role of cytochromes and conductive pili in DET Cytochromes, particularly multiheme c-type cytochromes, are essential for DET as they facilitate electron transfer across the bacterial cell envelope to the electrode. These proteins are embedded in the outer membrane and form a conductive pathway that bridges the intracellular electron donors and the extracellular electron acceptors (Kracke et al., 2015; Paquete et al., 2022). The presence of these cytochromes is a common feature among electroactive bacteria, including Geobacter and Shewanella species, which are well-known for their efficient DET capabilities (Zhao et al., 2020; Paquete et al., 2022). Conductive pili, also known as microbial nanowires, are another critical component in DET. These pili are filamentous structures that extend from the bacterial cell surface and can conduct electrons over micrometer distances. They enable bacteria to establish electrical connections with distant electrodes or other cells, thereby enhancing the overall electron transfer efficiency in bioelectrochemical systems (Aiyer, 2019; Lovley et al., 2021). The combination of cytochromes and conductive pili allows bacteria to effectively transfer electrons directly to the electrode, bypassing the need for soluble mediators (Lovley et al., 2021; Paquete et al., 2022). 4.3 Case studies highlighting DET in specific bacteria (e.g., Geobacter spp.) Geobacter species are among the most studied electroactive bacteria due to their remarkable ability to perform DET. These bacteria possess a high abundance of OM c-Cyts and conductive pili, which are crucial for their
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