Journal of Energy Bioscience 2024, Vol.15, No.2, 85-95 http://bioscipublisher.com/index.php/jeb 87 3 Key Electroactive Bacteria in MFCs 3.1 Identification of major electroactive bacterial species Microbial fuel cells (MFCs) rely on the unique capabilities of electroactive bacteria to transfer electrons to the anode, facilitating electricity generation. Among the diverse array of electroactive microorganisms, three key species have been extensively studied: Geobacter sulfurreducens, Shewanella oneidensis, and Pseudomonas species. Geobacter sulfurreducens is a well-known iron-reducing bacterium that produces high power densities in MFCs. This species is particularly effective in transferring electrons directly to the anode via conductive pili or nanowires, making it a model organism for studying extracellular electron transfer (EET) mechanisms (Logan et al., 2019; Li et al., 2021). Shewanella oneidensis is another prominent electroactive bacterium that utilizes both direct and mediated electron transfer mechanisms. It can produce redox-active compounds such as flavins, which facilitate electron transfer to the anode (Pinto et al., 2018; Aiyer, 2020). Pseudomonas species, although less studied, have shown significant electrochemical activity in MFCs, contributing to both electricity generation and bioremediation processes (Zhou et al., 2020). 3.2 Metabolic pathways facilitating electron transfer The metabolic pathways of electroactive bacteria are crucial for their ability to transfer electrons to the anode in MFCs. These pathways involve complex biochemical processes that enable the bacteria to extract energy from organic substrates and transfer the resulting electrons to external electron acceptors. In Geobacter sulfurreducens, the primary pathway involves the oxidation of organic compounds, such as acetate, coupled with the reduction of iron or other metal oxides. The electrons generated during this process are transferred to the anode via conductive pili or nanowires, which act as biological wires (Logan et al., 2019; Li et al., 2021). Shewanella oneidensis employs a similar strategy but also produces soluble redox mediators like flavins and quinones that shuttle electrons from the cell to the anode, enhancing the efficiency of electron transfer (Pinto et al., 2018; Aiyer, 2020). Pseudomonas species utilize a variety of metabolic pathways, including the production of pyocyanin, a redox-active compound that facilitates electron transfer to the anode (Liu et al., 2018; Zhou et al., 2020). 3.3 Genetic and physiological characteristics contributing to electroactivity The genetic and physiological characteristics of electroactive bacteria play a significant role in their ability to transfer electrons to the anode in MFCs. These characteristics include the presence of specific genes encoding for electron transfer proteins, the structure of the cell envelope, and the formation of biofilms. Geobacter sulfurreducens possesses genes encoding for outer membrane c-type cytochromes, which are essential for direct electron transfer to the anode. These cytochromes are located on the outer membrane and interact with conductive pili or nanowires to facilitate electron transfer (Logan et al., 2019; Li et al., 2021). The thick cell wall of Shewanella oneidensis contains multiple layers of cytochromes and other redox-active proteins that enable both direct and mediated electron transfer. The ability to produce redox mediators like flavins further enhances its electroactivity. Pseudomonas species exhibit a high degree of genetic diversity, with genes encoding for various redox-active compounds and electron transfer proteins. The production of biofilms by these bacteria also contributes to their electroactivity by providing a stable environment for electron transfer processes (Zhou et al., 2020; Liu et al., 2018). In conclusion, the study of electroactive bacteria in MFCs reveals the intricate mechanisms by which these microorganisms transfer electrons to the anode. Understanding the identification, metabolic pathways, and genetic and physiological characteristics of key electroactive bacteria such as Geobacter sulfurreducens, Shewanella oneidensis, and Pseudomonas species is essential for optimizing MFC performance and developing practical applications for sustainable energy generation and environmental remediation.
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