Optimizing Shewanella MFCs: Biofilm, EET, and Operational Factors

Microbial Attachment and Electron Transfer Mechanisms. The efficiency of Microbial Fuel Cells (MFCs) driven by facultative anaerobes like Shewanella hinges on two critical factors: robust microbial attachment to the electrode and efficient extracellular electron transfer (EET). Key to surface colonization are the mannose-sensitive hemagglutinin (MSH) pilin genes (mshA-D), which facilitate biofilm formation and are directly involved in current generation. Genetic studies reveal that manipulating surface structures can enhance performance; for instance, disrupting the polysaccharide biosynthesis gene SO_3177 in S. oneidensis MR-1 improves both adhesion and electrical output. The conductivity of the biofilm is ultimately dependent on the bacterium's ability to shuttle electrons to the electrode surface, a process Shewanella achieves through both direct and indirect mechanisms.

The Molecular Machinery of Electron Transfer. At the heart of Shewanella's EET capability is the specialized Mtr electron transport pathway. This molecular machinery includes key components such as the decaheme cytochromes MtrA and MtrC, which are embedded in the outer membrane by the beta-barrel porin MtrB. Homologs like MtrD, MtrE, and MtrF provide functional redundancy and are also essential for efficient current production. Beyond this core pathway, Shewanella genomes encode numerous accessory proteins that fine-tune MFC performance, and natural variation in EET gene sets among different strains contributes to their differing power outputs and coulombic efficiency.

Direct and Mediated Electron Transfer Pathways. Shewanella can transfer electrons to electrodes either directly via physical contact through the Mtr pathway or indirectly using exogenous or self-produced redox mediators. Electron shuttles such as thionine, methyl viologen, and humic acids can significantly enhance current generation. The balance between reducing the electrode and competing electron acceptors is complex; for example, the presence of oxygen, while initially a competitor for electrons, can stimulate the production of endogenous mediators like riboflavin, which subsequently act as powerful redox shuttles to boost electrical output. Optimizing this mediator environment is a key research area.

Influence of Electron Donors and Substrate Synergy. The type of electron donor provided fundamentally shapes the electrical performance of Shewanella MFCs. Understanding the effects of mixed organic carbon compounds is crucial for real-world applications, as wastewaters are inherently complex. Shewanella readily oxidizes simple carboxylic acids like lactate and formate. Intriguingly, these substrates can act synergistically; a combination of lactate and formate yields a higher electrical output than either alone, as lactate primarily serves as a carbon source while formate is efficiently oxidized as an electron donor. The full range of metabolizable substrates and their fate in current generation remains an active field of study.

Biofilm Enhancement and Electrode Material Effects. Biofilm formation is a critical determinant of MFC success, and Shewanella thrives in this role at circumneutral pH. Biofilm density and current output can be dramatically increased through genetic manipulation; for example, overexpressing the c-di-GMP biosynthesis geneydeH in S. oneidensis MR-1 results in biofilms three times thicker and a threefold increase in current density. The physical and chemical properties of the electrode itself also play a major role. The surface wettability of electrodes directly impacts EET, with superhydrophilic surfaces yielding substantially higher current than hydrophobic ones. Similarly, electrodes coated with semi-conductive materials like carbon-coated hematite prove more efficient than bare carbon cloth, leveraging the conductive properties of Fe(III) oxides.

Operational Parameters and Synergistic Consortia. Strategies to boost power density extend beyond pure cultures and electrode design. Isolating synergistic microbial consortia has proven highly effective; for instance, co-culturing Shewanella with the riboflavin-producingBacillus subtilis can skyrocket power densities from tens to hundreds of mW m⁻². Operational parameters like solution ionic strength are also critical, as they affect internal resistance, though the effect of specific ions like Ca²⁺ can be variable and system-dependent. Furthermore, the applied anode potential directly influences the physiological state of Shewanella, shifting its proteome to express higher levels of EET and energy-generating proteins at more positive potentials, while lower potentials may induce stress responses and alter metabolic priorities.