A kinetic perspective on extracellular electron transfer by anode-respiring bacteria

Area of Science:

• Microbiology
• Electrochemistry
• Bioenergetics

Background:

• Anode-respiring bacteria (ARB) oxidize organic matter to produce electrical current in microbial fuel cells and electrolysis cells (MXCs).
• Extracellular electron transfer (EET) is essential for ARB to transfer electrons to a solid anode.
• Existing EET mechanisms include direct cell-anode contact, soluble electron shuttles, and conductive extracellular matrices.

Purpose of the Study:

• To perform a kinetic analysis of different EET mechanisms used by ARB.
• To evaluate the feasibility of each EET mechanism for achieving high current densities (> 10 A m⁻²) with low anode potential loss (< few hundred millivolts) in MXCs.
• To identify the most promising EET mechanism for advancing MXC technology.

Main Methods:

• Literature review and kinetic analysis of published experimental results for various EET mechanisms.
• Comparative evaluation of EET mechanisms based on their ability to meet MXC performance goals.
• Theoretical assessment of electron transfer rates and potential losses associated with each mechanism.

Main Results:

• Direct cell-anode contact is limited by the number of bacteria that can physically connect to the anode, hindering high current densities.
• Soluble electron shuttles exhibit slow diffusion rates at typical concentrations, limiting current generation and causing significant anode potential loss.
• Electron transport through a solid conductive matrix is the only mechanism that explains the observed high current densities and low anode potential losses in MXCs.

Conclusions:

• A conductive extracellular matrix is critical for efficient electron transfer in ARB.
• Understanding the biological components that form conductive matrices is essential for optimizing ARB function in MXCs.
• Developing MXCs with high performance relies on harnessing EET mechanisms involving solid conductive matrices.