Bacterial Electron Transfer in a Microbial Fuel Cell

Electron Transfer Pathway

Bioanode Conditions

Substrate conc. S 5.0 mM

Anode potential E -100 mV

Biofilm density (pili conductivity) 1.00×

External Circuit

Load resistance R_ext 1.0 kΩ

Live Readouts

Current density j

0.00 A/m²

Cell current I

0.00 mA

Power at load (1 cm² electrode)

0.00 mW

How it works

Exoelectrogenic bacteria (e.g. Geobacter, Shewanella) oxidise an organic substrate and must dump the resulting electrons onto a solid electrode outside the cell — extracellular electron transfer (EET). Two real pathways exist:

Direct / nanowire — electrons hop through a network of outer-membrane cytochromes and conductive pili ("microbial nanowires") that make direct electrical contact with the anode. Low resistance, scales with biofilm thickness.
Mediated / shuttle — a diffusible redox molecule (e.g. a phenazine or quinone) is reduced at the cell, diffuses to the anode, is oxidised, and diffuses back. Diffusion-limited, so it caps out at a lower current.

Steady-state current density follows Nernst–Monod kinetics (Torres et al., 2008), combining substrate limitation with the redox-potential dependence of the electron-transfer step:

j = j_max · [S / (K_S + S)] · 1 / (1 + exp(-(F/RT)·(E − E_KA)))

S = substrate concentration, K_S = half-saturation constant, E = anode (working electrode) potential, E_KA ≈ formal potential of the terminal cytochrome, F/RT ≈ 38.9 V⁻¹ at 25 °C. The Monod term throttles current when fuel runs short; the Nernst term throttles it when the anode isn't poised positive enough to make electron donation thermodynamically favourable.

The electrons collected at the anode leave through the external wire and load resistor to the cathode, where they reduce oxygen (O₂ + 4H⁺ + 4e⁻ → 2H₂O). Charge balance inside the cell is carried not by electrons but by protons migrating through the electrolyte / membrane toward the cathode — shown here as the faint red stream crossing the blue membrane.

Pathway: Direct (nanowire)

Exoelectrogenic bacteria Conductive pili / nanowires Electron flow Proton (H⁺) flow

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