Electrochemical Impedance Spectroscopic Analysis of Diffusion-Layer Thickness Distribution Associated with Flagellar Motion of <i>Volvox carteri</i>

Although electrochemical methods have been used to relate microbial motility with ionic or redox signals, the evaluation of diffusion-layer variations induced by such motility using electrochemical impedance spectroscopy (EIS) remains limited.Thus, in this study, EIS is used to investigate how the flagellar activity of Volvox carteri modulates the diffusion layer above an electrode.Finite-element simulations and distributed equivalent-circuit modeling are performed for both uniform and non-uniform diffusion-layer thicknesses.Simulations predict that phototactic flagellar convection results in impedance spectra with a slope lower than 45 in the mid-frequency region and a pronounced finite-diffusion bend at low frequencies.These features are validated through experiments involving Volvox-immobilized electrodes under dark and illuminated conditions.Light irradiation reduces the effective diffusion-layer thickness from 2.4 10 -2 cm to 6.6 10 -3 cm and introduces a distributed thickness ranging from 4.5 10 -4 to 9.0 10 -3 cm, as extracted by transmission-line fitting.These results provide quantitative insights into bio-induced mass-transport modulation and demonstrate the applicability of distributed diffusion models in bioelectrochemical systems.