Disentangling the Sources of Spectral Induced Polarization of Microbial Systems: Experiments in Non-Geological Media

This study investigates the origin of spectral induced polarization (SIP) signals associated with microbial systems, aiming to distinguish direct microbial electrical effects from indirect geochemical processes. Although SIP has been proposed for detecting subsurface microbial activity, its interpretation is complicated by overlapping polarization mechanisms. We therefore performed controlled experiments in simplified, non-geological environments to isolate the roles of microbial presence, attachment, and activity. Using a custom millifluidic chip, SIP measurements were conducted with \textit{Pseudomonas putida} KT2440 under three configurations: bacteria suspended in solution, bacteria attached to metal surfaces, and bacteria affecting SIP through redox reactions.Bacterial cells in suspension did not produce a measurable SIP response: phase shifts were indistinguishable from those of the background medium, despite small increases in bulk conductivity. Reanalysis of previous studies indicates that interpretations based solely on imaginary conductivity can be misleading, and that phase shift is the more reliable indicator. Likewise, bacterial attachment to metal surfaces did not modify the SIP response, even though microscopy confirmed surface colonization.In contrast, strong and time-dependent SIP signals were observed when iron was immersed in oxygenated media, closely tracking oxygen consumption and attributable to surface oxidation. When oxygen was removed—by microbial respiration or chemical scavenging—the change in SIP response was significantly reduced, and systems with and without bacteria behaved similarly. Numerical simulations with redox-modified boundary conditions reproduced these trends and linked SIP evolution to changes in surface capacitance and electron transfer. Overall, SIP is sensitive not to microbial cells themselves, but to microbially mediated redox processes involving conductive materials.