Linking microbial dynamics to geophysical signals in porous media

Understanding near-surface biogeochemical dynamics is vital in the context of rapid environmental change. Spectral induced polarization (SIP), a non-invasive geophysical method, holds potential for monitoring microbial activity and abundance. A key driver of subsurface biogeochemical processes. We investigated the SIP responses of Shewanella oneidensis MR-1 in reactors packed with alginate beads as an inert porous medium. By analysing the imaginary conductivity ( response of the SIP signal, we found that SIP signals closely tracked microbial growth phases, as confirmed by measured colony forming units, CFUs, (R² = 0.80) and ATP (R² = 0.90) measurements. SIP responses peaked during exponential growth and declined in the stationary phase. Notably, the bead-packed systems provided stable signals due to reduced cell settling (often a drawback of pure suspension experiments). Our results demonstrate that SIP can effectively detect and monitor biologically driven changes in the electrical properties of porous media. By working with inert beads, we have isolated cells' responses from interactions between cells and mineral or organic colloids. This study supports the advancement of SIP as a high-resolution method for quantifying dynamic microbial activity in porous media. Ongoing research aims to upscale our findings to study the effect of microbial metabolic state on their electrical properties. Planned retentostat experiments are expected to improve our quantitative understanding of microbially generated SIP signals and contribute to their eventual deployment as an off-the-shelf monitoring tool for non-geophysicists.