Bacterial cell density shifts the low-frequency electrical polarization of sand, giving spectral induced polarization an in-situ geophysical fingerprint of microbial presence

Spectral induced polarization (SIP) measures how a porous medium stores and releases charge in response to a low-frequency alternating current — the same technique geophysicists apply to real sediments without drilling. Because microbial cell surfaces carry a net electrical charge, the presence of bacteria should, in principle, register as a shift in a sediment's polarization response. This is a foundational premise of biogeophysics, the subdiscipline that reads subsurface microbial activity through geophysical (electrical) signals.

A 2005 Geophysical Research Letters study by Ntarlagiannis and colleagues tested the premise directly in controlled sand columns seeded with bacterial cells. Relative to sterile sand, the measured polarization "decreased (up to 60%)" at low cell density and "increased (up to 15%)" at higher cell density — a non-monotonic, density-dependent signature that ties a measurable electrical response to microbial presence rather than to the mineral matrix alone. The result establishes that microbe-linked electrical activity is a real field-detectable signal, not merely a lab curiosity confined to isolated filaments.

This is the detection-side companion to the biology-side mechanism claim at claim-cable-bacteria-conduct-through-nickel-protein-wire: one note explains how a microbial structure conducts, this one shows that microbial electrical properties leave a remotely readable geophysical trace. The two together close a loop between a laboratory electron-transport mechanism and an in-situ measurement technique.