Spectral Induced Polarization Response of Bacteria Growth and Decay in Soil Column Experiments

Spectral induced polarization (SIP) exhibits potential to be a nonintrusive approach to monitor bacterial activity in biological hotspots associated with the critical zone of the earth. The polarization of bacteria in a low-frequency electrical field is related to the polarization of their electrical double layer coating their surface. However, few studies have quantified the induced polarization responses on both gram-negative (GN) and gram-positive (GP) bacteria in soil column experiments. To address this gap, 17 experiments using two strains, Pseudomonas aeruginosa O1 (PAO1, GN) and Brevibacillus centrosporus (L3, GP) are conducted. Complex conductivity spectra are collected in the frequency range 10 mHz-10 kHz during bacterial growth and decay phases in soils. The complex conductivity spectra are fitted using a double Cole-Cole model to remove the effect of Maxwell-Wagner polarization. The change in the magnitude of the polarization (quadrature conductivity or normalized chargeability of the low-frequency contribution) is linearly related to the bacterial density, regardless of the type of bacteria. The changes in the normalized chargeability and Cole-Cole relaxation time are directly proportional to the density of bacteria. Furthermore, it is inferred that the thickness of microcolonies plays a critical role in the relaxation time rather than the diameter of individual bacteria. This study expands the potential of SIP for in situ monitoring of microbial activity in soils.