Tracking microbial movement in saturated media with spectral induced polarization

Pathogenic microorganisms in the subsurface can contaminate soil and water supplies, potentially posing great danger to human health. Early contamination detection routines rely on sparse direct sampling which is spatiotemporally limited. Thus, the path of microorganisms in the subsurface remains ambiguous and this can cause delays in detection of biohazardous threats. The geophysical spectral induced polarization (SIP) technique, sensitive to microbes' presence and activity in porous media, is a promising method to monitor microbial transport pathways. Here we evaluated the efficiency of SIP in monitoring the chemotactic movement of Sporosarcina pasteurii in saturated porous media. A cylindrical sample holder was packed with Ottawa sand and saturated with sterile KCl solution. The sample holder was oriented vertically and S. pasteurii was introduced at the bottom, forcing the movement of the microbes against gravity, towards a carbon source available at the top of the column. Temporal SIP measurements were collected at 3 regions of the sample holder: bottom (microbial injection point), middle and top (carbon source). Both the real (σ^′) and imaginary (σ^″) conductivity parts of the SIP signal increased over time with the σ^″ showing a peak signal magnitude following the upward movement of the microbes. We repeated the experiment excluding the carbon source in experiment 2 and omitting microbial injection in experiment 3. However, we did not observe any significant SIP signal changes in these two experiments. This is the first study to indicate the strong SIP signal correlation with microbial chemotaxis.