Quadrature conductivity: A quantitative indicator of bacterial abundance in porous media

Abstract

The abundance and growth stages of bacteria in subsurface porous media affect the concentrations and distributions of charged species within the solid-solution interfaces. Therefore, spectral induced polarization (SIP) measurements can be used to monitor changes in bacterial biomass and growth stage. Our goal was to gain a better understanding of the SIP response of bacteria present in a porous material. Bacterial cell surfaces possess an electric double layer and therefore become polarized in an electric field. We performed SIP measurements over the frequency range of 0.1-1 kHz on cell suspensions alone and cell suspensions mixed with sand at four pore water conductivities. We used Zymomonas mobilis at four different cell densities (including the background). The quadrature conductivity spectra exhibited two peaks, one around 0.05-0.10 Hz and the other around 1-10 Hz. Because SIP measurements on bacterial suspensions are typically made at frequencies greater than 1 Hz, these peaks have not been previously reported. In the bacterial suspensions in growth medium, the quadrature conductivity at peak I was linearly proportional to the density of the bacteria. For the case of the suspensions mixed with sands, we observed that peak II presented a smaller increase in the quadrature conductivity with the cell density. A comparison of the experiments with and without sand grains illustrated the effect of the porous medium on the overall quadrature conductivity response (decrease in the amplitude and shift of the peaks to the lower frequencies). Our results indicate that for a given porous medium, time-lapse SIP has potential for monitoring changes in bacterial abundance within porous media.

Bacterial Stern layer diffusion: experimental determination with spectral induced polarization and sensitivity to nitrite toxicity

“…The slope of equation is two orders of magnitude lower than that between SIP measurements and cell densities reported in Zhang et al . (). The discrepancy between the relationship derived for S. oneidensis in non‐polarizing quartz sand (see Mellage et al .…”

“…recently prepared suspension versus cells under growth conditions), and the similar magnitude of the three cell densities in the regression analysis in Zhang et al . (). Future research must address these key factors with the goal of developing unified relationships for estimation of cell density from σ″ measurements.…”

“…) and Z. mobilis suspensions in Zhang et al . () is likely a function of several factors in their experiments, including the different potential electrode construction (i.e. encased in electrolyte versus in direct contact with sample), different bacterial strains, the metabolic/physiological state of the cells (i.e.…”

“…Because of this surface charge, bacteria in contact with water are surrounded by an electrical double layer (EDL; Zhang et al . ). Spectral induced polarization (SIP) is an active geophysical method that measures electric conduction and charge storage, based on the difference in properties between injected and detected waveforms (see Kemna et al .…”

“…Using suspensions of Zymomonas mobilis , Zhang et al . () showed the dependence of σ″ on variable cell concentrations. These authors proposed a linear relationship between cell density and σ″ , based on measurements performed at three cell concentrations.…”