Insights Into Biogeophysical Signatures Using Polarization Force Microscopy

The success of bioremediation strategies is dependent upon effective monitoring of microorganisms in the subsurface. Induced polarization (IP) may represent a cost-effective, complementary technique to existing borehole-based microbe detection schemes. Recent studies show a significant, yet poorly understood IP effect associated with the presence of bacteria in aqueous and porous media. This effect is believed to be rooted in the physicochemical surface interactions between cells and minerals which we probe using polarization force microscopy. Polarization force experiments were conducted on a hydrated mica surface using the gram positive bacterium Bacillus subtilis and the gram negative bacterium Escherichia coli. On all surfaces, polarization force maximums (Fmax) increase as relative humidity is increased and surface water content rises. The Fmax response exhibited by E. coli was higher than that of B. subtilis at relative humidities (RH\u3e75%), which suggests a unique effect due to the gram negative membrane structure of E. coli. The additional fluidity of the outer and inner membrane and the additional mobile charge within the gram negative periplasm are possible sources of the enhanced polarizability. Based on similarities between modeled, frequency-dependent permittivity trends on a bacterium and our experimental polarization force measurements, we propose the polarization force as a proxy for local permittivity at the cell-mineral interface. In this framework, unique dielectric dispersions with increasing frequency are exhibited by all three surfaces. Moreover, decay constants of the time evolution of the polarization force at low frequency reveal similar, relatively slow mobile ion response associated with both bacteria, and an overall faster mobile ion response on mica. This suggests either lower mobile ion density or higher intrinsic surface mobilities for the mica. Lower mobilities on the cells could be attributed to inhibited ion movement due to protein and lipopolysaccharide membrane structures. Overall, this work shows distinct differences in the mobile ion and polarization force response of bacteria and mica. The differences in the polarizability we observed for each surface provides nanoscale information on charge separation mechanisms that could potentially sum up to a bulk, i.e. column- or field- scale, biogeophysical IP response