Complex electrical conductivity is increasingly used to monitor subsurface processes associated with microbial activities as microbial cells mostly have surface charges and thus electrical double layers. While highly saline environments are frequently encountered in coastal and marine sediments, there are limited data available on the complex conductivity associated with microbial activities under a high salinity condition. Therefore, we present the spectral responses of complex conductivity of sand associated with bacterial growth and biofilm formation under a highly saline condition of ∼1% salinity and ∼2 S/m pore water conductivity with an emphasis on relaxation behavior. A column test was performed, in which the model bacteria Shewanella oneidensis MR-1 were stimulated for cell growth and biofilm formation in a sand pack, while the complex conductivity was monitored from 0.01 Hz to 10 kHz. The test results show that the real conductivity increases in the early stage due to the microbial metabolites and the increased surface conduction with cell growth, but soon begin to decrease because of the reduction of charge passages due to bioclogging. On the other hand, the imaginary conductivity significantly increases with time, and clear bell-shaped relaxation behaviors are observed with the peak frequency of 0.1- 1 Hz, associated with double layer polarization of cells and electrically conductive pili and biofilms. The Cole-Cole relaxation model appears to well capture such relaxation behaviors, and the modeling results show the gradual increases in normalized chargeability and decreases in relaxation time during bacterial growth and biofilm formation in the highly saline condition. Comparison with previous literature confirms that the high salinity condition further increases the normalized chargeability while it suppresses the phase shift and thus the imaginary conductivity. Our results suggest that the complex conductivity can effectively capture microbial biomass formation in sands under a highly saline condition.

We present a global atlas of 4,728 metagenomic samples from mass-transit systems in 60 cities over 3 years, representing the first systematic, worldwide catalog of the urban microbial ecosystem. This atlas provides an annotated, geospatial profile of microbial strains, functional characteristics, antimicrobial resistance (AMR) markers, and genetic elements, including 10,928 viruses, 1,302 bacteria, 2 archaea, and 838,532 CRISPR arrays not found in reference databases. We identified 4,246 known species of urban microorganisms and a consistent set of 31 species found in 97% of samples that were distinct from human commensal organisms. Profiles of AMR genes varied widely in type and density across cities. Cities showed distinct microbial taxonomic signatures that were driven by climate and geographic differences. These results constitute a high-resolution global metagenomic atlas that enables discovery of organisms and genes, highlights potential public health and forensic applications, and provides a culture-independent view of AMR burden in cities.