Towards Improved Non-Invasive Monitoring Techniques: Understanding the Geo-Electrical Signal of Microbial Abundance and Activity in Subsurface Porous Media (NIMM)

• Applicant: Professor Dr. Adrian Mellage
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 516752146

Microbial biomass (e.g. bacteria) that populates the subsurface is a vital component in elemental, nutrient and contaminant turnover in the environment, catalyzing turnover reactions and effectively biodegrading solutes of potential environmental concern. Changes in microbial abundance and activity are closely linked to the efficiency of biodegradation processes. Thus, monitoring their behavior in the subsurface is key to improving our understanding of turnover reactions and the predictive capabilities of water quality models. Measuring microbes, however, is not straightforward and traditional approaches are limited by the poor accessibility of the subsurface. Sampling relies on invasive approaches and isolating microbial cells from the sample requires further treatment and extraction procedures. So-called non-invasive approaches have gained traction in recent decades because of their potential to monitor microbes within the complex environments they inhabit, for example soils and groundwater, and at a much greater spatial and temporal resolution. In particular, the geo-electrical method spectral induced polarization (SIP) is sensitive to the surface charging properties of microbial cell surfaces and can serve as a proxy for microbial abundance and growth. The method’s potential has been showcased in a series of studies. However, open questions regarding the exact mechanisms driving the signals stemming from microbial growth and activity remain unanswered, hindering the method’s application outside of the field of applied geophysics. The proposed project aims to isolate the SIP signal of bacterial cells in the absence of parallel contributing mechanisms and link the combined effects of cell abundance and activity to the magnitude and spectral characteristics of SIP responses. The work proposed will combine targeted microbial growth experiments under static (batch), well-mixed (retentostat), and flow-through conditions. The experiments will quantify the signals of bacterial cells at various metabolic states, ranging from present and inactive, to active and growing. By performing measurements on cells in the absence of other charged media (e.g. sediment) the project will isolate the separate controls of the abundance versus the metabolic activity of cells. Upscaling from batch incubations to flow-through systems will be performed via systematic experiments of increasing complexity. Experiments in natural porous media, the highest level of complexity, will benefit from data of precursor experiments that will allow for microbial electrical signals to be isolated from parallel sediment-derived signals. The proposed work will ultimately fine-tune the applicability of SIP as a non-invasive monitoring tool that can make the jump between a promising tool showcased by geophysicists to a robust geophysical method applied by biogeochemists, geomicrobiologists and hydro(geo)logists.

• DFG Programme: Research Grants