Probing microbe-mineral-organic matter interactions in soils with photothermal infrared spectromicroscopy

Interactions among microbes, minerals, and organic matter are key controls on carbon, nutrient, and contaminant dynamics in soil and sediments. However, probing these interactions at relevant scales and through time remains an analytical challenge due to both their complex nature and the lack of tools permitting non-destructive time-step analysis. The recent development of optical photothermal infrared (O-PTIR) microscopy has opened the way for non-invasive analysis of these interactions at submicron resolution through time. Here we demonstrate the ability of O-PTIR microscopy to analyze mineral-organic microstructures down to 400 nm, without contact, allowing the time-resolved, non-destructive characterization of both mineral and organic components. Results showed that, while all these mineral-organic microstructures can be analyzed without measurable beam damage using the appropriate laser power, poorly crystalline minerals, and high-molecular-weight compounds are more sensitive to damage than crystalline minerals and low-molecular-weight compounds, respectively. Despite these differences in beam damage sensitivity, we found analytical conditions under which all materials were analyzed without damage and could therefore be analyzed repeatedly over time. With synthetic mineral-organic microstructures, we localized mineral-bound and unbound organic compounds, down to the sub-micron scale. Our results highlight the potential of and provides analytical recommendation for the application of O-PTIR microscopy to resolve microbe-mineral-organic matter interactions in soil and sediments.