Microbial Organic Carbon Coatings on Surfaces of Calcite and Biotite Correspond Spatially and Chemically with the Minerals Elemental Signal

Although macro-scale in nature, most biogeochemical processes are driven by interactions at the micro- and nanoscale. Here, we present an experimental approach for studying interactions of soil microbes with calcite and biotite, with a goal to identify the newly formed organo-mineral associations on mineral surfaces. We placed in-growth mesh bags with biotite and calcite in a ponderosa pine rhizosphere for 6 months and then we imaged and analyzed the spatial and chemical associations of minerals with the newly produced organic matter deposited on the mineral surfaces. First, we determined the composition of the microbial community for the bulk soil, calcite and biotite by the 16S and ITS2 rRNA gene sequencing. Then we analyzed the organic content of the samples by the Fourier transform ion cyclotron resonance mass spectrometry (FTICR). Both calcite and biotite showed predominantly lipid-like and unsaturated hydrocarbon compounds, whereas the bulk rhizospheric soil also additionally showed lignin-like and carboxylrich alicyclic molecules. These results indicate that the organic compounds made by the microbes in-situ in the mesh bags are arguably less stable than those found in the bulk soil. For the spatially resolved correlated methods, we first determined the regions of interest by TEM, focusing on finding microbes and layers of organic material on mineral surfaces. Then we subjected these same regions to the high-resolution imaging and elemental mapping by scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM, EDS). Additionally, to spectrally image the same regions, we used scanning transmission X-ray microscopy coupled with X-ray absorption near edge structure (STXM, XANES). Both the STEM /EDS high-resolution elemental mapping and the STXM/XANES x-ray spectral imaging evidenced the respective Ca and Fe elemental increase at the microbe-mineral interface, indicating biogenic release of these mineral cations. This correlative, multi-capability approach provided characterization of microbial interaction with minerals at the high spatial and spectral resolution, and it can bring a new detailed understanding of stabilization of microbial C in soils by a biomineralization mechanism.