Microbial Identification, High-Resolution Microscopy and Spectrometry of the Rhizosphere in Its Native Spatial Context

Abstract During the past decades, several stand-alone and combinatory methods have been developed to investigate the chemistry (i.e. mapping of elemental, isotopic and molecular composition) and the role of microbes in soil and rhizosphere. However, none of these approaches are currently capable of characterizing soil-root-microbe interactions simultaneously in their spatial arrangement. Here we present a novel approach that allows chemical and microbial identification of the rhizosphere at micro-to nano-meter spatial resolution. Our approach includes i) a resin embedding and sectioning method suitable for simultaneous correlative characterization of Zea mays rhizosphere, ii) an analytical work flow that allows up to six instruments/techniques to be used correlatively, and iii) data and image correlation. Hydrophilic, immunohistochemistry compatible, low viscosity LR white resin was used to embed the rhizosphere sample. We employed waterjet cutting and avoided polishing the surface to prevent smearing of the sample surface at nanoscale. Embedding quality was analyzed by Helium Ion Microscopy (HIM). Bacteria in the embedded soil was identified by Catalyzed Reporter Deposition-Fluorescence In Situ Hybridization (CARD-FISH) to avoid interferences from high levels of auto fluorescence emitted by soil particles and organic matter. Chemical mapping of the rhizosphere was done by Scanning Electron Microscopy (SEM) with Energy-dispersive X-ray analysis (SEM-EDX), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), nano-focused Secondary Ion mass Spectrometry (nanoSIMS), and confocal Raman spectroscopy (µ-Raman). High-resolution correlative characterization by six different techniques followed by image registration shows that this method can meet the demanding requirements of multiple characterization techniques to chemically map the rhizosphere and identify spatial organization of bacteria. Finally, we presented individual and correlative workflows for imaging and image registration to analyze data. We hope this method will be a platform to combine various 2D analytics for an ample understanding of the rhizosphere processes and their ecological significance. Graphical Abstract