2D Image Quantification of Microbial Iron Chelators (Siderophores) Using Diffusive Equilibrium in Thin Films Method.

Siderophores are natural metal chelating agents that strongly control the biogeochemical metal cycles such as Fe in the environment. This article describes a new methodology to detect and quantify at the micromolar concentration the spatial distribution at millimeter scale of siderophores within the root's system. The "universal" CAS assay originally designed for bacterial siderophores detection and later designed for fungus was adapted here for diffusive equilibrium in thin film gel techniques (DET). The method was calibrated against the marketed desferrioxamine mesylate (DFOM) siderophore and applied with experiments performed with sunflower ( Helianthus annuus) and wheat ( Triticum aestivum) cultivated on free iron agar medium plates. We present here the first results with 2D images of the siderophores distribution in the vicinity of the root system of plants. With this technique we detected (i) the production of siderophores on bacteria inoculated ( Pseudomonas fluorescens) environments and (ii) hotspots of natural iron binding ligands production up to 50 µM in the wheat rhizosphere. The lower detection limit in our experiment was 2.5 µmol/L. This new technique offers a unique opportunity to investigate the siderophore production in two dimensions in a wide range of applications from laboratory experiments to natural systems very likely using an in situ and nondestructive tool.

Simultaneous 2D imaging of dissolved iron and reactive phosphorus in sediment porewaters by thin-film and hyperspectral methods.

This study presents a new approach combining diffusive equilibrium in thin-film (DET) and spectrophotometric methods to determine the spatial variability of dissolved iron and dissolved reactive phosphorus (DRP) with a single gel probe. Its originality is (1) to postpone up to three months the colorimetric reaction of DET by freezing and (2) to measure simultaneously dissolved iron and DRP by hyperspectral imaging at a submillimeter resolution. After a few minutes at room temperature, the thawed gel is sandwiched between two monospecific reagent DET gels, leading to magenta and blue coloration for iron and phosphate, respectively. Spatial distribution of the resulting colors is obtained using a hyperspectral camera. Reflectance spectra analysis enables deconvolution of specific colorations by the unmixing method applied to the logarithmic reflectance, leading to an accurate quantification of iron and DRP. This method was applied in the Arcachon lagoon (France) on muddy sediments colonized by eelgrass (Zostera noltei) meadows. The 2D gel probes highlighted microstructures in the spatial distribution of dissolved iron and phosphorus, which are most likely associated with the occurrence of benthic fauna burrows and seagrass roots.