Production of microbial extracellular polymeric substances in soil-like systems – The role of nutrient imbalances, water availability and presence of mineral surfaces

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Extracellular polymeric substances (EPS) are composed of a diverse compound mixture including polysaccharides (EPS-PS) and proteins. EPS provide several functions for microbial life and certain soil functions such as aggregation and water retention have been proposed to be shaped by EPS. Although it is known that various soil microorganism are capable to produce EPS, it is still unclear if and how they do that in vitro. The interplay between microorganisms and soil environmental conditions is likely to control microbial EPS production. However, it is not clear how e.g. the lack of resources or an oversupply of nutrients, fluctuation in water availability and the presence of mineral surfaces affect EPS production. Thus, the main goal of this project is to identify and quantify the most important driving factors for EPS production (amount, composition, and spatial expansion) in soil-like systems. By this we aim to assess the quantitative importance of EPS in these systems. We will mainly study the supply of nutrients (C, N, and P), the availability of water within different pore sizes, and the presence of mineral surfaces as driving factors. EPS production will be studied by a series of experimental approaches with increasing complexity from liquid media, over so called "soil Chips" to artificial soil systems by a combination of extraction and visualization techniques. In addition, we will study the effectiveness of EPS extractions influenced by different clay contents and evaluate the usefulness of µCT imaging in combination with contrast agents to study spatial expansion of EPS in 3D. Firstly, production and composition of EPS-PS will be screened under various environmental conditions in liquid media by a microplate platform for high-throughput. Next to total carbohydrate contents, the complete monomer composition will be analyzed via ultra-high performance liquid chromatography coupled with ultra violet and electrospray ionization ion trap detection (UHPLC-UV-ESI-MS). Then, we will increase complexity by analyzing the nutrient supply and water availability within diverse pore size distributions with the “soil chip” approach. This novel technique allows us to visualize EPS production after lectin staining with confocal laser scanning microscopy and thus evaluate the spatial expansion of EPS driven by environmental factors. In the last step, we will further increase complexity and add the third dimension (3D) by using artificial soil column experiments (quartz sand and montmorillonite). In these 3D soil-like systems we study the drivers of EPS production as the availability of mineral surfaces, pore size distribution and nutrient imbalances in a combined way. With the unique combination of extraction and visualization techniques, we will determine the drivers of EPS production and asses the quantitative importance of EPS in soil-like systems. Moreover, our project will deliver the basis for protocols to study EPS in situ in soil science.