CLAY POLYSACCHARIDE OR SAND POLYSACCHARIDE ASSOCIATIONS AS MODELS FOR THE INTERFACE BETWEEN MICROORGANISMS AND SOIL - WATER RELATED PROPERTIES AND MICROSTRUCTURE

Many soil micorganisms are able to produce extracellular polysaccharides (EPS). Electron microscopic observations of soils demonstrated that EPS are produced in soils and are closely associated with the surrounding clay particles. In the present study, experimental clay-polysaccharide associations were taken as models for the soil/biota interface, and their microstructure and physical properties were investigated. In the methodology, special attention was given to control the water potential and to preserve, as far as possible, the organizations of the original hydrated conditions. EPS increased the water retention of clay minerals or sands on desiccation and on rehydration, and reduced desiccation and rehydration rates. This was explained by the strong water-holding properties of EPS. Cryo-scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used, and showed that the polysaccharides occurred as a network of strands in the interparticle porosity of clay minerals. Polysaccharides changed the clay microstructure into an organo-mineral network with extensive interparticle bridging. The present results show that the clay-polysaccharide associations exhibit specific physical properties and microstructures that should affect biological functions and survival, for example through storage of water and buffering against water potential fluctuations. Clay-polysaccharide sheaths also take part in the binding of aggregates by soil biota.