Diffusion of glucose in microbial extracellular polysaccharide as affected by water potential

Extracellular polysaccharide secretion (EPS) is a salient feature of many soil bacteria. This study investigates whether an EPS shell modulates the diffusion rate of nutrients in the immediate microenvironment of soil microorganisms. The diffusion of glucose through pure microbial polysaccharides xanthan and dextran, and through kaolinite and EPS-amended kaolinite was measured at several water potentials using a steady state method. A specific device was developed to simultaneously monitor water potential (Ψ) using polyethylene glycol (PEG) solutions, and measure diffusion rates. The diffusion rate of glucose increased with increasing Ψ and volumetric water content in both polysaccharides. At all Ψ values studied, glucose diffused faster in EPS or in EPS-amended clay than in pure clay, due to the higher volumetric water content of EPS and their water-saturated porosity. Water retention and diffusion characteristics in EPS may thus help soil bacteria to maintain physiological functions at low water potential.