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Extracellular Polymeric Substances Affecting Pore-Scale Hydrologic Conditions for Bacterial Activity in Unsaturated Soils

^a School of Architecture, Civil and Environmental Engineering, EPFL- Ecole Polytechnique Federale, Lausanne CH- 1016, Switzerland
^b Dep. of Civil and Environmental Engineering, Univ. of Connecticut, Storrs, CT 06269
^c Institute of Environment & Resources, Technical Univ. of Denmark, Kgs. Lyngby, DK-2800, Denmark

View larger version (19K): [in this window] [in a new window]   FIG. 1. Molecular structure of the model extracellular polysaccharide xanthan, produced by Xanthomonas campestris (Garcia-Ochoa et al., 2000).

View larger version (93K): [in this window] [in a new window]   FIG. 2. Morphological changes of bacterial extracellular polymeric substance on desiccation (top panel; Roberson and Firestone, 1992) and on rehydration (bottom panel; Shaw et al., 2003), where F denotes filaments of desiccated microcolony, S shows spaces in which filaments lie, and G is glycan (extracellular polysaccharide).

View larger version (15K): [in this window] [in a new window]   FIG. 3. Water-retention capacity of different polysaccharides and extracellular polymeric substance (EPS) analogs (modified from Chenu, 1993).

View larger version (18K): [in this window] [in a new window]   FIG. 4. (a) Relative diffusivity (De/Daq) of solute in biofilm (hydrated extracellular polymeric substance) as a function of solute molecular weight (Stewart, 2003), and (b) relative solute diffusivity in gel (Dg/Dw) as a function of solute hydrodynamic radius RA (Fatine-Rouge et al., 2004).

View larger version (15K): [in this window] [in a new window]   FIG. 5. Increased water-holding capability of sand amended with extracellular polymeric substances (EPS) (Roberson and Firestone, 1992).

View larger version (18K): [in this window] [in a new window]   FIG. 6. Effects of xanthan gum concentration on the saturated hydraulic conductivity of sand medium (modified from Yen, 2001), and of a pack of glass beads with mean diameter of 0.5 to 1.0 mm.

View larger version (15K): [in this window] [in a new window]   FIG. 7. Solute diffusion rates in unsaturated silt loam soil and kaolinite, and glucose diffusion rates in xanthan as a function of water potential (Chenu and Roberson, 1996; Moldrup et al., 2003).

View larger version (83K): [in this window] [in a new window]   FIG. 8. (a) Glass beads column (0.5–1.0 mm diam.) containing blobs of extracellular polymeric substance (EPS) prior addition of dyed water, (b) column saturated with dyed water for 15 min (no dye diffusion in EPS), (c) saturated column after 12 h (including saturated EPS blobs), and (d) column drained and flushed with water (retention of water by EPS blobs after drainage).

View larger version (13K): [in this window] [in a new window]   FIG. 9. Different rates of water potential decrease attributed to evaporation retardation in extracellular polymeric substance (EPS)–amended sand (modified from Roberson and Firestone, 1992).