MICROBIAL EXTRACELLULAR POLYMERS ON SILICATE MINERAL WEATHERING

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Observations of naturally weathered minerals associated with lichen communities show silicate mineral weathering reactions occurring in a complex microbial extracellular polysaccharide gel. Mineral dissolution experiments were performed with plagioclase feldspar (Ca0.75Na0.25Al1.75Si2.25O8) in solutions of naturally occurring polysaccharides (4 alginates, gum xanthan, pectin, starch) that have structures and compositions similar to extracellular polymers produced by microorganisms. Solutions were analyzed for dissolved Si and Al as an indicator of feldspar dissolution. At neutral pH, feldspar dissolution was inhibited by five of the acid polysaccharides compared to an organic free control. However, the undifferentiated alginate substantially enhanced both Si and Al release from the feldspar. Under acidic conditions, initial pH=4, all of the polymers enhanced feldspar dissolution compared to the inorganic controls. Pectin and gum xanthan increased feldspar dissolution by a factor of 10, and the alginates enhanced feldspar dissolution by a factor of 50 to 100. Si and Al concentrations continue to increase with time, even though solutions are super-saturated with respect to several possible secondary phases. The ability of microbial extracellular polymers to affect mineral weathering reactions depends not only on the types of functional groups but also on the orientation of these ligands. For example, the two monomers comprising alginate, mannuronic (M) and guluronic (G) acid, have their carboxylic groups in the equatorial position, resulting in glycosidic bonds at positions 1 and 4 that are equatorial for mannuronate and axial for guluronate. Poly-mannuronate forms a flat ribbon, with the acid functional group held out away from the molecule. This orientation should allow for the carboxyl groups in sequential sugars to bond to sites on mineral surfaces, and possibly inhibit dissolution. Poly-guluronate forms a buckled chain ("egg box structure"), with the carboxyl group oriented in towards the molecule. This site forms bidentate complexes with ions in solution. We would like to use 13C-NMR to determine the structure of the alginate polymers used in our dissolution experiments to determine why they have different effects on mineral weathering reactions. Alginates enriched in GG blocks should form strong complexes with ions in solution, thereby enhancing mineral weathering. We would also like to determine how these polymers form complexes with metal ions (Al+3) and if the polymer structures change as complexes form.