POSITIVE FEEDBACK AT THE MINERAL-MICROBE-WATER INTERFACE: A FEW EXAMPLES

Weathering occurs at the water-rock interface through the interaction of physical, chemical, and biological processes. Many of these processes are enhanced by positive feedback. For example, the rate of advancement of the saprolite-rock interface is controlled by the rate of permeation of water inward, and this rate may be controlled by dissolution of highly soluble minerals disseminated throughout the rock or by the distribution of fractures. As dissolution proceeds, the mineral-water interfacial area increases, causing enhanced interface advance downward into unaltered rock. The presence of microbiota also creates positive feedback within the regolith. Researchers have shown that, where growth of micro-organisms is limited by nutrients such as phosphorus, enhanced microbial growth occurs around apatite crystals. As these P-solubilizing organisms colonize the mineral surface, a biofilm may develop that hosts other organisms and further enhances the weathering rates under the biofilm. Similarly, where Fe limitation occurs in soils, Fe-solubilizing micro-organisms can colonize Fe-containing minerals by secreting siderophores. Siderophores enhance dissolution of Fe oxides and silicates, and in some cases, can enhance growth of other organisms. Where N limits growth, Mo may become limiting due to its use as a catalytic center in most nitrogenases. Nitrogen-fixing organisms may enhance the release of Mo from minerals through the secretion of Mo-complexing ligands. In some cases, secretion of extracellular polymeric material by micro-organisms may be enhanced when such metal-complexing ligands are secreted. Ligand concentrations in the polymeric material may be increased under nutrient-limited conditions, creating micro-environments of lower pH and higher reactivity. Thus, enhanced weathering in micro-environments creates the physical conditions to maintain water at the mineral surface and promote growth of more organisms, enhancing rates of weathering over abiotic systems.