Biological impact on mineral dissolution: Application of the lichen model to understanding mineral weathering in the rhizosphere

Microorganisms modify rates and mechanisms of chemical and physical weather ing and clay growth, thus playing fundamental roles in soil and sediment fo rmation. Because processes in soils are inherently complex and difficult to study, we employ a model based on the lichen-mineral system to identify th e fundamental interactions. Fixed carbon released by the photosynthetic sym biont stimulates growth of fungi and other microorganisms, These microorgan isms directly or indirectly induce mineral disaggregation, hydration, disso lution, and secondary mineral formation. Model polysaccharides were used to investigate direct mediation of mineral surface reactions by extracellular polymers. Polysaccharides can suppress or enhance rates of chemical weathe ring by up to three orders of magnitude, depending on the pll, mineral surf ace structure and composition, and organic functional groups. Mg, Mn, Fe, A l, and Si are redistributed into clays that strongly adsorb ions. Microbes contribute to dissolution of insoluble secondary phosphates, possibly via r elease of organic acids, These reactions significantly impact soil fertilit y. Below fungi-mineral interfaces, mineral surfaces are exposed to dissolve d metabolic byproducts. Through this indirect process, microorganisms can a ccelerate mineral dissolution, leading to enhanced porosity and permeabilit y and colonization by microbial communities.