Illustrations of the occurrence and diversity of mineral-microbe interactions involved in weathering of minerals

Microorganisms are widespread in all natural environments where, in order to generate energy to form new cell structures, they oxidize and reduce organic and inorganic materials, and form gaseous, liquid and solid metabolic compounds which they excrete into their environment. The energetic and chemical activities of microorganisms are involved in the solubilization or fixing or precipitation of inorganic elements, in the weathering of minerals (silicates, phosphates, carbonates, sulphides, oxides) and in the formation of mineral deposits. Both autotrophic (chemolithotrophic) and heterotrophic (chemo-organotrophic) microorganisms are involved and participate directly (mainly by oxidation-reduction processes) or indirectly (by metabolic products) in the weathering, transformation and evolution of minerals in soils and sediments. Some examples are provided by: (1) the weathering of layer silicates in the rhizosphere of plants (root environment) as influenced by microorganisms (bacteria and mycorrhizal fungi) associated with the roots; (2) the autotrophic bacteria (Thiobacilli) which solubilize sulphides by oxidation of Fe and S that depend on the contact between bacteria and mineral and of mineral surface properties and electrochemical parameters; and (3) heterotrophic bacteria (Bacilli, Clostridia, etc.) which are able, using the available soil organic matter as source of C and energy, to dissolve ferric oxides (hematite, goethite) by reduction of insoluble ferric iron in soluble ferrous iron with rates depending on mineral element substitution in the oxide structure. Such examples illustrate the importance, the interest and the diversity of 'microorganism-mineral interactions' and allow us to underline different incidences, applications and perspectives of research and development.