Microbial Roles in Mineral Transformations and Metal Cycling in the Earth's Critical Zone

Microbes play key geoactive roles in the biosphere particularly in the areas of element biotransformations and biogeochemical cycling, metal and mineral transformations, decomposition, bioweathering, soil and sediment formation. All kinds of microbes, including prokaryotes and eukaryotes and their symbiotic associations with each other and "higher organisms", can contribute actively to geological phenomena, and central to many such geomicrobial processes are metal and mineral transformations. Microbial roles in mineral transformations and metal cycling are especially important in the Earth's Critical Zone (CZ) and microbes possess a variety of properties that can effect changes in metal speciation, toxicity and mobility, mineral formation or mineral dissolution or deterioration. Such mechanisms are important components of natural biogeochemical cycles for metals as well as associated elements in organic matter, soil, rocks and minerals, e. g. sulfur and phosphorus, and metalloids, actinides and metal radionuclides. Apart from being important in natural biosphere processes, metal and mineral transformations can have beneficial or detrimental consequences for human society. Bioremediation refers to the application of biological systems to the clean-up of organic and inorganic pollution with bacteria and fungi being the most important organisms for reclamation, immobilization or detoxification of metallic and radionuclide pollutants. Some biominerals or metallic elements deposited by microbes have catalytic and other properties in nanoparticle, crystalline or colloidal forms, and these are relevant to the development of novel biomaterials for structural, technological, environmental and antimicrobial purposes. In contrast, metal and mineral transformations by microbes may result in spoilage and destruction of natural and synthetic materials, rock and mineral-based building materials, e. g. concrete, acid mine drainage and associated metal pollution, biocorrosion of metals, alloys, and related substances, and adverse effects on radionuclide speciation, mobility and containment, all with immense social and economic consequences. In view of the ubiquity and importance of microbes in such global processes, it can be proposed that an additional definition of the CZ could be "that portion of the terrestrial environment characterized by a significant microbial influence on metal and mineral transformations, organicmatter decomposition, and the cycling of other elements".