MICROSCOPIC TO MACROSCOPIC PERSPECTIVES ON MICROBIAL MINERAL FORMATION

Microorganisms contribute to the formation of a wide variety of authigenic minerals including oxides, phosphates, carbonates, sulfides, and silicates. The mechanisms of microbial mineral precipitation are diverse, but generally involve two distinct stages. These are nucleation and crystal growth. Nucleation is the most critical stage for mineral precipitation and occurs either homogeneously or heterogeneously. In homogeneous reactions, mineral nuclei are formed by the random collisions of ions in a saturated solution. In many instances, microbial metabolic activity indirectly sustains the onset of saturation through the production of reactive inorganic ligands (e.g., sulfide, phosphate, dissolved inorganic carbon), or directly through enzyme mediated changes in redox state (e.g., oxidation of reduced iron or manganese). Conversely, heterogeneous nucleation involves the formation of crystal nuclei on the surfaces of reactive sorbent solids like microbial cells. Once a stable nucleus has formed, crystal growth can proceed spontaneously providing that the concentration of ions in solution continues to exceed the solubility product of the solid phase mineral (i.e., the solution must remain saturated). The geochemical significance of bacterial mineral precipitation is emphasized by the fact that microorganisms are the most abundant life form on earth and live in a wide variety of environments. Bacteria exist wherever there is liquid water up to temperatures of at least 110 °C, and thrive even in the harshest environments, from the high Arctic to the depths of the oceans and porous terrestrial strata. Moreover, fossil and geochemical evidence of bacterial life is as old as the geologic record, extending back at least 3.8 billion years. The implications of these observations are unambiguous. Bacterial mineral preciptiation is significant on a global scale, and has helped shape the planet throughout geological time.