INGERSON LECTURE: EATING AND BREATHING AT THE WATER-ROCK INTERFACE

Micro-organisms colonized the land surface more than 2 billion years ago and have subsequently perfected techniques for extracting elements for nutrients or harvesting energy from metal redox reactions at the mineral surface. Trace metals such as Fe, Mn, Cu, Zn, V, Mo, Ni, and Co are extracted from earth materials by micro-biota for enzymes, coenzymes, and cofactors, and some serve as terminal electron acceptors or electron donors. Many of these processes are enhanced by molecules secreted by micro-organisms into the environment, but other than the common phenomenon wherein micro-organisms secrete acids to lower the pH, relatively little is known about microbial exudates that dissolve minerals. For example, microbially enhanced extraction of metals from minerals by cation-specific high-affinity ligands has only been documented for Fe. However, we have recently shown that Methanobacterium thermoautotrophicum, a methanogen with a requirement for Ni, enhances release of Ni from Ni-silicate. The Fe-reducing bacterium Shewanella putrefaciens secretes compounds that complex iron and secretes an electron shuttle to reduce iron oxides. Azotobacter vinelandii, a nitrogen-fixing soil bacterium known to produce Fe-complexing ligands, may also produce Mo-complexing ligands in order to extract Mo for use in nitrogenase enzyme. Cell-free supernatant from A. vinelandii cultures is also capable of extracting Mo from silicate, consistent with the importance of secreted ligands. In contrast, filtered supernatant solution from cultures of the cyanobacterium Anabaena are not as efficient at dissolving apatite crystals as dissolution in cultures with cells present. The cyanobacterium coats itself with extracellular polymeric material when P-limited, and etching of apatite under biofilms of P-limited Anabaena is extensive. The identification of metal-specific ligands, electron shuttles, and other mechanisms of microbial solubilization of minerals, including their effects upon the chemistry and isotope signatures of dissolved species and mineral surfaces, promises to open a new chapter in our understanding of interactions between the biological and the geological world.