LIFE ON THE ROCKS: MICROBIAL ACQUISITION OF METAL MICRONUTRIENTS

Although the average concentrations in soil waters of Fe, Mn, Zn, Ni, Cu, Co, and Mo are extremely low, these metals are used in bacterial enzymes, coenzymes, and cofactors. To extract Fe from their environment, bacteria commonly secrete siderophores; however, to our knowledge no such high-affinity complexation mechanisms have been documented for other metals. We have investigated microbial extraction of metals by synthesizing Fe-Ni-Mo-containing silicate glasses for incubation with three soil microbes in metal-deficient medium. All three microbes, like virtually all known organisms, need Fe. Bacillus mycoides, a heterotrophic aerobic soil microbe with no known need for Mo, releases catecholate siderophores to take up Fe in pH-neutral conditions. In experiments with B. mycoides + glass + medium, release of Fe is enhanced but release of Mo is not enhanced from glass compared to abiotic experiments. Azotobacter vinelandii, a nitrogen-fixing siderophore-producing aerobe, needs Fe and Mo for the nitrogenase I enzyme. When the glass is incubated with the azotobacter, enhanced release of both metals is observed. Finally, Methanobacterium thermoautotrophicum, a methanogen with a significant requirement for Ni for enzymes of methanogenesis, enhances release of Ni from the glass. The mechanisms by which these microbes extract metals is under investigation: for example, isotopic composition (56Fe/54Fe) of Fe in solution after siderophore-promoted dissolution of hornblende crystal is isotopically lighter than Fe released to solution without organic ligands. Investigation of other metal isotopic signatures, mineral surface chemistry, and the organic chemistry of bacteria-mineral systems should elucidate mechanisms of metal cycling by bacteria. Even though one-fifth to one-third of all elements are bioessential, much remains to be learned about such biological cycling. New analytical techniques will allow quantitative analysis of these processes.