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Project Progress

Fe, Mn, Zn, Ni, Cu, Co, and Mo are extremely low in abundance in natural waters, but each of these metals is used in bacterial enzymes, coenzymes, and cofactors. While it is well known that microbes secrete siderophores to extract Fe from their environment, it is not understood how these siderophores attack minerals to provide the FeIII, nor is it understood how bacteria extract other micronutrients. In previously reported work, we have shown that microbes can mobilize Mo (Azotobacter vinelandii), Ni (Methanobacterium thermoautotrophicum), and Cu (Bacillus mycoides) from silicates.

Mineral dissolution experiments with a siderophore-producing soil bacterium (closely related to Bacillus mycoides) from Gore Mountain, NY, show that this microbe enhances Fe release from goethite. Dissolved Fe released from goethite in the presence of the bacillus yields δ⁵⁶Fe = -1.6‰. The Fe isotope fractionation is attributed to several factors: isotopic equilibration between Fe(II) and Fe(III) in solution followed by uptake of Fe(III) by bacteria; adsorption of isotopically heavy Fe(II) on goethite surfaces. We also have completed adsorption experiments abiotically to show that adsorption of Fe(II) is an important mechanism in the fractionation of Fe isotopes.

Last year we documented that Mo that is taken up by azotobacter into cell mass is isotopically light compared to the Mo source material (an Fe-containing silicate). The Mo released to solution in the presence of azotobacter is not isotopically fractionated. This documents that fractionation occurs during cellular uptake, but not during extraction from the host material. Mo extraction has now been shown to be related to the production of aminochelin, a siderophore with high affinity for Mo known to be produced by azotobacter.

We continue to search to identify the ligand responsible for extraction of Ni from an Fe silicate glass in the presence of a methanogen. We have documented for the first time that supernatants from the methanogen cultures are also capable of extraction of Ni. We are now working fulltime on identification of the putative Ni ligand.

Anabaena, a cyanobacterium, was grown with fluorapatite as sole phosphorous (P-) source for 7 days. Cell growth was compared with cultures that contained a dissolved P-source, as well as a P-depleted control. An inorganic control, apatite + medium, was also run without inoculation. At the end of the experiment, fluorapatite grains from the various treatments were compared using a scanning electron micrograph (SEM). Enhanced etching of the apatite in the presence of the cyanobacterium as compared to the abiotic control, despite the same pH in each experiment, documents that either i) extracellular ligands or polymers secreted by the Anabaena attack the mineral surface or ii) uptake of P by the Anabaena changes the affinity of the reaction and enhances apatite dissolution. Dissolution of apatite in spent supernatant (cells filtered out) is not enhanced relative to medium alone, contrary to our earlier observations. The ability of microorganisms to change the affinity of reacting solutions may be an important characteristic that has generally been ignored.

Very preliminary results have been collected investigating the isotopes of Cu related to bacterial chalcopyrite and chalcocite oxidation. This work is with Joaquin Ruiz and Ryan Mathur.

We have exciting new results from work with biochemists Ming Tien and Shane Ruebush of Penn State, where we are working on isolating the enzymes responsible for iron reduction by dissimilatory iron reduction. Tien and Ruebush, in collaboration with Brantley and Icopino, have shown reduction in vitro of goethite, birnessite, hematite, and pyrolusite. These are the first known mineral reductions in vitro.