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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 d⁵⁶Fe = –1.6 ‰. The Fe isotope fractionation is attributed to a kinetic isotope effect produced during ligand-promoted dissolution of Fe. Isotopic fractionation is accelerated by continuously agitating reacting mixtures, contradicting a fractionation mechanism related to Fe reduction, and corroborating a mechanism wherein mineral-water mixing allows greater preferential attack at the mineral surface. Extractions of the soil from which the bacteria were isolated show that the exchangeable Fe is lighter than Fe in both oxide minerals and hornblende in the soil by -1.1‰ and -1.5‰, respectively. The results of this study suggest a mechanism for biological Fe isotope fractionation and show that Fe isotope signatures may be present in modern soil systems. Specifically, we predict that if our results can be extrapolated to soils, primary minerals that have been heavily leached of Fe should be isotopically heavy, while zones of partial reprecipitation of leached materials should be isotopically light. These signatures could be useful in tracing biological or abiological Fe transformations in the environment.

This year we also documented that the 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 may be related to siderophores that complex with Mo (as well as Fe) in solution; however, azotochelin, a siderophore with high affinity for Mo known to be produced by azotobacter, is not responsible for measurable Mo extraction from the silicate glass in our experiments. Either the azotobacter produces another ligand (not a siderophore) that complexes Mo, or protochelin, a pyoverdin-type siderophore, is responsible for Mo release from the glass. We hope to document the nature of the Mo-extracting ligand.

We are similarly searching to identify the ligand responsible for extraction of Ni from an Fe silicate glass in the presence of a methanogen. We have tried several avenues in this project, without success. However, we recently teamed with a biochemist and we have new approaches to pursue.

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 extracellular ligands or polymers secreted by the Anabaena attack the mineral surface. Dissolution of apatite in spent supernatant (cells filtered out) is also rapid compared to the medium alone, but not as fast as observed in experiments with cells present.