Gold and Silver Trapping by Uncultured Magnetotactic Cocci

We studied the sites of gold and silver trapping by uncultured magnetotactic cocci from microcosms using transmission electron microscopy and energy-dispersive X-ray analysis. Two morphotypes were found to trap gold or silver. Morphotype 1 had large magnetite crystals frequently twinned in an unusual way and contained phosphorus-rich granules and electron-lucent inclusions probably composed of polyhydroxyalkanoates. Morphotype 2 presented smaller crystals with smaller width/length ratios and granules containing C, O, P, S, Cl, Na, Mg, Ca, and Fe, called phosphorus-sulfur-iron granules due to the presence of relatively large amounts of phosphorus, sulfur and iron. Gold was found in morphotype 2 bacteria, mainly in phosphorus-sulfur-iron granules. Additionally, the capsule presented small deposits that seemed to be composed of elemental gold. Silver was found in both spherical and rosette-shaped crystalline deposits also containing sulfur at the cell envelope of morphotype 1 bacteria. The rosette-shaped deposits had six subunits, suggesting that a homohexameric macromolecular assembly might be involved in their nucleation process. This seems to be an example of a highly organized structure mineralized incidentally by a biologically induced biomineralization process.

Keywords:

• biomineralization
• gold
• heavy metals
• magnetotactic bacteria
• polyphosphate granules
• silver
• X-ray microanalysis

Acknowledgments

We are thankful to F. F. Freitas and U. Lins for discussion and M. M. M. Oliveira for technical assistance. We thank also Laboratório de Ultraestrutura Cellular Hertha Meyer for electron microscopy facilities. This work was supported by Brazilian grants from CNPq, CAPES, and FAPERJ.