Mineralized Filaments Help Power Microbes

Bacteria in an abandoned Wisconsin iron mine have been found to extrude and shed alginate polysaccharide filaments that serve as templates to form extraordinarily long iron oxyhydroxide (FeOOH) nanocrystals. The researchers who made the discovery believe the microbes generate the mineralized strands as part of an elaborate metabolic process to produce energy [Science, 303, 1656 (2004)].

Graduate student Clara S. Chan and geochemist Jillian F. Banfield at the University of California, Berkeley; physics professor Gelsomina De Stasio at the University of Wisconsin, Madison; and colleagues used high-resolution synchrotron X-ray microscopy and other techniques to study the filaments. The team also reproduced the filaments in the lab by adding an iron solution to an alginate solution.

The researchers believe bacterial enzymes catalyze iron oxidation and oxygen reduction at the cell membrane, leading to mineral formation. Amorphous FeOOH precipitates on the polymer filaments (shown, arrows point to cell wall) and over time forms elongated nanocrystals (bottom) that are surrounded by amorphous and crystalline FeOOH.

It has been unclear how or why some microbes produce these extracellular mineral structures. The researchers propose that the generation of hydrogen ions during FeOOH formation lowers the pH surrounding the cells, creating a gradient to move protons across the cell membrane to generate energy via adenosine triphosphate. The work should aid understanding of biomineralization, they note, and could lead to new clues about ancient life.