Scientists have long been fascinated by the organic-mineral filaments that extend from the surface of some iron-oxidizing bacteria living in environments from streams to hydrothermal vents. But researchers still don’t know exactly how they are formed.
Now, using helium ion microscopy (HIM), a relatively new method with higher resolution than scanning electron microscopy (SEM), James M. Byrne of the University of Tübingen and his colleagues have captured these stalks forming in clearer detail than ever before (Environ. Sci. Technol. Lett. 2018, DOI: 10.1021/acs.estlett.8b00077). Unlike with SEM, imaging organic substances with HIM doesn’t require coating the material with platinum or gold, which can distort the structures being studied.
The group cultured bacteria isolated from low-oxygen sediments in a Denmark bay. By sampling the iron-containing culture repeatedly over a month and analyzing the samples with HIM, they documented how the structures changed. First, a spiral-shaped stalk formed, and over days it was covered with mineral crystals that eventually coated the spiral so heavily that its shape was no longer discernible. Subsequent elemental analysis showed that the spirals were mostly organic while the mineral deposits were lepidocrocite, an iron oxide-hydroxide mineral.
“It really blew my mind when we started to see these objects forming,” Byrne says. “You can see the individual fibers being produced by the bacteria.”
Some researchers hypothesize that bacteria make these structures to protect themselves by removing more toxic, dissolved iron(II) from their surroundings as insoluble iron(III), and the authors note that more work may solve that mystery, too.