A team from Lawrence Livermore National Laboratory (LLNL), UC Berkeley, and Lawrence Berkeley National Laboratory say the bacteria are binding and immobilizing the nanoparticles themselves, which are potentially toxic to the bacteria.

Sulfate-reducing bacteria can cause heavy metals such as zinc to precipitate and form nanoparticles. However, these particles move freely because they are so small (typically 2-6 nanometers in diameter) and redissolve if conditions change.

Researchers showed that the mine bacteria cause nanoparticles to aggregate, limiting their movement and thereby protecting the bacteria. When the metal nanoparticles aggregate, they don't move as easily and are less soluble.

Using ion-mass and infra-red spectrometry, as well as transmission electron microscopy, and infra-red spectroscopy, the scientists determined that protein contributes to the formation of densely aggregated nanoparticulate zinc-sulfide spheroids, and that various amino acids induce rapid aggregation in metal-sulfide nanoparticles.

"Potentially we can use proteins for nanoparticle clean up," says researcher Peter Weber of LLNL. "With the boom in nanoscience, people are asking questions about potential environmental impact. Here, we see that naturally produced nanoparticles can be naturally controlled."

Zinc-sulfide nanoparticles in the presence of cysteine clump together.