Magnetotactic bacteria remove and recover rare elements

The microbial world (much larger than even originally imagined, as demonstrated in the new Tree of Life) contains an extremely wide array of biochemical reactions that various organisms use to acquire energy, release waste products, and detoxify the surrounding environment. These broad abilities allow microbes to grow in some of the harshest conditions known on Earth – from sulphuric springs to frozen tundra, there are few places that are truly sterile in this world.

While our human biochemical reactions are limited, our ingenuity is not, and scientists are able to exploit microbes for our benefit, such as in chemical spills. Using microbes to degrade or sequester toxic molecules is one form of bioremediation, and has many various applications. Famously, scientists used bacteria that degrade alkanes and polycyclic aromatic hydrocarbons during cleanup of the 2010 BP Deepwater Horizon oil spill. But what happens when the job is done and the toxic molecules are removed? Leaving the bacteria in place may be harmless, but ideally the cells would be removed along with any toxic chemicals they still contain.

A new study published in Applied and Environmental Microbiology used a clever approach to recover bacterial cells along with the removed molecules: a magnetic field. The research team, led by first author Masayoshi Tanaka and senior author Tadashi Matsunaga, used magnetotactic bacteria, which generate magnetic particles inside their cells. They grew these bacteria in a medium containing selenium oxyanion (SeO₃^2-), with the goal of accumulating the Se atoms around the bacterial cells without disrupting magnetic particle synthesis.

Selenium is a rare element that is a useful semiconductor; as our use of technologies that use semiconductors has increased, so has selenium’s value – and its water contamination levels, which can have toxic effects. Collecting selenium via bacteria, which can then themselves be collected via magnetic fields, would therefore serve two purposes: to clean polluted waters and to recycle valuable elements whose mining damages the earth. Before putting the bacteria to work, however, the authors first had to show that Magnetospirillum magneticum could grow in the presence of SeO₃^2- (or at least wasn’t inhibited/killed by it) and that the bacterial magnetite crystal synthesis wasn’t disrupted by SeO₃^2- presence.

As bacterial cell counts increase, contamination decreases

The research team measured selenium concentration in the growth medium, and found that as the number of bacterial cells increased, the concentration of SeO₃^2- in the medium decreased (see figure, left). Where did the SeO₃^2- go? The researchers needed to determine whether the selenium is adsorbed to the bacterial surface or taken into the bacterial cytoplasm. Not only is this an important biological question, but if the scientists hope to recover the selenium sequestered by the bacteria, knowing its location will determine how to retrieve it. Microscopy analysis showed that bacterial cells grown in SeO₃^2- contained spherical granules, and elemental mapping indicated these were reduced selenium (see figure, right). They also noted no selenium contamination of the magnetite crystals that confer magnetic properties to M. magneticum.

Elemental analysis shows no Se in magnetite granules

Finally, the scientists wanted to test whether the bacteria containing selenium granules could be recovered by an external magnetic field. They collected the cells grown in the presence of SeO₃^2- and measured the amount of selenium: 3.6x10⁸ atoms per cell recovered. This proof-of-principle experiment showed that this method of biorecovery for selenium may someday be applied to water purification protocols.

This same group has studied magnetotactic bacteria as a means to remove other toxic substances from water, including toxic heavy metals like cadmium, but this study showed that M. magneticum incorporates selenium much more efficiently. However, magnetosome formation can be conferred to other bacterial species ($) with the transfer of key genetic regions – suggesting that other species that have higher tolerance for certain elements, or ability to sequester different elements could be genetically altered to be magnetically recoverable. This novel removal-and-recovery strategy may therefore someday have extended application to other valuable rare elements.

-- Julie Wolf

Photo credits: Figures from AppEnvMicro study

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