Transform a toxic metal into gold, no problem for Cupriavidus metallidurans. Among their many roles in life on Earth, it turns out that some microbes are also experts in the purification of precious metals. An international team of researchers is now highlighting how this bacterium, discovered in 2009, manages to produce gold by ingesting toxic metal compounds.
Like many other elements, gold can follow a biogeochemical cycle: it dissolves, moves and concentrates back into the sediments of the earth. Microbes, of paramount importance in the mechanics of life, are also involved in every step of this process. But then, how do these bacteria do not poison themselves with these highly toxic compounds? How do they “detoxify” the elements?
C. metallidurans thrive in soils that contain both hydrogen and a range of toxic heavy metals. On the one hand, it means that competition with other organisms easily poisoned in such an environment is rare. “If an organization chooses to survive here, it must find a way to protect itself from these toxic substances,” says microbiologist Dietrich H. Nies, of Martin Luther University Halle-Wittenberg (Germany) and lead author of this study .
It turns out that the bacterium has a rather ingenious protection mechanism, which involves not only gold, but also copper. Compounds containing these two elements can easily enter C. metallidurans cells. Once inside, they interact in such a way that the copper and gold ions are transported deep inside the bacteria, where they could potentially wreak havoc. But to deal with this problem, the bacteria use enzymes to move the offending metals out of their cells.
For copper, this enzyme is called CupA. But the presence of gold causes a new problem. “When gold compounds are also present, the enzyme is removed and the toxic compounds of copper and gold remain inside the cell,” Nies explains. At this point, other bacteria could simply give up and go live in a less toxic environment, but not C. metallidurans. The bacterium actually has another enzyme to play in its favor, which scientists have labeled CopA.
With this molecule, the bacteria can convert copper and gold compounds into compounds that will then be less easily absorbed by the cell. “The bacteria is less poisoned, and the enzyme that pumps the copper can then remove the excess copper unimpeded,” notes the researcher. Not only does this process allow the microbe to lose all this unwanted copper, but it also results in tiny gold nanoparticles in the form of nuggets on the bacterial surface.