Scientists say that a bacteria that protects itself by turning its environment to gold could help in gold extraction.
Delftia acidovorans lives in sticky biofilms that form on top of gold deposits, but exposure to dissolved gold ions can kill it. That's because although metallic gold is unreactive, the ions are toxic.
To protect itself, the bacterium has evolved a chemical that detoxifies gold ions by turning them into harmless gold nanoparticles. These accumulate safely outside the bacterial cells.
Nathan Magarvey of McMaster University in Hamilton, Ontario, who led the team that uncovered the bugs' protective trick said that the discovery could be used to dissolve gold out of water carrying it, or to design sensors that would identify gold-rich streams and rivers.
When the bugs sense gold ions, they secrete a protein dubbed delftibactin A into the surroundings and it chemically changes the ions into particles of gold 25 to 50 nanometres across. The particles accumulate wherever the bugs grow, creating patches of gold.
Because the nanoparticle patches do not reflect light in the same way as bigger chunks of the metal - they are deep purple in colour.
When Magarvey deliberately snipped out the gene that makes delftibactin A, the bacteria died or struggled to survive exposure to gold chloride. Adding the protein to the petri dish rescued them.
The bacterium Magarvey investigated is one of two species that thrive on gold, identified by Frank Reith of the University of Adelaid in 2009.
Cupriavidus metallidurans survives using the slightly riskier strategy of changing gold ions into gold inside its cells.
"If delftibactin is selective for gold, it might be useful for gold recovery or as a biosensor. But how much dissolved gold is out there is difficult to say," said Reith.