Microbial alchemists under the microscope

A particular type of bacteria can precipitate gold from an aqueous solution to form those elusive nuggets, Australian researchers have found. The discovery is the latest episode in a geological debate about the possible bacterial role in the formation of gold deposits. 

Frank Reith and his colleagues at the Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRC LEME), Bentley, took samples of gold grains from the soil surrounding two Australian gold mines. Microscopic imaging techniques identified live bacterial biofilms on these grains.

DNA testing of these biofilms identified the bacteria as Ralstonia metallidurans, an organism that has previously been implicated in the formation of gold precipitates from aqueous solution. But this is the first study to image live biofilms on gold grains and to identify the organisms likely to be involved. 

Bacteria can precipitate a whole range of metals in order to eliminate heavy metal toxicity from their environment. Ralstoinia metallidurans is particularly resistant to heavy metal pollution.

  Gold has significant implications for the Australian economy. Reith says that these findings could aid mineral explorers in detecting and interpreting transported secondary gold nuggets, and aid the detection of underlying gold mineralization. He adds that understanding these mechanisms may also lead to the development of environmentally friendly methods of gold recovery, and aid research into the biosynthesis of gold nanoparticles.

The first evidence of ’gold-fixing’ by bacteria was found by US Geological Survey scientist John Watterson in 1992 but since then Watterson himself has cast doubt over his research technique, stimulating an ongoing debate about the bugs’ role.

Reith, however, is still convinced: ’I have frequently talked to gold prospectors here in Australia, who claim to have found substantial nuggets in areas that they had intensively searched for gold years before. Having thought that they had found everything there, they returned a couple of years later and new nuggets had turned up. Maybe microbes made them.’