Golden Nuggets

by Elio

Cop­per, ura­nium, gold, and other met­als are mined by bac­te­ria and ar­chaea, and this takes place on an indus­trial scale. (For two re­views, click here and here.) Con­sortia of these or­gan­isms leach min­er­als from ores – even low-grade ores – and thus have com­mer­cial bles­sing. This is an old story. Rio Tinto, a river in South­ern Spain, has been known since pre-Ro­man times to be the site of bac­te­r­ial min­eral min­ing. Newer is the re­al­iza­tion that bac­te­ria also do the op­po­site of leach­ing, namely they can min­er­al­ize var­i­ous min­er­als in­clud­ing gold.

Scan­ning elec­tron im­age of bac­te­ri­o­form gold on a gold grain from the Hit or Miss Mine in north­ern Queens­land. Cour­tesy of CSIRO Aus­tralia.

In two dis­tant mines in Aus­tralia (one with the appro­pri­ate name of Hit or Miss), soil over­ly­ing the min­er­al­ized zones con­tains an un­ex­pected amount of gold grains and nuggets. Be­low, the gold is se­questered in non-gran­u­lar form in quartz. In or­der to be trans­ported to the sur­face, it must be sol­u­bi­lized and then rem­iner­al­ized. It is thought that bac­te­ria and ar­chaea carry out both steps. A re­cent Aus­tralian study probes the rem­iner­al­iza­tion process in some de­tail. Nuggets col­lected from the sur­face, rang­ing in size from 0.1 to 2.5 mm, were found to be cov­ered with a biofilm con­sist­ing of per­haps as many as 30 dif­fer­ent bac­te­r­ial species. The dom­i­nant or­gan­ism found on all the golden grains but not in the sur­round­ing soil was a close rel­a­tive of Ral­sto­nia met­al­lidu­rans. This beta-pro­teobac­terium is known to be highly re­sis­tant to heavy met­als and to be able to re­duc­tively pre­cip­i­tate sil­ver, cad­mium, and zinc.

John Stone with Gold Min­ing Pan ca. 1939. Credit: Cali­fornia Gold: Folk Mu­sic from the Thir­ties. Col­lected by Cow­ell, Li­brary of Con­gress.

The re­searchers de­ter­mined that in the lab­o­ra­tory R. me­tallidurans can also re­duc­tively pre­cip­i­tate highly toxic gold chlo­ride, al­though 90% of the cells die ini­tially. The pre­cip­i­tated gold is de­posited in dis­tinct patches within some of the cells and dis­trib­uted all over the sur­face of oth­ers. Since gold is toxic to most or­gan­isms, Ral­sto­nia may have lit­tle com­pe­ti­tion while it detox­i­fies its lo­cal en­vironment.

While some may want to use this or­gan­ism as an indi­ca­tor of gold, mi­cro­bi­ol­o­gists might use gold as an in­di­ca­tor of Ral­sto­nia. Is Ral­sto­nia the ful­fill­ment of an alchemist's dream, the em­bod­i­ment of the philosopher's stone? Well, not quite, be­cause Ral­sto­nia doesn't turn base met­als into gold, which is what that elu­sive mag­i­cal stone can do. But it comes close.