Australia is rightly famous for its gold nuggets and some, such as the ‘Golden Eagle’ found at Coolgardie, were as big as a gap-year’s rucksack. The curious thing about them is that they are generally found in the most featureless parts of the continent, Western Australia being a case in point. What sharpens the paradox is that these flat areas have been peneplains for up to a billion years. A nugget found in a Yukon or Californian stream is easily attributed to high-energy transport in water, and indeed most of those show signs of long transport in water: they are rounded and pitted. The one kilogram and weightier nuggets from Australia could never have been physically moved across the featureless plains, and most of them come from the alluvium deposited by sluggish Cenozoic drainages, now as dry as a bone — the ‘deep leads’ famous for their gold rushes in the past. They are also oddly shaped, the ‘Golden Eagle’ having wing-like flanges, which any physical transport would bend into conformity, for gold is of course very malleable. One long-held hypothesis is that they formed by precipitation from the extremely noxious groundwater that still persists tens of metres beneath the surface, gold being water-transportable in the form of complex ions such as those involving Au and Cl. But it now seems that the mediator is bacterial in origin (Reith, F. et al. 2006. Biomineralization of gold: biofilms on bacterioform gold. Science, v. 313, p. 233-236).
Frank Reith and his Australian colleagues collected soils that contain small gold grains from goldfields across the continent. A great many have strangely knobbly surfaces and branching structure when scanned under an electron microscope, whereas fine gold grains from primary deposits in hard rock often shows signs of gold’s crystal symmetry, or at least highly angular surfaces. The soil-gold particles do look as though they formed in association with living processes. Using stains that fluoresce when bonded to organic matter the researchers found numerous associations between gold and organisms of some kind. When organic material was leached from separated gold grains it revealed DNA closely similar to a bacterium that is known experimentally to precipitate gold from dissolved Au-Cl complexes. Ordinary soil grains showed no such genetic tracers. It looks as if Reith et al. have discovered living biofilms coating the gold grains that the constituent bacteria are in the process of growing. Amazingly, they also found gold-plated living bacterial cells. The probable explanation is that the bacteria live in water so rich in gold (by no means a great deal of it, however) that they are defending themselves from gold’s known toxicity — Ralstonia metallidurans, as its Latin name suggests, is a highly metal-tolerant organism. Nuggets may well form as a result of bacterial defence mechanisms.