Shift work on the gold nugget
In some life forms we hate bacteria - slimy biofilms don't exactly evoke positive associations. Microbes are also socialized in the soil in this way. Here, however, a species wraps itself extraordinarily noble.
In Coolgardie, Australia, when the gold rush began, all you had to do was bend down. Because, so the story goes, the coveted precious metal lay right at your feet and glittered alluringly in the sun. The nuggets also lured in the soil and especially in the streams of the old gold mining town. This so-called secondary gold was not always there, but only formed during the weathering of primary gold deposits.
Researchers from the Australian National University in Acton have unearthed gold nuggets from the bottoms of two gold mines – the Tomakin and the Miss Goldmine. Taking a closer look under the electron microscope, they discovered structures that had caught the eye of other researchers before them: the surface of the grains, which were up to 2.5 millimeters in size, was reminiscent of a sponge, consisting of tiny clusters of small, gold-coated spheres. The biologists were able to confirm that these were preserved bacterial relics: They succeeded in detecting traces of genetic material in the spheres – DNA that, as it turned out, came from over thirty different bacterial species. These lived long ago in a biofilm that covered the metal. One microbe in particular, as the genetic analyzes showed, was native to all the gold nuggets examined: Ralstonia metallidurans. But what exactly is the bacterium doing on all the gold nuggets?
Ralstonia metallidurans is known to survive in environments rich in heavy metals. Reith was able to show for the first time that the bacterium reduces dissolved gold chloride in the laboratory to such an extent that insoluble, elemental gold (Au) is formed. In doing so, the microbe somehow injects electrons into the compound. Reith also believes he knows why the microorganism behaves this way: it detoxifies its metabolism because the soluble gold compound is toxic to Ralstonia, but the elemental gold is not. In fact, the researchers observed that the living bacteria carried tiny nuggets of gold around with them. Some deposited the noble metal just below the cell surface, others in turn gilded their entire cell membrane - the latter probably deposited the metal in a complex with sulfur and phosphorus.
Reith assumes that the bacterium also behaves in the same way in nature. He first sees a number of helpers, other soil bacteria and fungi, which mobilize dissolved gold compounds with the water in the soil. Thus, oxidized metal (Au+III or Au+I) would first reach distant areas and be mineralized there by Ralstonia and other microorganisms over the years. Layer by layer, the bacteria deposit the almost invisible grains of gold. Then, when the microbes die sooner or later, their golden cell membranes remain immortalized. The biofilm of a new generation of bacteria then stretches over these – the growth is similar to that of a bacterial coral reef, consisting of gold-coated cell membranes.
Gold nuggets can also be formed by biomineralization - that's what the researchers call the microbial process. And maybe Ralstonia metallidurans really was the cash cow in Coolgardie. Certainly not every gold nugget comes from the bacterial forge. However, as Reith emphasizes, some organisms would definitely produce gold. The research team now wants to investigate what exactly happens when Ralstonia settles in the rich soil. In principle, it would be enough if you pour soluble gold compounds into the soil, add the bacteria and then wait patiently. Who knows, maybe there really will be gold nuggets. In nature, however, the process may have taken a few million years.