In 2001, Kashefi and collaborators published an article in Applied and Environmental Microbiology reporting the surprising finding that several iron-reducing microbes can use gold as an electron acceptor for their respiration. These microbial alchemists included both mesophilic and thermophilic bacteria as well as hyperthermophilic archaea. The beauty of this process is that the oxidized form of gold provided to the microbes, Au(III), is soluble, whereas its reduced form, Au(0), is insoluble. Hence, the microbes respire soluble gold and precipitate it as gold nanoparticles on their outer surface plus, in the case of the Gram-negative bacteria, in the periplasmic space as well. These studies provided the first experimental evidence supporting the role of microbes in the formation of gold deposits in both hydrothermal and cooler environments, thus challenging the prevailing view that gold mineralization was an abiotic process.
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