Under the surface of the Earth, microorganisms are responsible for most of the metal cycling and mineral formation. They use the metal ions for nutrition and metabolic energy. Unlike most metals, Au is not needed by the microorganisms because it does not have any free ions when in the surface conditions. This means that the bacteria only has to disperse and reconcentrate the Au. Since the bacteria is not known to take in anything from the Au, it is argued that the bacteria are passive in the process and use abiogenic pathways. These researchers worked to find out if this process is actually active and energy dependent. To conduct the experiment, the authors incubated operable, inactive, and dead C. metallidurans cultures with Au and hydroxychloride complexes with pH levels between 5.0 and 8.0. This helped them examine potential biochemical pathways for speciation. They looked at the individual speciation in the Au cells, as well as the bacteria’s response to the Au complexes. The metabolic state of the cells was also under observation. The active bacteria in higher pH conditions collected less Au than active bacteria in lower pH conditions. The cells that were inactive or dead had no considerable changes. The bacteria that was metabolically active gathered the most Au, especially in conditions with a pH of 7.0 or higher. The researchers gathered that the bacteria C. metallidurans plays an active role in the biogeochemical cycling of gold and other metals. This disproves earlier assumptions that secondary gold formation is an abiotic process. Their discoveries of secondary enrichment zones in surface environments during the formation of precious metals with this bacterium indicate that PGE can also undergo biogeochemical cycling. Now that it is understood that bacteria can have an active role in biomineralization, new technology can be developed. New biosensors that are Au specific could make it easier to pull the precious metals out of the bacteria, and improve the hydrometallurgical process overall.

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