Studies with a diversity of hyperthermophilic and mesophilic dissimilatory Fe(III)-reducing Bacteria andArchaea demonstrated that some of these organisms are capable of precipitating gold by reducing Au(III) to Au(0) with hydrogen as the electron donor. These studies suggest that models for the formation of gold deposits in both hydrothermal and cooler environments should consider the possibility that dissimilatory metalreducing microorganisms can reductively precipitate gold from solution.A wide diversity of both Bacteria and Archaea have the ability to transfer electrons to Fe(III) (11,12,14). Many of these Fe(III)-reducing microorganisms are also capable of transferring electrons to other metals and metalloids. Microbial reduction of Fe(III) and other metals can influence the fate of metals in aquatic sediments, submerged soils, and the subsurface (10,11,13). One of the more geologically significant impacts of microbial metal reduction is the formation of minerals that can be important geological signatures of the activity of metal-reducing microorganisms and, in some instances, that may represent economically important ore deposits. For example, the formation of magnetite during Fe(III) oxide reduction (21) has been considered to be an indication of the activity of Fe(III)-reducing microorganisms in aquatic sediments (4), in the deep, hot biosphere (5), and on Mars (22). The massive magnetite accumulations formed in the Precambrian period, presumably as the result of microbial activity (1,8,27), represent an important source of iron ore. It has also been suggested that microbial reduction of U(VI) to U(IV), which precipitates uranium from solution (19), might account for the formation of some uranium ores (6, 17).Like uranium, gold is soluble in the oxidized form, Au(III), but the reduced form of gold, Au(0), is insoluble (23). The finding that addition of Au(III) to cell suspensions of Geobacter metallireducens oxidized c-type cytochromes, which are thought to be involved in electron transport to metals (15), suggested that this dissimilatory metal reducer might be able to transfer electrons to Au(III) (9). Furthermore, the c 3 -cytochrome of Desulfovibrio vulgaris, known to be involved in the reduction of U(VI) (20) and Cr(VI) (18), can also transfer electrons to Au(III) (9). However, the possibility that whole cells of either G. metallireducens, D. vulgaris, or other dissimilatory Fe(III)-reducing microorganisms reduced Au(III) was not further evaluated.In order to determine the potential for dissimilatory Fe(III)-reducing microorganisms to precipitate gold from solution, the following organisms were cultured (800 ml) in 1-liter bottles, using strict anaerobic techniques as previously described (16 (ATCC 35115), "Desulfitobacterium metallireducens" (laboratory culture collection), and the acetate-oxidizing hyperthermophile strain 234 (laboratory culture collection). Cell suspensions were prepared as previously described (26). Briefly, cells were harvested anaerobically by centrifugation, resuspended in 8...

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Aromatic compounds are an important component of the organic matter in some of the anaerobic environments that hyperthermophilic microorganisms inhabit, but the potential for hyperthermophilic microorganisms to metabolize aromatic compounds has not been described previously. In this study, aromatic metabolism was investigated in the hyperthermophile Ferroglobus placidus. F. placidus grew at 85 degrees C in anaerobic medium with a variety of aromatic compounds as the sole electron donor and poorly crystalline Fe(III) oxide as the electron acceptor. Growth coincided with Fe(III) reduction. Aromatic compounds supporting growth included benzoate, phenol, 4-hydroxybenzoate, benzaldehyde, p-hydroxybenzaldehyde and t-cinnamic acid (3-phenyl-2-propenoic acid). These aromatic compounds did not support growth when nitrate was provided as the electron acceptor, even though nitrate supports the growth of this organism with Fe(II) or H2 as the electron donor. The stoichiometry of benzoate and phenol uptake and Fe(III) reduction indicated that F. placidus completely oxidized these aromatic compounds to carbon dioxide, with Fe(III) serving as the sole electron acceptor. This is the first example of an Archaea that can anaerobically oxidize an aromatic compound. These results also demonstrate for the first time that hyperthermophilic microorganisms can anaerobically oxidize aromatic compounds and suggest that hyperthermophiles may metabolize aromatic compounds in hot environments such as the deep hot subsurface and in marine and terrestrial hydrothermal zones in which Fe(III) is available as an electron acceptor.

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No hyperthermophilic microorganisms have previously been shown to anaerobically oxidize acetate, the key extracellular intermediate in the anaerobic oxidation of organic matter. Here we report that two hyperthermophiles, Ferroglobus placidus and "Geoglobus ahangari," grow at 85°C by oxidizing acetate to carbon dioxide, with Fe(III) serving as the electron acceptor. These results demonstrate that acetate could potentially be metabolized within the hot microbial ecosystems in which hyperthermophiles predominate, rather than diffusing to cooler environments prior to degradation as has been previously proposed.