Abstract
Many different species of acidophilic prokaryotes, widely distributed within the domains Bacteria and Archaea, can catalyze the dissimilatory oxidation of ferrous iron or reduction of ferric iron, or can do both. Microbially mediated cycling of iron in extremely acidic environments (pH < 3) is strongly influenced by the enhanced chemical stability of ferrous iron and far greater solubility of ferric iron under such conditions. Cycling of iron has been demonstrated in vitro using both pure and mixed cultures of acidophiles, and there is considerable evidence that active cycling of iron occurs in acid mine drainage streams, pit lakes, and iron-rich acidic rivers, such as the Rio Tinto. Measurements of specific rates of iron oxidation and reduction by acidophilic microorganisms show that different species vary in their capacities for iron oxido-reduction, and that this is influenced by the electron donor provided and growth conditions used. These measurements, and comparison with corresponding data for oxidation of reduced sulfur compounds, also help explain why ferrous iron is usually used preferentially as an electron donor by acidophiles that can oxidize both iron and sulfur, even though the energy yield from oxidizing iron is much smaller than that available from sulfur oxidation. Iron-oxidizing acidophiles have been used in biomining (a technology that harness their abilities to accelerate the oxidative dissolution of sulfidic minerals and thereby facilitate the extraction of precious and base metals) for several decades. More recently they have also been used to simultaneously remediate iron-contaminated surface and ground waters and produce a useful mineral by-product (schwertmannite). Bioprocessing of oxidized mineral ores using acidophiles that catalyze the reductive dissolution of ferric iron minerals such as goethite has also recently been demonstrated, and new biomining technologies based on this approach are being developed.
Highlights
Many different species of acidophilic prokaryotes, widely distributed within the domains Bacteria and Archaea, can catalyze the dissimilatory oxidation of ferrous iron or reduction of ferric iron, or can do both
Measurements of specific rates of iron oxidation and reduction by acidophilic microorganisms show that different species vary in their capacities for iron oxido-reduction, and that this is influenced by the electron donor provided and growth conditions used
In contrast to iron oxidation at low pH, the ability of acidophiles to grow via the dissimilatory reduction of ferric iron was only discovered in the late 1980s/early 1990s, with this trait being first reported for an autotrophic bacterium (At. ferrooxidans; Pronk et al, 1991) and heterotrophic bacteria (Acidiphilium spp.; Johnson and McGinness, 1991)
Summary
Fe(OH) is −38.6 at 25 ̊C (Monhemius, 1977) so that the theoretical maximum concentrations of non-complexed ferric iron at pH 2 is 140 mg L−1, compared to 140 ng L−1 at pH 4 and 0.14 fg L−1 at pH 7. In contrast to iron oxidation at low pH, the ability of acidophiles to grow via the dissimilatory reduction of ferric iron was only discovered in the late 1980s/early 1990s, with this trait being first reported for an autotrophic bacterium (At. ferrooxidans; Pronk et al, 1991) and heterotrophic bacteria (Acidiphilium spp.; Johnson and McGinness, 1991). [quoted to be Sulfolobus (S.) acidocaldarius, though this is unlikely as this archaeon does not grow autotrophically as described in the paper] coupled sulfur oxidation to ferric iron reduction Both classified genera of iron-oxidizing euryarchaeotes (Ferroplasma and Acidiplasma) can reduce ferric iron, though the electron donor here is organic (e.g., yeast extract) rather than elemental sulfur (Dopson et al, 2004; Golyshina et al, 2009).
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