Iron is the fourth most abundant element in the Earth's crust, and is the most abundant element in the Earth as a whole (Ehrlich, 1996). Microbially mediated transformations of this element are of major importance to the evolution of the Earth. The most plausible source of energy to support early organisms is geochemical energy obtained from water-rock reactions (Jakosky & Shock, 1998). On modern Earth, aerobic iron based lithotrophy may occur at low pH or, under microaerophilic conditions, at neutral pH. Anaerobic oxidation of Fe(II) by non-sulphur bacteria implies that oxygen-independent biological iron oxidation was possible before the evolution of oxygenic photosynthesis (Widdel et al., 1993).
We are investigating microbial and geological details of two inactive mines. The first one is a flooded zinc mine of neutral pH near Madison (Wisconsin), where large aggregates of Gallionella spp. and yet unidentified bacteria could be found. Sometimes, the water body is divided into two zones. Rock contacting with water of the bottom zone is of red colour, indicating an iron-oxidizing process.
The second point of interest is an acid mine drainage site in northern California. The site contains a pyrite rich ore body. This is dissolving to produce sulfuric acid solutions (typically 35-45 °C, pH 0.5 - 1.0). Oxidative dissolution of pyrite is mediated by lithotrophic organisms through catalysis of ferrous iron oxidation. Microbial populations have been characterized by amplification and sequencing of DNA extracted from natural samples and by probing of the ribosomal RNA of individual cells using fluorescent domain-, group-, genus- and species-specific oligonucleotides. Populations, typically dominated by iron oxidizing organisms, had limited diversity in specific samples, indicating a high stress potential (Atlas et al., 1991). But, communities varied from site to site within the mine. Dominant organisms detected at the mine were mostly novel and these included Archaea of the genus Ferroplasma, and Bacteria of the genera Sulfobacillus and Leptospirillum. A novel type of Leptospirillum dominated some particular subaerial slimes within the mine. Sulfobacillus spp. were confined to the hotter environments investigated. Ferroplasma spp. were dominant members of communities in environments of high conductivity and low pH. Ferroplasma isolate fer1 grew heterotrophically, but oxidized Fe(II) with an optimal temperature of 45ºC and an optimal pH of 1.2. It was capable of growth at pH 0 to 3. Fer1 can contribute to acid mine drainage generation through catalysis of the reaction:
FeS2 + 14 Fe3+ + 8 H2O --> 15 Fe2+ + 2 SO42- + 16 H+.
In the laboratory pyrite dissolution kinetic experiments at pH 1.5 and 37 °C using mixed cultures and iron- and sulfur-oxidizing isolates were conducted. Dissolution rates normalized to cell numbers and surface area were determined and the contribution of microbially induced dissolution, at the mine, could be estimated.
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