Carbon, Iron and Sulfur Metabolism in Acidophilic Micro-Organisms
Section snippets
Definition, Habitats and General Physiological Characteristics
While there is no formal definition of what constitutes an acidophilic micro-organism, there is a general consensus that optimal growth of an acidophile occurs at a pH that is significantly less than 7. Sub-division between “moderate” and “extreme” acidophiles, with the former having pH growth optima of 3–5 and the latter of <3, has been proposed (Johnson, 2007). Besides these obligate acidophiles, low-pH environments may contain metabolically active “acid tolerant” micro-organisms that have pH
Carbon Metabolism in Obligately Autotrophic Acidophiles
As described above, a general classification of acidophilic prokaryotes may be based on their carbon metabolism. Obligately autotrophic acidophiles are those that obtain cellular carbon from an inorganic carbon source, carbon dioxide, in contrast to obligate heterotrophs that obtain carbon from organic sources and facultative autotrophs that can use both inorganic and organic carbon. In most acidic environments in the absence of light, autotrophic iron- and sulfur-oxidizing micro-organisms are
Dissimilatory Oxidation of Iron
Due to the widespread occurrence of pyrite and other iron sulfides, iron is often present at elevated concentrations in the low pH of typical acidic environments, where most (cationic) metals are highly soluble. In addition to being highly soluble, ferrous iron is only very slowly oxidized by molecular oxygen at pH<4 (Stumm and Morgan, 1981), so iron-oxidizing acidophiles do not have to compete with chemical oxidation of their growth substrate. Because the free energy associated with the
Dissimilatory Oxidation of Elemental Sulfur and Reduced Inorganic Sulfur Compounds
Naturally acidic environments are formed in areas of high sulfur concentration, and are usually associated with areas of geothermal activity. Those sites that have low neutralization potential (e.g. lack carbonate-containing minerals) become acidic due to the oxidation of sulfide by acidophilic micro-organisms, producing sulfate and protons (e.g. sulfuric acid). In addition, geothermal sites may also contain sulfur, which is formed by the condensation of hydrogen sulfide with sulfur dioxide.
APPLIED AND ECOLOGICAL ASPECTS
Acidophilic micro-organisms are the key drivers of a rapidly expanding area of biotechnology, generically referred to as “biomining.” In addition, their role in generating acidic, metal-rich effluents that drain abandoned mine spoils and mineral tailings have been documented for over 50 years. The extremophiles exploit natural as well as man-made acidic environments, such as solfatara fields in geothermal areas, and sites where there are surface exposures of sulfide-rich rocks (gossans). In
ACKNOWLEDGMENTS
We acknowledge the financial support provided by the European Commission under the Sixth Framework Programme for Research and Development in the frame of the research projects BioMinE (European project contract NMP1-CT-500329-1) and Bioshale (European project contract NMP2-CT-2004 505710). DBJ is grateful to the Royal Society (U.K.) for the provision of an Industrial Fellowship.
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