Carbon, iron and sulfur metabolism in acidophilic micro-organisms

Acidophilic micro-organisms are those (mostly prokaryotes) that grow optimally at pH <3 (extreme acidophiles) or at pH 3-5 (moderate acidophiles). Although once considered to comprise relatively few species of bacteria and archaea, the biodiversity of extreme acidophiles is now recognized as being extensive, both in terms of their physiologies and phylogenetic affiliations. Chemolithotrophy (the ability to use inorganic chemicals as electron donors) is widespread among extreme acidophiles, as ferrous iron and sulfur represent two major available energy sources in many natural and man-made extremely acidic environments. Dissimilatory reduction of iron and sulfur (as a consequence of their use as electron acceptors in oxygen-limited and anoxic environments) are also a major biogeochemical processes in low-pH environments. Acidophiles display considerable diversity in how they assimilate carbon; some are obligate autotrophs, others obligate heterotrophs, while a large number use either organic or inorganic carbon, depending on the availability of the former. This review describes the intricate relationships between carbon, iron and sulfur transformations by acidophilic micro-organisms, and how these are significant in both industrial and environmental contexts.