Potential importance of dissimilatory Fe(III)-reducing microorganisms in hot sedimentary environments

Abstract

Fe(III) is available as an electron acceptor in many modern hot (80-110°C) sedimentary environments which hyperthermophilic microorganisms inhabit and Fe(III) may have been an important electron acceptor as microbial life evolved on hot, ancient Earth. Recent studies have demonstrated that the ability to reduce Fe(III) is a highly conserved characteristic of hyperthermophiles and that the metabolic capabilities of some hyperthermophiles are greatly expanded when Fe(III) is available as an electron acceptor. An increasing diversity of novel hyperthermophiles, including some that are known to be important in the environment from molecular studies, are being recovered from hot environments when Fe(III) oxide is used as the electron acceptor for enrichment and isolation. These include the first hyperthermophiles documented to anaerobically oxidize acetate, a key intermediate in anaerobic carbon and electron flow, as well aromatic compounds and long-chain fatty acids. This suggests that complex organic matter can be oxidized to carbon dioxide with Fe(III) serving as the sole electron acceptor in hot microbial ecosystems. In addition to reducing Fe(III), some hyperthermophiles can reduce a variety of other metals, including U(VI) and Au(III), providing a potential explanation for the deposition of metals in some hot environments. Although the study of Fe(III) reduction in hyperthermophiles is still in its infancy, it is clear that Fe(III) reduction is central to the metabolism of many hyperthermophiles and that when Fe(III) is available in hot, microbial ecosystems, it has the potential to be an important electron acceptor for the anaerobic oxidation of hydrogen and organic matter.