Dissimilatory Fe(III) reduction (i.e., iron respiration) among hyperthermophilic microorganisms may be an ancient and widespread form of metabolism on earth and is a good candidate metabolism for putative life elsewhere. Iron respiration coupled with H2 oxidation at 100°C is highly favorable thermodynamically and can occur independent of photosynthesis. Hyperthermophilic iron reducers have been isolated from terrestrial hot springs, deep-sea hydrothermal vents, and deep (> 1,000 m) subsurface samples such as petroleum reservoirs, geothermal pools and mining operations. The hyperthermophilic archaeon Pyrobaculum aerophilum can reduce both soluble (Fe(III) citrate) and insoluble (poorly crystalline Fe(III) oxide) forms of iron. Direct contact is not necessary for reduction of insoluble iron suggesting the organism uses either an extracellular electron shuttle or a chelator for iron reduction. Growth on iron by a newly isolated Pyrobaculum sp. was measured at pH 5 where growth on O2, NO3- and S° no longer occurred, which broadened the pH range for growth of the organism. Environmental biomarkers of iron reducers may include extracellular iron chelators and electron shuttles, lipids, and 16S rRNAs. Markers of iron respiration may include magnetite formation and stable iron isotope fractionation. The identification of biomarkers for iron respiration at high temperatures is in its infancy but could provide insight into the microbial ecology of the subsurface biosphere, past and present, and provide targets for missions to other planets and moons.