Bacteria capable of aerobic respiration on ferrous ions are spread throughout eubacterial and archaebacterial phyla. Comparative spectroscopic analyses revealed that phylogenetically distinct organisms expressed copious quantities of spectrally distinct redox-active biomolecules during autotrophic growth on soluble iron. Thiobacillus ferroxidans, Leptospirillum ferrooxidans, Sulfobacillus thermosulfidooxidans, and Metallosphaera sedula possessed iron respiratory chains dominated by a blue copper protein, a novel red cytochrome, a novel yellow protein, and a novel yellow cytochrome, respectively. Further investigation of each type of respiratory chain will be necessary to deduce the advantages and disadvantages of each.