Thermococcus litoralis is a strictly anaerobic archaeon (archaebacterium) that grows at temperatures up to 98 degrees C by fermenting peptides. Its growth is stimulated by tungsten, and a tungsten-containing iron-sulfur protein that has formaldehyde ferredoxin oxidoreductase (FOR) activity has been purified. FOR is a homotetramer with a subunit M(r) of 70,000. It contains approximately four irons, four acid-labile sulfides, and one tungsten atom per subunit. The tungsten appears to be present as a pterin cofactor, and the Fe/S seems to comprise an unusual [4Fe-4S] cluster that in the reduced state exists in a pH-independent S = 3/2 form and a pH-dependent S = 1/2 form. FOR catalyzed the oxidation of C1-C3 aldehydes with a temperature optimum > or = 90 degrees C and used T. litoralis ferredoxin as an electron acceptor. It did not oxidize aldehyde phosphates, utilize CoASH, or reduce NAD(P). The N-terminal sequence of FOR shows homology with the tungsto-iron-sulfur aldehyde ferredoxin oxidoreductase previously purified from the saccharolytic, hyperthermophilic archaeon Pyrococcus furiosus, in which it is proposed to function in a novel pyroglycolytic pathway (Mukund, S., and Adams, M. W. W. (1991) J. Biol. Chem. 266, 14208-14216). We show here that P. furiosus, which will also grow on peptides, albeit poorly, contains a second aldehyde-oxidizing enzyme analogous to FOR. Similarly, T. litoralis, which utilizes saccharides if limited for peptides, contains low concentrations of an enzyme analogous to AOR. It is proposed that formaldehyde (apparent Km, 62 mM) is not the true substrate for FOR; rather, the enzyme has an as yet unknown role in peptide fermentation in hyperthermophilic archaea.