The fundamental issue addressed by this research is how and why do organisms choose to use Mo or W at the catalytic sites of key enzymes. The model organism is the hyperthermophilic archaeon Pyrobaculum aerophilum, and the model system is respiratory nitrate reduction. Nitrate reductase (NR) catalyzes a high potential reaction (Eo' = + 420 mV), and in organisms such as Escherichia coli the physiological electron donor is formate via formate dehydrogenase (FDH) (Eo' = - 430 mV) in the formate-nitrate respiratory chain. Both of these enzymes are molybdoenzymes in E. coli, yet W is the preferred element in enzymes in hyperthermophiles. However, FDH has also been purified as a naturally-occurring tungstoenzyme from various thermophilic bacteria, but NR had not been previously purified from a thermophilic organism, let alone one growing at 100 °C. Growth of P. aerophilum at 98 °C in the presence of nitrate requires tungstate and is inhibited by the presence of molybdate in higher or equimolar concentrations. However, both oxyanions support growth at low (78 °C) temperature. Elemental analyses showed that both oxyanions were taken up by the organism under both of these growth conditions. The results are the first to show a temperature-dependent difference in W and Mo requirements. Purification of NR from P. aerophilum revealed two forms of the enzyme, NR (NR1 and NR2) and two forms of a NR-FDH complex (NR1-FDH1 and NR2-FDH2). The purified NR enzymes and the NR/FDH enzyme complexes of P. aerophilum all exhibited maximal activity at 90 ºC. NR1 is a Mo-containing enzyme with a specific activity of 534 U/mg and resembles other respiratory NRs found in mesophilic microorganisms not only in metal content, but also in subunit composition. NR2 is the first example of a W-containing NR and has a specific activity of 223 U/mg. The NR1/FDH1 complex contained 0.6 g-atoms Mo and 0.3 g-atoms W per mol of complex with specific activities of 712 and 33 U/mg for NR and FDH, respectively. The NR2/FDH2 complex contained 0.6 mol W and 0.2 g-atoms Mo per mol of complex, with specific activities of 373 and 3 U/mg for NR and FDH, respectively.