The only organisms known to require tungsten for life are some species of hyperthermophilic archaea (Pyrococcus furiosus and Thermococcus litoralis). However, tungsten-containing enzymes have been discovered in several other species of archaea and bacteria. The biological role of tungsten has been reviewed in detail by Kletzin and Adams (1996). Tungsten-containing enzymes in prokaryotes are functionally diverse and include formate dehydrogenase and acetylene hydratase. Some molybdoenzymes can retain function when molybdenum is replaced with tungsten (Stewart et al, 2000), and some organisms have both tungsten-dependent and molybdenum-dependent isoenzymes that are expressed under different environmental conditions (Hochheimer et al, 1998). Despite having a functional role in many prokaryotes, tungsten can inhibit growth in some bacteria (Sugio et al, 2001)

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