TUNGSTEN AND LIFE NEAR 100°C IN HYDROTHERMAL SYSTEMS

Hyperthermophiles are microorganisms that grow at temperatures of 90°C and above. They have been isolated from geothermally-heated environments, including deep sea hydrothermal vents. The majority of the more than twenty genera of hyperthermophile that are known are classified as archaea (formerly archaebacteria) rather than as bacteria, and they appear to be the most slowly evolving of all life forms. Most hyperthermophilic species cannot grow in the presence of oxygen and are strict anaerobes. Some obtain energy for growth either by producing methane, by oxidizing iron, or by reducing sulfate, and they obtain cellular carbon by reducing carbon dioxide. The majority, however, utilize proteins as sources of both energy and as cellular carbon, and some also use carbohydrates. These protein-dependent organisms also reduce elemental sulfur (S°) to hydrogen sulfide and most show little if any growth if S° is not present.

The best studied of the S°-reducing hyperthermophiles is Pyrococcus furiosus (Pf), which grows optimally near 100°C. Curiously, the growth of Pf is dependent upon tungsten (W), an element that is rarely used in biological systems. In fact, virtually all life forms require the analogous element molybdenum (Mo) for growth, but this metal does not appear to be utilized by Pf. Four W-containing enzymes have been purified from Pf and they all catalyze redox reactions of low reduction potential They are thought to play key roles in the primary metabolic pathways by which Pf metabolizes peptides and carbohydrates. Crystal structures of two of the Pf tungstoenzymes show that W is coordinated to these proteins in a manner similar to that in which Mo-containing enzymes coordinate Mo. However, the two enzyme types show no other structural similarity and represent completely different enzyme families. A comparison of the chemical properties of W and Mo show that W is much more suited to catalyze low potential reactions under anaerobic conditions at extreme temperatures. In contrast to Mo, W is present in normal seawater at an exceedingly low concentration. However, marine hydrothermal systems (including vent fluids and smoker chimneys) contain much higher W concentrations, more than sufficient to support hyperthermophilic life.