THE METABOLISM OF HYDROGEN BY EXTREMELY THERMOPHILIC, SULFUR-DEPENDENT BACTERIA

In just the last few years, a group of bacteria have been discovered that have the remarkable property of growing near and above 100°C. These extremely thermophilic organisms, defined here as having the ability to grow at 90°C with optimum growth at 80°C and above, have been isolated mainly from sulfur-rich, marine geothermal environments, both shallow and deep sea. They comprise over a dozen different genera, and except for one novel eubacterium, all may be classified as archaebacteria. The majority of the extremely thermophilic genera metabolize elemental sulfur (S°) and a survey of the various organisms reveals that most of them also depend upon the oxidation of hydrogen gas (H2) as an energy source. In addition, two extremely thermophilic genera are known that actively produce H2 as end-products of novel fermentative metabolisms. The enzyme hydrogenase, which is responsible for catalysing H2 activation and H2 production, appears to play several roles in electron and energy transfer during the growth of these organisms. Hydrogenase has so far been purified from only one extremely thermophilic species, from Pyrococcus furiosus (Topt = 100°C), and hydrogenase activity has been exmained in cell-free extracts of only a few others. However, a comparison of their properties with those of hydrogenases from mesophilic bacteria suggests that (a) the hydrogenase responsible for catalysing H2 oxidation in extremely thermophilic organisms may be an extremely thermostable version of the mesophilic enzyme, and (b) a new type of 'evolution' hydrogenase, lacking the NiS or FeS catalytic sites of the mesophilic enzymes, is required for catalysing H2 evolution at temperatures near and above 100°C. © 1990.