Metabolism in hyperthermophilic microorganisms

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Abstract
Hyperthermophilic microorganisms grow at temperatures of 90 °C and above and are a recent discovery in the microbial world. They are considered to be the most ancient of all extant life forms, and have been isolated mainly from near shallow and deep sea hydrothermal vents. All but two of the nearly twenty known genera are classified asArchaea (formerly archaebacteria). Virtually all of them are strict anaerobes. The majority are obligate heterotrophs that utilize proteinaceous materials as carbon and energy sources, although a few species are also saccharolytic. Most also depend on the reduction of elemental sulfur to hydrogen sulfide (H2S) for significant growth. Peptide fermentation involves transaminases and glutamate dehydrogenase, together with several unusual ferredoxin-linked oxidoreductases not found in mesophilic organisms. Similarly, a novel pathway based on a partially non-phosphorylated Entner-Doudoroff scheme has been postulated to convert carbohydrates to acetate, H2 and CO2, although a more conventional Embden-Meyerhof pathway has also been identified in one saccharolytic species. The few hyperthermophiles known that can assimilate CO2 do so via a reductive citric acid cycle. Two So-reducing enzymes termed sulfhydrogenase and sulfide dehydrogenase have been purified from the cytoplasm of a hyperthermophile that is able to grow either with or without So. A scheme for electron flow during the oxidation of carbohydrates and peptides and the reduction of So has been proposed. However, the mechanisms by which So reduction is coupled to energy conservation in this organism and in obligate So-reducing hyperthermophiles is not known.

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