The biochemical diversity of life near and above 100 degrees C in marine environments

Hyperthermophilic micro-organisms grow at temperatures above 90 degrees C w ith a current upper limit of 113 degrees C. They are a recent discovery in the microbial world and have been isolated mainly from marine geothermal en vironments, which include both shallow and deep sea hydrothermal vents. By 16S rRNA analyses the are the most slowly evolving of all extant life forms , and all but two of the nearly 20 known genera are classified as Archaea ( formerly Archaebacteria). Almost all hyperthermophiles are strict anaerobes . They include species of methanogens, iron-oxidizers and sulphate reducers , but the majority are obligate heterotrophs that depend upon the reduction of elemental sulphur (S degrees) to hydrogen sulphide for significant grow th. The heterotrophs utilize proteinaceous materials as carbon and energy s ources, although a few species are also saccharolytic. A scheme for electro n flow during the oxidation of carbohydrates and peptides and the reduction of S degrees has been proposed. Two S degrees-reducing enzymes have been p urified from the cytoplasm of one hyperthermophile (T-opt 100 degrees C) th at is able to grow either with and without S degrees. However, the mechanis ms by which S degrees reduction is coupled to energy conservation in this o rganism and in obligate S degrees-reducing hyperthermophiles is not known. In the heterotrophs, sugar fermentation is achieved by a novel glycolytic p athway involving unusual ADP-dependent kinases and ATP synthetases, and nov el oxidoreductases that are ferredoxin-rather than NAD(P)-linked. Similarly , peptide fermentation involves several unusual ferredoxin-linked oxidoredu ctases not found in mesophilic organisms. Several of these oxidoreductases contain tungsten, an element that is rarely used in biological systems. Tun gsten is present in exceedingly lon concentrations in normal sea water, but hydrothermal systems contain much higher tungsten concentrations, more tha n sufficient to support hyperthermophilic life.