The unique physiological property of hyperthermophiles is their ability to grow fastest at very high temperatures (≥80°C). In their ecosystems, they form complex food webs. Within these communities, hyperthermophiles function as primary producers and/or as consumers of organic material. The primary producers are chemolithoautotrophs, using a variety of inorganic compounds as electron donors and acceptors (Table 4). Under aerobic growth conditions, molecular hydrogen is converted to water (‘Knallgas reaction’) and elemental sulfur, sulfides or thiosulfate are oxidized to sulfuric acid. In the absence of oxygen, elemental sulfur, sulfate, thiosulfate, sulfite, nitrate, nitrite and CO2 are suitable electron acceptors for growth, while molecular hydrogen is the electron donor in these energy-yielding reactions (Table 4). Instead of hydrogen, Ferroglobus placidus is able to use ferrous iron as electron donor for nitrate reduction (Fig. 1c; Table 4) .
Table 4. Energy conservation in chemolithoautotrophic hyperthermophiles
|H2+1/2O2H2O||Aquifex, Thermocrinis Sulfolobusa, Acidianusa, Metallosphaeraa, Pyrobaculuma|
|2S°+3O2+2H2O2H2SO4||Aquifex, Sulfolobusa, Acidianusa, Metallosphaeraa|
|2FeS2+7O2+2H2O2FeSO4+2H2SO4=‘metal leaching’||Sulfolobusa, Acidianusa, Metallosphaera|
|H2+6FeO(OH)2Fe3O4+4H2O||Pyrobaculum sp. nov.|
|H2+S°H2S||Ignicoccus, Acidianus, Stygiolobus, Pyrodictiuma, Pyrobaculuma, Thermoproteusa|
|4H2+CO2CH4+2H2O||Methanopyrus, Methanothermus, Methanococcus|
Recently, it was shown that heavy metal compounds are further suitable electron acceptors for anaerobic growth of hyperthermophiles. Several members of Pyrobaculum can reduce ferric iron to form magnetite . By coexistence with anaerobic iron oxidizers like F. placidus, a hot iron cycle can be postulated, which may have existed already within early life forms on Earth .
Autotrophic hyperthermophiles are able to obtain cellular carbon through the reduction of carbon dioxide via the reductive tricarboxylic acid cycle (performed by e.g. Aquifex pyrophilus, Thermoproteus neutrophilus) or the reductive acetyl CoA pathway (e.g. Archaeoglobus lithotrophicus, F. placidus) [34,35]. Recently, a new CO2 fixation pathway, the 3-hydroxypropionate cycle, was described for members of the Sulfolobales (Metallosphaera sedula, Sulfolobus metallicus and Acidianus infernus) . Several chemolithoautotrophs are facultatively heterotrophic, alternatively using organic matter for growth. These two different modes of metabolism were identified for example in representatives of the genera Sulfolobus, Metallosphaera, Acidianus or Pyrobaculum (Table 5).
Table 5. Energy conservation in heterotrophic hyperthermophiles
|Type of metabolism||External electron acceptor||Energy-yielding reaction||Genera|
|Respiration||S°||2[H]+S°H2S||Pyrodictium, Thermoproteus, Pyrobaculum, Thermofilum, Desulfurococcus, Stetteria,Thermodiscus, Stygiolobus, Sulfurisphaera|
| ||SO42− (S2O32−; SO32−)||8[H]+H2SO4H2S+4H2O||Archaeoglobus|
| ||O2||2[H]+1/2O2H2O||Sulfolobus, Metallosphaera, Acidianus, Pyrobaculum, Aeropyrum|
|Fermentation||–||peptidesisovalerate, isobutyrate, butanol, CO2, H2, etc.||Pyrodictium, Hyperthermus, Thermococcus, Pyrococcus, Thermoproteus, Thermosphaera, Sulfophobococcus, Desulfurococcus, Staphylothermus|
| ||–||glucosel(+)-lactate+acetate+H2+CO2||Thermotoga, Thermosipho, Fervidobacterium|
| ||–||cellobiose, maltose or pyruvateacetate+alanine+H2+CO2||Pyrococcus|
A variety of obligate heterotrophs are known which grow by different types of respiration or fermentation. So far, Aeropyrum pernix is the only hyperthermophile gaining energy exclusively by aerobic respiration of complex organic matter . For anaerobic respiration, sulfur, sulfur-containing compounds or nitrate may be used as electron acceptors (Table 5). Depending on the organism, complex organic compounds (e.g. yeast extract, meat extract, water-extracted crude oil, cell homogenates of archaea or bacteria) or defined substrates (e.g. maltose, glucose, arachnic acid, l(+)-lactate, acetate) are used as growth substrates [2,13]. Fermentatively growing hyperthermophiles are present within the archaeal and bacterial domains. During growth on peptides or carbohydrates, different organic acids, carbon dioxide and hydrogen are formed as major products (Table 5) . Molecular hydrogen, a potent inhibitor of growth for members of Thermotoga, Thermococcus or Pyrococcus[38,39], for example, can be removed by gassing with nitrogen or argon. Alternatively, the inhibition can be prevented by the addition of sulfur, cystine or thiosulfate, whereupon H2S is formed instead of hydrogen. Thermotoga maritima may use Fe(III) in place of sulfur as an electron sink to get rid of inhibitory hydrogen during fermentation. Another possibility to eliminate hydrogen is by interspecies hydrogen transfer during co-cultivation of fermentative and hydrogen-consuming hyperthermophiles [40,41]. Thermococcus stetteri, Thermococcus celer, Staphylothermus marinus and Pyrococcus furiosus exhibited excellent growth, when cultivated together with Methanococcus thermolithotrophicus or Methanococcus jannaschii. A further strategy to prevent hydrogen production was identified in P. furiosus. Instead of acetate, alanine is formed as a reduced end product by reductive amination of pyruvate.