Domain Bacteria includes some unique hyperthermophilic species. They exhibit remarkable adaptations that enable survival in extreme environments.
Thermotoga species are rod-shaped, gram-negative, non-sporulating hyperthermophiles that form a sheath-like envelope called a toga. They ferment sugars or starch, producing lactate, acetate, CO₂, and H₂, and can also grow via anaerobic respiration using H₂ and ferric iron. Found in hot springs and hydrothermal vents, over 20% of their genes show strong homology to Archaea due to horizontal gene transfer.
Thermodesulfobacterium is a thermophilic sulfate-reducing bacterium that strictly anaerobically oxidizes lactate, pyruvate, and ethanol to reduce SO₄²⁻ to H₂S. It contains ether-linked lipids, a trait common in Archaea, though its lipids differ structurally.
Aquifex, the most thermophilic known bacterium (up to 95°C), is a chemolithotrophic autotroph that oxidizes H₂, elemental sulfur, or thiosulfate, using O₂ or nitrate as electron acceptors. It follows the reverse citric acid cycle for autotrophy and has a small genome (1.55 Mbp). Its placement on early phylogenetic branches suggests H₂ was a key electron donor in early life.
Thermocrinis, a relative of Aquifex, grows optimally at 80°C, oxidizing H₂, thiosulfate, or sulfur with O₂ as an electron acceptor. Thermocrinis ruber, discovered in Yellowstone by Thomas Brock in the 1960s, forms pink “streamers” in hot springs and adapts its morphology based on growth conditions. Brock’s findings contributed to the discovery of many hyperthermophilic Bacteria and Archaea.
The Thermales order consists mainly of thermophilic and hyperthermophilic bacteria. One notable species, Thermus aquaticus, was first identified in the mid-1960s by Thomas Brock in a hot spring at Yellowstone National Park. Since its discovery, T. aquaticus has been widely studied as a model organism for understanding life in extreme heat environments and has been found in various geothermal ecosystems.
A major breakthrough attributed to T. aquaticus is its Taq DNA polymerase, an enzyme that remains stable at high temperatures. This stability was crucial in the development of the polymerase chain reaction (PCR) technique, enabling full automation of DNA amplification. The introduction of PCR has significantly transformed biological research and applications across multiple fields.
Hyperthermophilic bacteria optimally grow in temperature ranges of 70 to 95°C. They are typically found in terrestrial hot springs and marine hydrothermal vents.
Thermotoga species are rod-shaped, nonsporulating, fermentative anaerobes that form a sheath-like envelope called a toga.
Thermodesulfobacterium is a sulfate-reducing genus. Its species produce ether-linked lipids, contributing to their unique lipid profile, which shows features of both Bacteria and Archaea.
Species in Aquifex are obligate chemolithotrophic and autotrophic. They utilize the reverse citric acid cycle to capture energy by oxidizing hydrogen, thiosulfate, and sulfur. They are the most thermophilic species of all known Bacteria, growing at temperatures up to 95°C.
Thermus species grow optimally at 70°C, and some species are readily cultivated in laboratories to obtain thermostable enzymes.
The species Thermus aquaticus produces Taq DNA polymerase, which is crucial in the polymerase chain reaction technique.
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