グラム陽性細菌：クロストリジウム目および乳酸菌目

Hyperthermophilic bacteria optimally grow in temperature ranges of 70 to 95°C. These organisms are typically found in terrestrial hot springs and marine hydrothermal vents, where extreme heat is a defining environmental characteristic. Their ability to thrive at such high temperatures represents a remarkable adaptation to extreme environments.

Thermus aquaticus produces Taq DNA polymerase, an enzyme that remains stable at high temperatures. This stability was crucial in developing the polymerase chain reaction (PCR) technique, enabling full automation of DNA amplification. Since its discovery in Yellowstone in the mid-1960s, T. aquaticus has transformed biological research and applications across multiple fields.

Aquifex species are obligate chemolithotrophic autotrophs capable of growing at temperatures up to 95°C, making them the most thermophilic bacteria known. They utilize the reverse citric acid cycle to capture energy by oxidizing hydrogen, thiosulfate, and sulfur. Their small genome and placement on early phylogenetic branches suggest hydrogen was a key electron donor in early life.

Thermotoga species are rod-shaped, gram-negative, non-sporulating hyperthermophiles that form a distinctive sheath-like envelope called a toga. They ferment sugars or starch, producing lactate, acetate, CO₂, and H₂. Over 20% of their genes show strong homology to Archaea due to horizontal gene transfer, reflecting their unique evolutionary history.

Thermodesulfobacterium and other hyperthermophilic bacteria produce ether-linked lipids, a trait common in Archaea that contributes to their unique lipid profile. These lipids show features of both Bacteria and Archaea, enabling membrane stability at extreme temperatures. This adaptation is crucial for survival in geothermal ecosystems where conventional lipids would denature.

Hyperthermophilic bacteria employ diverse metabolic strategies. Thermotoga species ferment sugars and can grow via anaerobic respiration using H₂ and ferric iron. Thermodesulfobacterium strictly anaerobically oxidizes lactate, pyruvate, and ethanol to reduce sulfate to H₂S. Aquifex oxidizes hydrogen, sulfur, or thiosulfate using oxygen or nitrate as electron acceptors, utilizing the reverse citric acid cycle for autotrophy.

Hyperthermophilic bacteria in Domain Bacteria exhibit remarkable adaptations enabling survival in extreme environments. Their genetic and biochemical features often resemble those of Archaea, suggesting evolutionary connections. Understanding these organisms within the three domain system of life provides insights into early life evolution and the diversity of microbial adaptation strategies.