Chapter 17: Diversity of Archaea

Archaea share distinctive cellular features such as ether linked membrane lipids, the absence of peptidoglycan in their cell walls, and complex transcription machinery resembling that of eukaryotes. Genomic evidence suggests that early archaeal lineages were hyperthermophilic chemolithotrophs that used hydrogen as an energy source and fixed carbon dioxide. Modern archaeal groups exhibit extensive metabolic diversity, functioning as chemoorganotrophs or chemolithotrophs and growing under aerobic or anaerobic conditions. One of the most distinctive archaeal metabolisms is methanogenesis, the biological production of methane carried out exclusively by methanogenic archaea that inhabit anoxic environments such as sediments, wetlands, and animal digestive systems. The phylum Euryarchaeota includes several well studied groups such as extremely halophilic archaea adapted to hypersaline environments, methanogens that generate methane through several metabolic pathways, and thermoacidophilic species such as Thermoplasma that thrive in hot acidic habitats. Other archaeal lineages include Thaumarchaeota, widespread ammonia oxidizing chemolithotrophs that play important roles in the global nitrogen cycle, and members of the DPANN superphylum such as Nanoarchaeota, organisms with extremely reduced genomes that often live in symbiotic or parasitic associations with other archaea. The TACK superphylum includes lineages such as Korarchaeota and Crenarchaeota, many of which inhabit geothermal environments and rely on sulfur based respiration or other chemolithotrophic metabolisms. Hyperthermophilic crenarchaeotes found in volcanic systems and hydrothermal vents demonstrate extraordinary adaptations to extreme heat, including thermostable proteins, specialized membrane lipids, reverse DNA gyrase that stabilizes DNA at high temperatures, and molecular chaperones that maintain protein structure. Recent discoveries of Asgard archaea containing eukaryotic signature genes suggest a close evolutionary relationship between archaea and eukaryotes, providing new insight into the origin of complex cellular life. Collectively, these diverse archaeal lineages illustrate how microorganisms have evolved a wide range of physiological and molecular strategies that allow life to persist in extreme and varied environments across Earth.