Archaea: Diversity and Characteristics

Domain Archaea

• Archaea are named for the Archaean eon, the period of geological history when life first spread across Earth
• During the Archaean, Earth was enveloped by high temperatures and an atmosphere lacking oxygen but with toxic gases
• Archaea were once considered remnants of this early era, as many live in extreme environments such as volcanic systems and salt ponds
• Archaea are single-celled organisms with prokaryotic cell structure, but are genetically and physiologically distinct from Bacteria
• They share more features with Eukarya
• Archaeal cells likely contributed to the origin of Eukarya

Diversity and Cultivation

• Archaea are nearly as diverse as Bacteria, although many are difficult to cultivate
• Most well-characterized Archaea come from only two phyla: Euryarchaeota and Crenarchaeota
• Several species have also been isolated from the phylum Thaumarchaeota
• Korarchaeota and Nanoarchaeota are represented only by strains grown in cocultures or by enrichment techniques

Phylogenetic Relationships

• A phylogenetic tree (Figure 17.1) shows the relationships between major archaeal orders within the five major archaeal phyla (studied in pure or highly enriched cultures) and other archaeal taxa
• There are four major branches: Euryarchaeota, DPANN, TACK, and Asgard. The exact position of the root of the archaeal tree remains uncertain.

Common Traits

• Common traits of Archaea include ether-linked lipids, a lack of peptidoglycan in cell walls, and structurally complex RNA polymerases similar to those of Eukarya
• Archaea can be chemoorganotrophs or chemolithotrophs, and they can be respiratory or fermentative, aerobic, or anaerobic, using diverse electron donors and acceptors

Superphyla

• Broader taxonomic sampling has led to the clustering of archaeal phyla into superphyla
• Currently, three named superphyla are recognized: Asgard, DPANN, and TACK
• DPANN and TACK are named based on the phyla present within them

Unusual Characteristics of Archaea

• Methane production (methanogenesis) is a unique characteristic of Archaea, called methanogens
• Methanogenesis evolved early in archaeal evolution, with well-characterized methanogens belonging to the phylum Euryarchaeota
• Methanogenesis is globally important as it produces most natural gas and significantly affects climate due to methane's "greenhouse gas" effect
• Methanogens are important in various anoxic environments such as freshwater sediments, wetlands, rice paddies, wastewater treatment plants, geothermal systems, and within animal guts

Hyperthermophilic Methanogens

• Methanopyrus, a hyperthermophilic methanogen, is a rod-shaped organism that shares properties with both hyperthermophiles and methanogens
• It was isolated from hot sediments near submarine hydrothermal vents and from "black smoker" hydrothermal vent chimneys
• This organism produces methane only from hydrogen and carbon dioxide, and grows rapidly at high temperatures

Extremophiles

• Archaea include many examples of extremophiles, including hyperthermophiles (growth optimized above 80°C), halophiles, acidophiles, and psychrophiles
• These organisms live in extreme environments

Halophilic Archaea

• Extremely halophilic Archaea (haloarchaea) live in environments with high salt concentrations
• These environments include naturally salty environments, solar salt evaporation ponds, marine salterns, and artificial saline habitats like heavily salted foods

Other Microorganisms in Salt Environments

• Eukaryotic algae, such as Dunaliella, are often the dominant oxygenic phototrophs in salt lakes
• In highly alkaline soda lakes, anoxygenic phototrophic purple bacteria like Ectothiorhodospira and Halorhodospira dominate

Additional Characteristics

• Haloarchaea stain gram-negative, reproduce by binary fission, and do not form spores
• Cells can be rod-shaped, cocci, cup-shaped, or even square-shaped, and some contain gas vesicles
• Some strains of extreme halophiles are weakly motile, while most lack archaella

Acidophilic and Thermophilic Examples

• Other Archaea, such as Thermoplasma, Ferroplasma, and Picrophilus, are thermophilic and acidophilic
• Some of these organisms show the capability of growth below pH 0, in acidic conditions
• Thermoplasma and Ferroplasma are cell wall-less

Explore the fascinating domain of Archaea, organisms that emerged during the Archaean eon when life first spread across Earth. Learn about their unique characteristics, extreme environments, and the diversity within this group. Discover their relationships with Bacteria and Eukarya, and how they contribute to our understanding of life's origins.