6.2 Archaea

This section explores the two domains of prokaryotic life: Archaea and Bacteria. It details their unique features, habitats, and the distinct characteristics of key groups like cyanobacteria.

Domain Archaea

Archaea are prokaryotic cells, once called archaebacteria, known for inhabiting extreme environments similar to those on early Earth. They are genetically and molecularly distinct from bacteria.

Prokaryotic and Eukaryotic Cells→

Unifying Features of Archaea

• Membrane Lipids: Possess unique lipids with branched carbon chains linked to glycerol by ether linkages. This provides stability in extreme temperatures and pH.
• Cell Wall: Lacks peptidoglycan, which is a key differentiator from bacteria. Instead, they may have protein-based or carbohydrate-based walls.
• Metabolism:
  • Some are methanogens, producing methane gas in anaerobic conditions.
  • Many are autotrophs, capable of producing their own food using inorganic molecules like hydrogen, sulfur, and carbon dioxide.
• Genetics: Exhibit traits similar to eukaryotes, such as complex RNA polymerase and the presence of introns in some genes.

Comparison: Eubacteria vs. Archaea

Types of Archaea (Based on Environment)

Archaea are often referred to as extremophiles because they thrive in conditions that would be lethal to most other life forms.

• Methanogens: Methane-producers found in anaerobic environments like swamps, marshes, and the digestive tracts of animals (e.g., Methanobacterium smithii in the human gut).
• Halophiles: Salt-lovers that thrive in highly saline environments such as salted fish or the Dead Sea (e.g., Halobacterium halobium).
• Thermoacidophiles: Heat and acid-lovers that inhabit hot, acidic places like hot springs, geysers, and volcanic vents (e.g., Thermoplasma acidophilum, Thermus aquaticus).

Domain Bacteria (Eubacteria)

This vast group of prokaryotes is found in nearly every habitat on Earth and plays a crucial role in global ecosystems. The study of bacteria is known as bacteriology.

Cell Wall and Plasma Membrane→

Structural Adaptations for Survival

• Flagella: Whip-like appendages that allow for motility, enabling bacteria to move towards favorable conditions (e.g., nutrients) or away from harm.
• Pili: Hair-like structures on the surface that facilitate adhesion to surfaces (e.g., rocks, intestinal lining) and other bacteria, crucial for colonization.
• Endospores: A dormant, tough, non-reproductive structure produced by some bacteria. The outer cell disintegrates, but the endospore can survive extreme conditions (heat, cold, desiccation, poisons) and resume growth when conditions become favorable.
• Filaments: A mass of branching cell chains, characteristic of actinomycetes. This structure helps them bridge dry gaps between soil particles, aiding in nutrient absorption.

Common Phylogenetic Groups of Bacteria

Cyanobacteria

Often called blue-green algae, cyanobacteria are a prominent group of Gram-negative, photosynthetic bacteria.

Key Features

• Habitat: Extremely diverse, found in damp soil, freshwater, saltwater, on rocks, and in symbiotic relationships (e.g., forming lichens with fungi).
• Cell Structure:
  • Typical prokaryotic features: no nuclear membrane or membrane-bound organelles.
  • Cell wall contains muramic acid.
  • Often surrounded by a mucilaginous sheath.
  • Photosynthesis occurs on an extensive internal membrane system (thylakoids).
• Photosynthesis:
  • They perform oxygenic photosynthesis (release oxygen).
  • They use accessory pigments called phycobilins, with the blue pigment phycocyanin being predominant.
  • Evolutionarily significant for being the first organisms to introduce oxygen into Earth's primitive atmosphere.
• Asexual Reproduction:
  • Cell Division: In unicellular forms.
  • Fragmentation: Filaments break into smaller pieces called hormogonia, which grow into new filaments.
  • Akinetes: Formation of thick-walled resting cells filled with food reserves that can survive harsh conditions.
  • Spore Formation: Endospores can form within a heterocyst.
• Nitrogen Fixation:
  • About one-third of cyanobacteria can fix atmospheric nitrogen into ammonia.
  • This process often occurs in specialized, thick-walled cells called heterocysts.
  • Examples like Nostoc and Anabaena are used in agriculture to improve soil fertility.