# Domain Archaea and Domain Bacteria 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. ### 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 | Main Features | Eubacteria | Archaea | | :--- | :--- | :--- | | **rRNA sequences** | Many are unique to Eubacteria | Many sequences match those of Eukaryotes | | **RNA polymerase** | Relatively small and complex | Complex, similar in structure to Eukaryotes | | **Introns** | Absent | Present in some genes | | **Peptidoglycan in cell wall** | **Present** | **Absent** | | **Membrane lipids** | Carbon chains are unbranched | Carbon chains are *branched* | ### 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**. ### 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 | BACTERIAL GROUP | CHARACTERISTICS | | :--- | :--- | | **Aquificales** | Extremely thermophilic; considered the oldest branch of bacteria. E.g., *Aquifex*. | | **Green Non-Sulfur Bacteria** | Mostly photosynthetic, some are thermophilic. E.g., *Chloroflexus*. | | **Green Sulfur Bacteria** | Photosynthetic but do not produce oxygen. E.g., *Chlorobium*. | | **Proteobacteria** | Gram-negative. Includes photosynthetic (non-oxygen producing), chemotrophic, and nitrogen-fixing members. E.g., *Escherichia*, *Salmonella*. | | **Gram-positive Bacteria** | Includes endospore-formers, lactic acid bacteria, etc. E.g., *Staphylococcus*, *Clostridium*, *Bacillus*. | | **Cyanobacteria** | Photosynthetic and *produce oxygen*. E.g., *Nostoc*, *Anabaena*. | | **Bacteroides** | Gram-negative, rod-shaped bacteria. E.g., *Bacteroides*. | ## 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.