Archaebacteria are ancient prokaryotic organisms believed to be among the earliest life forms on Earth. They are mostly extremophiles.
General Characteristics
- Cell Type: Prokaryotic (no true nucleus, no membrane-bound organelles).
- Cell Wall:
- Lacks peptidoglycan
- Made of pseudopeptidoglycan, polysaccharides, or proteins
- Cell Membrane:
- Lipids are ether-linked (not ester-linked)
- Branched isoprenoid chains (unique feature)
- Habitat:
- Extreme environments: high salt, high temperature, acidic or anaerobic conditions
- Genetic Machinery:
- Transcription and translation machinery resembles eukaryotes
- Metabolism:
- Chemolithotrophic or chemoautotrophic
- No known pathogenic archaea
- Reproduction:
- Asexual (binary fission, budding, fragmentation)
- Ribosomes:
- 70S ribosomes (like bacteria) but structurally unique
Major Types
- Methanogens – produce methane (found in rumen, swamps)
- Halophiles – live in high-salt environments
- Thermoacidophiles – live in hot and acidic environments
Distribution of Archaebacteria
Archaebacteria have a restricted but highly specialized distribution. They are mainly found in extreme and unusual environments, where other organisms cannot survive.
Major Habitats
- High Salt Areas (Halophiles)
- Salt lakes (e.g., Sambhar Lake, Dead Sea)
- Salt pans, saline soils
- High Temperature Areas (Thermophiles)
- Hot springs
- Volcanic vents
- Geothermal areas
- Anaerobic Environments (Methanogens)
- Marshes and swamps
- Sewage treatment plants
- Rumen of cattle and other ruminants
- Paddy fields
- Acidic or Alkaline Conditions
- Acidic sulfur springs
- Highly alkaline lakes
Ultrastructure of Archaebacteria (Archaea)
Archaebacteria possess a prokaryotic but highly specialized ultrastructure, adapted to extreme environments.
1. Cell Envelope
- Cell Wall
- Lacks peptidoglycan
- Composed of pseudopeptidoglycan (pseudomurein), polysaccharides, proteins or glycoproteins
- Provides rigidity and resistance to extreme conditions
- Cell Membrane
- Lipids are ether-linked (not ester-linked)
- Branched isoprenoid chains
- May form monolayer (especially in thermophiles) → high thermal stability
2. Cytoplasm
- Dense, granular cytoplasm
- Contains enzymes adapted to extreme pH, temperature and salinity
- No membrane-bound organelles
3. Genetic Material (Nucleoid)
- Single circular, double-stranded DNA
- DNA associated with histone-like proteins (similar to eukaryotes)
- Helps in DNA stability under extreme conditions
4. Ribosomes
- 70S ribosomes (50S + 30S)
- Structurally closer to eukaryotic ribosomes
- Resistant to many bacterial antibiotics
5. Appendages
- Archaella (flagella-like structures)
- Structurally different from bacterial flagella
- Used for locomotion
6. Inclusion Bodies
- Storage granules of glycogen, polyphosphate, sulfur etc.
Special Ultrastructural Features
- Absence of pathogenic forms
- Unique transcription machinery resembling eukaryotes
- Exceptional membrane stability
Nutrition in Archaebacteria (Archaea)
Archaebacteria show mainly autotrophic and chemotrophic modes of nutrition, well adapted to extreme environments.
1. Chemolithoautotrophic Nutrition
- Energy obtained from oxidation of inorganic substances
- Examples:
- Oxidation of hydrogen (H₂), sulfur compounds
- Carbon source: CO₂
- Seen in thermophiles and acidophiles
2. Methanogenic Nutrition
- Special type of anaerobic chemoautotrophy
- Substrates: CO₂ + H₂, acetate, formate
- End product: Methane (CH₄)
- Occurs in methanogens (swamps, rumen of cattle, sewage plants)
3. Photoheterotrophic Nutrition (Halophiles)
- Light energy captured by bacteriorhodopsin
- No chlorophyll, no oxygen evolution
- Organic compounds act as carbon source
4. Heterotrophic Nutrition
- Some archaea utilize organic compounds
- Mostly saprophytic
Important Points
- No known parasitic or pathogenic archaea
- Unique metabolic pathways absent in eubacteria
Reproduction
Archaea are prokaryotic microorganisms that reproduce asexually. They do not show true sexual reproduction, but they possess efficient mechanisms for cell multiplication and genetic exchange, enabling survival in extreme environments.
I. Modes of Reproduction in Archaea
1. Binary Fission (Most Common Method)
- Similar to bacterial binary fission but with unique molecular machinery
- Steps:
- Replication of circular DNA
- Cell elongation
- Formation of division plane
- Separation into two genetically identical daughter cells
- Cell division proteins in archaea are often closer to eukaryotic proteins than bacterial ones
- Common in many archaeal groups
Significance:
✔ Rapid multiplication
✔ Maintains genetic stability
2. Budding
- A small outgrowth (bud) forms on the parent cell
- Bud enlarges and separates to form a new individual
- Observed in some archaea like Methanobacterium
- Daughter cell may initially be smaller than parent
Significance:
✔ Useful in stable but nutrient-limited habitats
3. Fragmentation
- Filamentous archaea break into smaller fragments
- Each fragment develops into a new individual
- Seen in certain filamentous or multicellular archaeal forms
Significance:
✔ Rapid colonization of new habitats
4. Multiple Fission (Rare)
- Nucleus (DNA) divides repeatedly before cytoplasmic division
- Produces more than two daughter cells
- Occurs rarely under favorable conditions
II. Genetic Recombination in Archaea (Not True Sexual Reproduction)
Although archaea lack sexual reproduction, they exhibit genetic exchange mechanisms that increase variability.
1. Transformation
- Uptake of naked DNA from the environment
- DNA integrates into archaeal genome
2. Conjugation-like Mechanisms
- DNA transfer via cell-to-cell contact
- Does not involve true sex pili like bacteria
3. Virus-mediated Gene Transfer
- Archaeal viruses transfer genetic material between cells
- Similar to bacterial transduction
Significance:
✔ Genetic diversity
✔ Adaptation to extreme conditions
✔ Evolutionary advantage