Bacteria and Archaea: Structure, Function, and Diversity

Bacteria and Archaea: Structure, Function, and Diversity

Introduction to Prokaryotes

Prokaryotes, which include the domains Bacteria and Archaea, are the most abundant and diverse organisms on Earth. Their remarkable adaptability allows them to thrive in environments that are too extreme for most other life forms, such as highly acidic, salty, cold, or hot habitats.

• Unicellular: Most prokaryotes are single-celled organisms.

• Size: Typically range from 0.5–5 µm in diameter.

• Shapes: Common shapes include cocci (spherical), bacilli (rod-shaped), and spirilla (spiral).

Adaptations for Success

Prokaryotes have evolved a variety of structural and functional adaptations that contribute to their ecological success and large population sizes.

• Rapid Reproduction: Prokaryotes reproduce quickly by binary fission, leading to exponential population growth under favorable conditions.

• Mutation and Evolution: Although mutation rates are low, the rapid rate of reproduction allows mutations to accumulate quickly, promoting genetic diversity and rapid evolution.

• Diverse Adaptations: Prokaryotes can survive in extreme environments due to specialized adaptations such as endospores and unique metabolic pathways.

Cell-Surface Structures

The cell wall is a key feature of prokaryotic cells, providing structural support and protection.

• Bacterial Cell Walls: Most contain peptidoglycan, a polymer of sugars and amino acids.

• Archaeal Cell Walls: Composed of various polysaccharides and proteins, but lack peptidoglycan.

• Gram Staining: Bacteria are classified as Gram-positive (thick peptidoglycan layer) or Gram-negative (thin peptidoglycan layer and outer membrane).

• Capsule: Many prokaryotes have a sticky layer of polysaccharide or protein outside the cell wall, aiding in adherence and protection.

Specialized Structures: Endospores, Fimbriae, Pili, and Flagella

Prokaryotes possess specialized structures that enhance survival and interaction with their environment.

• Endospores: Metabolically inactive, resistant cells that can survive harsh conditions for centuries.

• Fimbriae: Hair-like appendages that allow cells to adhere to surfaces or other cells.

• Pili: Longer appendages used for DNA exchange during conjugation.

• Flagella: The most common structures for movement, enabling taxis (directed movement in response to stimuli).

Internal Organization and Genetic Material

Prokaryotic cells lack membrane-bound organelles but may have specialized internal membranes for metabolic functions.

• Nucleoid: Region where the circular chromosome is located.

• Plasmids: Small, independently replicating DNA molecules.

• Ribosomes: Sites of protein synthesis, structurally different from those in eukaryotes.

Genetic Diversity: Reproduction and Recombination

Prokaryotes reproduce asexually by binary fission, but genetic diversity is enhanced by mutation and genetic recombination.

• Binary Fission: Asexual reproduction resulting in genetically identical offspring.

• Mutation: Main source of genetic variation due to rapid reproduction.

• Genetic Recombination: Includes transformation (uptake of foreign DNA), transduction (gene transfer by bacteriophages), and conjugation (direct transfer between cells).

Nutritional and Metabolic Diversity

Prokaryotes exhibit a wide range of nutritional strategies, classified by their energy and carbon sources.

Mode	Energy Source	Carbon Source	Types of Organisms
Photoautotroph	Light	CO2, HCO3-, or related compound	Photosynthetic prokaryotes (e.g., cyanobacteria), plants, certain protists
Chemoautotroph	Inorganic chemicals (e.g., H2S, NH3, Fe2+)	CO2, HCO3-, or related compound	Certain prokaryotes (e.g., Sulfolobus)
Photoheterotroph	Light	Organic compounds	Certain aquatic and salt-loving prokaryotes (e.g., Rhodobacter, Chloroflexus)
Chemoheterotroph	Organic compounds	Organic compounds	Many prokaryotes (e.g., Clostridium), fungi, animals, some plants

The Role of Oxygen in Metabolism

• Obligate aerobes: Require oxygen for cellular respiration.

• Obligate anaerobes: Poisoned by oxygen; use fermentation or anaerobic respiration.

• Facultative anaerobes: Can use oxygen if present or switch to anaerobic metabolism if not.

Nitrogen Metabolism

• Nitrogen fixation: Conversion of atmospheric N2 to ammonia (NH3), a process essential for supplying usable nitrogen to plants and other organisms.

• Example: Rhizobium forms nodules on legume roots, fixing nitrogen for the plant.

Prokaryotic Diversity and Classification

Prokaryotes are classified into two domains: Bacteria and Archaea. Their evolutionary relationships are not always reflected in their shape, nutritional mode, or motility due to horizontal gene transfer.

• Bacteria: Includes major groups such as Proteobacteria, Chlamydias, Spirochetes, Cyanobacteria, and Gram-positive bacteria.

• Archaea: Includes extremophiles such as extreme halophiles and thermophiles.

Major Groups of Bacteria

• Proteobacteria: Large, diverse group of Gram-negative bacteria; includes photoautotrophs, chemoautotrophs, and heterotrophs.

• Chlamydias: Parasitic bacteria that live within animal cells (e.g., Chlamydia trachomatis).

• Spirochetes: Helical, Gram-negative heterotrophs; some are pathogenic (e.g., Treponema pallidum causes syphilis).

• Cyanobacteria: Gram-negative photoautotrophs; important for oxygen production and as ancestors of chloroplasts.

• Gram-positive bacteria: Includes important decomposers and pathogens (e.g., Streptomyces, Bacillus anthracis).

Major Groups of Archaea

• Extreme halophiles: Thrive in highly saline environments.

• Extreme thermophiles: Thrive in very hot environments (e.g., Pyrococcus furiosus).

Summary Table: Domains of Life

Domain	Major Groups
Bacteria	Proteobacteria, Chlamydias, Spirochetes, Cyanobacteria, Gram-positive bacteria
Archaea	Euryarchaeotes, Thaumarchaeotes, Aigarchaeotes, Crenarchaeotes, Korarchaeotes
Eukarya	All eukaryotic organisms

Additional info: Horizontal gene transfer is a major evolutionary force in prokaryotes, complicating the reconstruction of their evolutionary history. Many prokaryotes are essential for ecosystem functioning, including nutrient cycling and symbiotic relationships with plants and animals.