Prokaryotic Diversity, Structure, and Evolution: Bacteria and Archaea

Prokaryotic Diversity, Structure, and Evolution

Introduction

• Carl Woese and Georgi Fox made groundbreaking discoveries by analyzing ribosomal RNA sequences, leading to the recognition of Archaea as a distinct domain of life, separate from Bacteria and Eukarya.

• This fundamentally changed our understanding of the tree of life and microbial diversity.

Diversity of Prokaryotic Structure

Distinguishing Structural Features of Prokaryotic Cells

• Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles.

• Key features include:

  • Cell wall: Provides structural support and shape; composition varies (e.g., peptidoglycan in Bacteria).

  • Plasma membrane: Encloses the cytoplasm and regulates transport.

  • Ribosomes: Sites of protein synthesis.

  • Nucleoid: Region containing the circular DNA molecule.

  • Some have additional structures such as flagella, pili, or capsules.

Prokaryotic Cell Shapes

• Common shapes include:

  • Cocci: Spherical

  • Bacilli: Rod-shaped

  • Spirilla/Spirochetes: Spiral-shaped

  • Other shapes: Vibrio (comma-shaped), filamentous, etc.

• Shapes are relatively limited compared to eukaryotes, but adaptations exist for specific environments.

Prokaryote Reproduction

Binary Fission

• Binary fission is the primary mechanism by which prokaryotes reproduce.

• Process:

  • DNA replicates.

  • Cell elongates and divides into two genetically identical daughter cells.

• This asexual process allows for rapid population growth.

Genetic Variation in Prokaryotes

• Binary fission alone does not generate genetic diversity.

• Genetic variation arises through Horizontal Gene Transfer (HGT):

  • Transformation: Uptake of free DNA from the environment.

  • Transduction: Transfer of DNA by bacteriophages (viruses).

  • Conjugation: Direct transfer of DNA between cells via a pilus.

  • Genetic recombination (e.g., plasmids, transposons) increases diversity.

Major Lineages of Bacteria and Archaea

Key Features of Major Bacterial Lineages

• Bacteria are classified into several phyla, including:

  • Actinobacteria: High G+C Gram-positive bacteria, important for antibiotics production.

  • Firmicutes: Low G+C Gram-positive bacteria, includes lactic acid bacteria and pathogens.

  • Proteobacteria: Large, diverse group; includes Escherichia coli, nitrogen-fixers, and pathogens.

  • Cyanobacteria: Photosynthetic, oxygen-producing bacteria (formerly "blue-green algae").

  • Spirochaetes: Spiral-shaped, includes pathogens like Treponema pallidum (syphilis).

  • Chlamydiae: Obligate intracellular parasites.

  • Gram-positive bacteria: Thick peptidoglycan cell wall, stain purple in Gram stain.

  • Gram-negative bacteria: Thin peptidoglycan layer, outer membrane, stain pink.

Major Taxa of Archaea

• Archaea are divided into several major groups:

  • Euryarchaeota: Includes methanogens (produce methane), halophiles (salt-loving), and some thermophiles.

  • Crenarchaeota: Many are thermophilic or acidophilic.

  • Thaumarchaeota: Important in nitrogen cycling.

• Archaea are often extremophiles, thriving in extreme environments (high temperature, salinity, acidity).

Key Terms and Definitions

Term	Definition
Actinobacteria	High G+C Gram-positive bacteria, important for soil ecology and antibiotics production.
Antibiotic	Substance that kills or inhibits the growth of microorganisms, especially bacteria.
Archaea	Domain of prokaryotes distinct from Bacteria, often found in extreme environments.
Bacteria	Domain of prokaryotes with peptidoglycan cell walls, diverse in metabolism and habitat.
Bioremediation	Use of microorganisms to degrade environmental pollutants.
Fermentation	Metabolic process that generates energy without oxygen, producing organic end products.
Firmicutes	Phylum of Gram-positive bacteria, includes lactic acid bacteria and some pathogens.
Gram stain	Laboratory technique to differentiate bacteria by cell wall structure (Gram-positive vs. Gram-negative).
Gram-negative	Bacteria with thin peptidoglycan layer and outer membrane; stain pink in Gram stain.
Gram-positive	Bacteria with thick peptidoglycan cell wall; stain purple in Gram stain.
Pathogen	Microorganism that causes disease.
Peptidoglycan	Polymer forming the cell wall of most bacteria.
Photosynthesis	Process by which organisms convert light energy into chemical energy.
Proteobacteria	Large, diverse phylum of Gram-negative bacteria.
Cell wall	Rigid structure outside the cell membrane, provides support and shape.
Chlamydiae	Phylum of obligate intracellular bacterial parasites.
Crenarchaeota	Phylum of Archaea, many are thermophiles or acidophiles.
Cyanobacteria	Photosynthetic bacteria, important for oxygen production.
Euryarchaeota	Phylum of Archaea, includes methanogens and halophiles.
Extremophile	Organism that thrives in extreme environmental conditions.
Gram-positive bacteria	Bacteria with thick peptidoglycan cell wall, retain crystal violet stain.
Koch's postulates	Set of criteria to establish a causative relationship between a microbe and a disease.
Methanogen	Archaea that produce methane as a metabolic byproduct.
Microbe	Microscopic organism, including bacteria, archaea, fungi, and viruses.
Microbiome	Community of microorganisms living in a particular environment, such as the human body.
Monophyletic group	Group of organisms that includes an ancestor and all its descendants.
Seeding	Introduction of microorganisms into an environment for bioremediation or other purposes.
Spirochaetes / Spirochetes	Phylum of spiral-shaped bacteria, some are pathogenic.
Thaumarchaeota	Phylum of Archaea, important in ammonia oxidation and nitrogen cycling.
Thermophile	Organism that thrives at high temperatures.
Tree of life	Diagram representing evolutionary relationships among all living organisms.

Additional info:

• Prokaryotes play essential roles in ecosystems, including nutrient cycling, decomposition, and as pathogens or mutualists.

• Understanding prokaryotic diversity is crucial for biotechnology, medicine, and environmental science.