3.1 - Prokaryotes

Learning Objectives

  • List some common properties of prokaryotes.
  • Explain the diversity of prokaryotes despite their asexual reproduction.
  • Describe the different nutritional modes of prokaryotes.
  • Discuss the ecological roles of prokaryotes.
  • Explain how prokaryotes can affect humans.

What are Prokaryotes?

  • Unicellular organisms typically lacking internal organelles.
  • Size ranges from 0.5 to 5 µm.
  • Common shapes:
    • Spherical (coccus)
    • Rod-shaped (bacillus)
    • Spiral (spiro-)

Extreme Environments and Prokaryotes

  • Certain prokaryotes thrive in extreme habitats:
    • Halobacterium in hyper-saline lakes (e.g., Lake Hillier, WA):
    • Pumps K+ ions to balance osmotic pressure.
    • Utilizes carotenoids for photosynthesis.
    • Deinococcus radiodurans: Survives 3 million rads of radiation.
    • Picrophilus oshimae: Grows at extremely low pH (0.03).
    • Pyrococcus furiosus: Thrives in high-temperature environments (100°C).

Cell Structure of Prokaryotes

  • Many possess a protective cell wall made of peptidoglycan (unlike eukaryotic cellulose or chitin).
  • Gram-positive vs. Gram-negative:
    • Gram-positive: Thick peptidoglycan layer; retains crystal violet stain.
    • Gram-negative: Thin peptidoglycan layer; can lose the crystal violet stain easily.
  • Additional structures:
    • Capsule: A sticky layer of polysaccharides/proteins aiding adhesion.
    • Fimbriae: Hairlike appendages that enhance attachment.
    • Endospore: Protective dormant structure formed under harsh conditions.
    • Flagella: Enables movement, can swim rapidly (up to 50× body length/sec).

Genetic Diversity in Prokaryotes

  • Prokaryotes reproduce by binary fission, which does not allow for sexual reproduction; however, they show diversity through:
    • Rapid reproduction: Some can divide every 20 minutes.
    • Mutations: Errors in DNA replication lead to variations.
    • Genetic recombination: Processes include:
    • Conjugation: Transfer of DNA between prokaryotes via a mating bridge.
    • Uptake of foreign DNA and plasmid exchanges.

Antibiotic Resistance

  • Emergence of superbugs due to:
    • Mutant bacteria resistant to antibiotics.
    • Overuse of antibiotics leading to selective pressure, fostering resistant strains.
    • Example: 1950s in Japan, treatments failing against resistant bacterial strains.

Nutritional Modes of Prokaryotes

  • Prokaryotes display various nutritional and metabolic strategies:
    • Autotrophs vs. Heterotrophs:
    • Photoautotrophs: Use light for energy.
    • Chemolithoautotrophs: Use inorganic compounds.
    • Chemoheterotrophs: Obtain energy from organic compounds.
  • Notable carbon and nitrogen metabolisms:
    • Nitrogen fixation: Conversion of atmospheric nitrogen (N2) into ammonia (NH3) by cyanobacteria and methanogens, essential for plant growth.

Phylogeny of Prokaryotes

  • Domains: Bacteria and Archaea:
    • Domain Bacteria:
    • Clades include:
      • Proteobacteria: Nitrogen-fixing bacteria.
      • Chlamydias: Parasitic, depend on hosts.
      • Spirochetes: Free-living and some pathogenic.
      • Cyanobacteria: Perform oxygen-generating photosynthesis.
      • Gram-positive bacteria: Diverse, includes antibiotic producers like Streptomyces.
    • Domain Archaea:
    • Known for extremophiles and unique properties that somewhat link them to both Bacteria and Eukarya.

Ecological Roles of Prokaryotes

  • Chemical recycling:
    • Chemoheterotrophic prokaryotes decompose organic material, releasing carbon and nutrients.
    • Autotrophic bacteria facilitate organic compound formation from CO2.
    • Nitrogen fixation enables new nitrogen forms usable by other life forms.
  • Prokaryotes participate in various interactions:
    • Mutualistic (+/+): Essential gut bacteria.
    • Commensal (+/0): Skin bacteria.
    • Parasitic (+/-): Pathogenic bacteria leading to diseases.

Human Interactions with Prokaryotes

  • Beneficial:
    • Microbiome in human gut aids in digestion, synthesizes Vitamins B7 and K.
  • Pathogenic:
    • Bacteria responsible for numerous diseases, including tuberculosis and cholera.
  • Utilization in Industry:
    • Bacteria used in food production (cheese, yogurt) and bioremediation (oil spills, sewage treatment).

Summary of Key Learning Points

  • Prokaryotes are highly adaptable, diverse, and crucial for ecological balance and human health.
  • They reproduce rapidly and exhibit high genetic variability, facilitating evolutionary success despite asexual reproduction.
  • Their varied metabolism supports their survival in many environments, while their roles include recycling nutrients and forming essential partnerships with other organisms.