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.