Overview
This lecture covers the evolutionary history, diversity, adaptability, and culturing challenges of prokaryotes, emphasizing extremophiles and biofilms.
Prokaryotic Evolution and Early Life
- Prokaryotes were Earth's first cellular life, predating plants and animals by billions of years.
- Early Earth had little oxygen and harsh conditions like high temperatures, volcanic activity, and strong radiation.
- Microbial mats, layered sheets of prokaryotes, appeared about 3.5 billion years ago.
- Stromatolites are fossilized microbial mats, providing physical evidence of ancient life.
The Ancient Atmosphere and Photosynthesis
- Early atmosphere was anoxic (lacking molecular oxygen), so only anaerobic organisms could survive.
- Phototrophs, organisms using sunlight for energy, appeared within a billion years of Earth's formation.
- Cyanobacteria evolved later, producing oxygen and gradually oxygenating the atmosphere.
- Elevated oxygen enabled more complex life and ozone formation to shield Earth from UV radiation.
Prokaryotic Adaptability and Extremophiles
- Prokaryotes survive in various environments due to adaptations like cell walls and endospore formation.
- Extremophiles are prokaryotes that thrive under extreme conditions (e.g., acidity, temperature, salt, radiation).
- Types include acidophiles (low pH), alkaliphiles (high pH), thermophiles (hot), psychrophiles (cold), halophiles (salty), and osmophiles (high sugar).
- Example: Halophilic prokaryotes live in the Dead Sea, tolerating high salt, divalent ions, and acidic pH.
Culturing Prokaryotes and Unculturable States
- Culturing prokaryotes requires specific nutrients and conditions; most prokaryotes are unculturable in labs.
- Over 99% of prokaryotes remain unculturable due to unknown or complex requirements.
- Some become temporarily unculturable by entering a dormant, viable-but-non-culturable (VBNC) state under stress.
- Presence of unculturable prokaryotes is detected using molecular techniques like PCR.
Biofilms and Prokaryotic Communities
- Prokaryotes often form biofilms: sticky microbial communities held in a matrix of secreted substances.
- Biofilms attach to surfaces, facilitating survival and resistance to antibiotics and disinfectants.
- Biofilm development occurs in five stages: attachment, irreversible attachment, maturation I, maturation II, and dispersal.
- Biofilms are difficult to eliminate and play roles in disease, industry, and household contamination.
Key Terms & Definitions
- Prokaryote — Single-celled organism lacking a membrane-bound nucleus, including bacteria and archaea.
- Microbial mat — Multi-layered sheet of prokaryotes, often found at material interfaces.
- Stromatolite — Layered sedimentary structure formed by mineral precipitation in microbial mats.
- Extremophile — Organism adapted to live in extreme environmental conditions.
- Endospore — Dormant, tough, non-reproductive structure produced by some bacteria for survival.
- Halophile — Organism that thrives in high-salt environments.
- VBNC state — Viable-but-non-culturable, a dormant state where prokaryotes survive stress but can't be cultured.
- Biofilm — Community of microorganisms in a self-produced matrix attached to a surface.
- Exopolysaccharide (EPS) — Sugar-rich matrix secreted by biofilm-forming microbes.
Action Items / Next Steps
- Review the table of extremophiles and their growth conditions.
- Consider why bacteria in biofilms resist antibiotics more than free-living bacteria.
- Prepare for discussion or quiz on prokaryotic diversity and biofilm ecology.