Chapter 13.1: Prokaryotic Diversity – Structure, Evolution, and Impact

Chapter 13: Diversity of Microbes, Fungi, and Protists

13.1 Prokaryotic Diversity

This section explores the evolutionary history, structure, ecological roles, and medical significance of prokaryotes, focusing on bacteria and archaea. Prokaryotes are among the most ancient and diverse life forms on Earth, thriving in a wide range of environments and playing essential roles in nutrient cycling, disease, and biotechnology.

Introduction to Prokaryotes

• Prokaryotes are present everywhere there is sufficient moisture, living on and inside other organisms, including humans.

• They thrive in environments inhospitable to most life forms and recycle essential nutrients, driving the evolution of new ecosystems.

Evolutionary History and Early Life on Earth

• Prokaryotes were the first forms of life on Earth, existing for billions of years before plants and animals.

• Earth is about 4.54 billion years old, with the earliest record of life dating back to about 3.5 billion years ago (microbial mats).

• Early Earth’s atmosphere was anoxic (lacking oxygen), so only anaerobic organisms could live.

• Phototrophs (organisms that convert solar energy into chemical energy) appeared about one billion years after Earth’s formation; cyanobacteria (blue-green algae) later began the oxygenation of the atmosphere, enabling the evolution of other life forms.

Adaptations and Extremophiles

• Early prokaryotes adapted to high temperatures and harsh conditions, such as strong radiation and volcanic activity.

• Extremophiles are prokaryotes that thrive in extreme environments (temperature, pH, salinity, pressure, radiation, etc.).

• Many extremophiles cannot survive in moderate environments, but their study opens possibilities for new drugs and industrial applications.

Biofilms

• A biofilm is a structured community of prokaryotes held together by a gummy matrix of polysaccharides, proteins, and nucleic acids.

• Biofilms grow attached to surfaces and are difficult to destroy, causing issues like pipe clogging and food contamination.

Characteristics of Prokaryotes

• All cells share four common structures:

  • Plasma membrane

  • Cytoplasm

  • Genetic material (DNA/RNA)

  • Ribosomes

• Prokaryotes come in three main shapes:

  • Cocci (spherical)

  • Bacilli (rod-shaped)

  • Spirilli (spiral-shaped)

The Prokaryotic Cell

• Prokaryotes are unicellular and lack membrane-bound organelles.

• They have a single circular chromosome located in the nucleoid region of the cell.

• Some have additional structures:

  • Capsule (protection and attachment)

  • Flagella (motility)

  • Pili (attachment and conjugation)

  • Plasmids (small, circular DNA pieces)

Cell Wall Structure

• Most prokaryotes have a cell wall outside the plasma membrane, providing shape and protection.

• Bacterial cell walls contain peptidoglycan; archaea have different chemical compositions.

• Bacteria are classified as Gram-positive (thick peptidoglycan layer) or Gram-negative (thin layer, outer membrane) based on Gram staining.

Comparison: Eubacteria vs Archaebacteria

Archaebacteria	Eubacteria
Often found in extreme environments	Found in moderate environments
Cell wall lacks peptidoglycan	Cell wall contains peptidoglycan
Membrane lipids are branched	Membrane lipids are unbranched
Genetic machinery resembles eukaryotes	Genetic machinery is distinct

Reproduction in Prokaryotes

• Prokaryotes reproduce asexually by binary fission:

$\text{Parent Cell} \rightarrow \text{Replication of DNA} \rightarrow \text{Segregation of DNA} \rightarrow \text{Cell Splitting into Two}$

• Binary fission does not provide genetic recombination, but prokaryotes can exchange DNA via:

  • Transformation (uptake of DNA from environment)

  • Transduction (virus-mediated DNA transfer)

  • Conjugation (direct transfer via pili)

How Prokaryotes Obtain Energy and Carbon

• Prokaryotes are metabolically diverse and fill many functional niches:

• They participate in nutrient cycles (nitrogen, carbon), decompose dead organisms, and grow inside living hosts.

• Energy sources:

  • Phototrophs: use sunlight

  • Chemotrophs: use chemical compounds (e.g., sugars, lactose)

Bacterial Diseases in Humans

• Pathogenic prokaryotes (mainly bacteria) have caused devastating diseases and plagues throughout history.

• There are no known pathogenic Archaea.

• Improved sanitation, clean food, and water have reduced disease transmission.

Historical Perspective: Pandemics and Antibiotics

• Records of infectious diseases date back to 1,000 B.C.

• Pandemics like the plague of Justinian and the Black Death were caused by Yersinia pestis (bacterium carried by fleas).

• Antibiotics have reduced mortality rates, but misuse has led to antibiotic-resistant bacteria (e.g., MRSA).

The Antibiotic Crisis

• Antibiotics are chemicals that inhibit the growth of other organisms.

• Overuse and incorrect use of antibiotics have led to the evolution of superbugs resistant to treatment.

• Only resistant forms survive and reproduce, increasing the proportion of resistant bacteria.

Foodborne Diseases

• Prokaryotes readily colonize food surfaces, causing foodborne illnesses through contamination by bacteria, viruses, molds, and parasites.

• Proper sterilization and canning procedures reduce the incidence of these diseases.

Summary Table: Prokaryotic Cell Features

Feature	Prokaryotes	Eukaryotes
Nucleus	No	Yes
Organelles	No	Yes
Chromosome	Single, circular	Multiple, linear
Cell Wall	Peptidoglycan (bacteria), varied (archaea)	Cellulose (plants), chitin (fungi)

Example: MRSA (Methicillin-resistant Staphylococcus aureus)

• Staphylococcus aureus is a common bacterium that can live on and in the human body.

• MRSA is resistant to many antibiotics and poses a significant health risk, especially in healthcare settings.

Additional info: These notes cover the foundational aspects of prokaryotic diversity, including evolutionary history, cell structure, ecological roles, and medical relevance, as outlined in a General Biology college course.