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Chapter 20: The Diversity of Prokaryotes and Viruses

Introduction

This chapter explores the diversity, structure, metabolism, and ecological roles of prokaryotes (Bacteria and Archaea) and viruses. Prokaryotes are the most abundant and diverse organisms on Earth, occupying nearly every habitat and playing essential roles in ecosystems and human health.

Three Domains of Life

Bacteria, Archaea, and Eukarya

  • Bacteria and Archaea are both prokaryotic, lacking membrane-bound organelles.

  • Eukarya includes all eukaryotic organisms (plants, animals, fungi, protists).

  • Bacteria and Archaea share a common ancestor but are fundamentally different in cell structure and genetics.

20.1 Which Organisms Are Members of the Domains Archaea and Bacteria?

Prokaryotic Diversity and Abundance

  • Prokaryotes are the most abundant group of organisms on Earth.

  • They are found in diverse environments, including the human body (mouth, gut, skin).

  • Prokaryotes have three common shapes due to their rigid cell wall:

    • Spherical (cocci): e.g., Streptococcus pyogenes, Staphylococcus aureus

    • Rod-like (bacilli): e.g., Bacillus subtilis

    • Spiral (spirilla): e.g., Vibrio cholerae

Bacterial Cell Structure

  • Key components: flagellum, pilus, capsule, cell wall, plasma membrane, nucleoid (DNA), cytoplasm, ribosomes.

Differences Between Bacteria and Archaea

  • Bacteria have cell walls made of peptidoglycan; Archaea have cell walls without peptidoglycan, often with polysaccharides.

  • Membrane lipids, ribosomal RNA, and enzymes differ between the two domains.

  • Superficially similar, but structurally and biochemically distinct.

Table: Differences Between Organisms in the Domains Archaea and Bacteria

Feature

Archaea

Bacteria

Cell Wall Composition

No peptidoglycan; polysaccharides

Peptidoglycan

Membrane Lipids

Branched hydrocarbons

Unbranched fatty acids

RNA Polymerase

Several types

One type

Initiator Amino Acid for Protein Synthesis

Methionine

Formylmethionine

Introns in Genes

Present in some genes

Absent

Response to Antibiotics

Resistant

Usually inhibited

Classification of Prokaryotes

  • Modern classification relies on DNA sequence data.

  • Traditional methods used morphological and biochemical features (e.g., Gram staining).

20.2 How Do Prokaryotes Survive and Reproduce?

Adaptations and Motility

  • Prokaryotes are highly adaptable, occupying nearly every habitat.

  • Many are motile, using flagella for movement.

  • Flagella may be single, paired, or scattered over the cell surface.

  • Bacterial flagella are structurally different from eukaryotic flagella; they rotate like a wheel.

Quorum Sensing and Biofilms

  • Bacteria can estimate their numbers using quorum sensing—the release and detection of signaling molecules.

  • Quorum sensing can trigger toxin production or bioluminescence.

  • Biofilms are communities of prokaryotes embedded in a sticky matrix, often resistant to disinfectants and antibiotics.

  • Dental plaque is a common biofilm formed by bacteria in the mouth.

Endospores

  • Some bacteria (e.g., Bacillus, Clostridium) form endospores—dormant, tough structures that protect against extreme conditions.

  • Endospores can survive boiling, radiation, and desiccation; some remain viable for millions of years.

  • Endospores present a challenge in food safety due to their resistance and potential for toxin accumulation.

Metabolism of Bacteria and Archaea

  • Prokaryotes exhibit diverse metabolic strategies:

    • Phototrophic: Use light energy (e.g., cyanobacteria).

    • Chemotrophic: Use chemical energy from organic or inorganic compounds.

    • Autotrophic: Use CO2 as a carbon source.

    • Heterotrophic: Use organic compounds as carbon source.

Types of Metabolism

Type

Energy Source

Carbon Source

Photoautotroph

Light

CO2

Chemoautotroph

Inorganic chemicals

CO2

Photoheterotroph

Light

Organic compounds

Chemoheterotroph

Organic chemicals

Organic compounds

Environmental Adaptations

  • Prokaryotes occupy extreme environments:

    • Thermophiles: High temperature (hot springs)

    • Halophiles: High salt concentration

    • Barophiles: High pressure (deep sea)

    • Psychrophiles: Cold environments (Antarctic sea ice)

Oxygen Requirements

  • Strict aerobes: Require O2 (e.g., humans)

  • Strict anaerobes: Killed by O2 (e.g., Clostridium tetani)

  • Facultative anaerobes: Can switch between aerobic and anaerobic respiration

Types of Cooperation in Microbes and Hosts

Symbiotic Relationships

  • Mutualistic: Both partners benefit (e.g., Rhizobium in legume roots, E. coli in human gut).

  • Parasitic: One benefits, one is harmed (e.g., pathogenic E. coli, Vibrio cholerae).

  • Commensal: One benefits, the other is unaffected.

  • Predation: Some bacteria actively prey on others.

Figures and Case Studies

Case Study: Unwelcome Dinner Guests

  • Foodborne illness caused by contaminated food (e.g., bacteria in burritos).

  • Symptoms can be severe, including paralysis and death.

  • Bacteria can grow in a wide variety of foods, making food safety a major concern.

Figures

  • Bacterial cell structure: Shows key components of a typical bacterium.

  • Three common prokaryote shapes: Cocci, bacilli, spirilla.

  • Cause of tooth decay: Biofilm-forming bacteria.

  • Spore formation: Protective endospores in bacteria.

  • Photosynthetic prokaryotes: Cyanobacteria and their role in oxygen production.

  • Extreme environments: Thermophiles in hot springs.

Key Terms and Concepts

  • Prokaryote: Unicellular organism lacking a nucleus and membrane-bound organelles.

  • Archaea: Domain of prokaryotes distinct from bacteria, often found in extreme environments.

  • Bacteria: Domain of prokaryotes with peptidoglycan cell walls.

  • Endospore: Dormant, resistant structure formed by some bacteria.

  • Biofilm: Community of microorganisms embedded in a self-produced matrix.

  • Quorum sensing: Cell-to-cell communication in bacteria to coordinate behavior.

  • Flagellum: Whip-like structure used for motility.

  • Symbiosis: Interaction between two different organisms living in close physical association.

Equations and Scientific Notation

  • Generalized equation for aerobic respiration:

  • Photosynthesis in cyanobacteria:

Summary

Prokaryotes are essential to life on Earth, exhibiting remarkable diversity in structure, metabolism, and ecological roles. Understanding their biology is crucial for fields ranging from medicine to environmental science.

Additional info: Some context and examples were inferred to provide a complete, self-contained study guide suitable for exam preparation.

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