Bacteria and Archaea: Structure, Diversity, and Roles in Biology

Chapter 27: Bacteria and Archaea

Three Domains of Life

The classification of life is based on three major domains: Bacteria, Archaea, and Eukarya. This system, proposed by Woese in 1977, reflects evolutionary relationships among all living organisms.

• Prokaryotes: Includes Bacteria and Archaea; lack a nucleus and membrane-bound organelles.

• Eukaryotes: Includes all multicellular life and protists; possess a nucleus and organelles.

• Protists are paraphyletic (ancestor and some descendants), while multicellular life is polyphyletic (multiple independent origins).

Prokaryotes: Two Major Clades

Prokaryotes are divided into two clades: Bacteria and Archaea. These groups share ancestral characteristics and provide insight into early life on Earth.

• Modern prokaryotes help us understand evolutionary transitions to eukaryotes.

• Prokaryotes have existed on Earth from approximately 3.6 to 2.1 billion years ago.

Prokaryote Body Plan

Prokaryotes are among the smallest and most abundant organisms on Earth, with three basic shapes:

• Spherical (cocci)

• Rod-shaped (bacilli)

• Spiral

They are difficult to classify due to their vast diversity and the limitations of traditional identification methods.

Cell Walls and Gram Staining

Most prokaryotes have a cell wall that maintains shape and protects the cell. Bacterial cell walls are primarily composed of peptidoglycan.

• Gram-positive bacteria: Thick peptidoglycan layer; stains purple (e.g., Staphylococcus aureus).

• Gram-negative bacteria: Thin peptidoglycan layer and outer membrane; stains pink (e.g., Escherichia coli).

Adhesion and Locomotion

Prokaryotes possess specialized structures for movement and attachment:

• Capsule: Sticky coat for protection and adhesion.

• Endospore: Highly resistant, dormant cell form.

• Fimbriae: Hair-like appendages for attachment.

• Flagella: Used for locomotion; can be located at various positions on the cell.

• Taxis: Directed movement toward or away from stimuli (e.g., chemotaxis, phototaxis).

Cellular Organization

Prokaryotes lack compartmentalization but perform all essential cellular functions:

• Single circular chromosome located in the nucleoid region.

• May contain plasmids: small, independently replicating DNA molecules.

• No nucleus or membrane-bound organelles.

Energy and Carbon Sources

Prokaryotes exhibit diverse metabolic strategies, classified by energy and carbon sources:

	Energy Source: Light	Energy Source: Chemical
Carbon Source: CO2	Photoautotrophs photosynthetic prokaryotes, protists, plants	Chemoautotrophs certain prokaryotes
Carbon Source: Organic Compounds	Photoheterotrophs some prokaryotes	Chemoheterotrophs many prokaryotes, fungi, animals

Oxygen Requirements

Prokaryotes vary in their oxygen requirements:

• Obligate aerobes: Require oxygen for respiration.

• Obligate anaerobes: Poisoned by oxygen.

• Facultative anaerobes: Can use either aerobic or anaerobic metabolism.

Asexual Reproduction: Binary Fission

Prokaryotes reproduce rapidly by binary fission, leading to large populations and high mutation rates.

• Generation times can be as short as 20 minutes under optimal conditions.

• Large populations increase the likelihood of mutations, which drive evolution.

Genetic Variation: Recombination

Prokaryotes can acquire genetic variation through several mechanisms:

• Transformation: Uptake of DNA from the environment.

• Horizontal gene transfer: Movement of genes between species.

• Transduction: Viral transfer of DNA via bacteriophages.

Diversity of Prokaryotes

Prokaryotes are highly diverse, with many unknown species. Bacteria are better studied due to their medical relevance.

Diversity of Bacteria

• Proteobacteria: Includes nitrogen fixers, pathogens (e.g., Salmonella).

• Chlamydias: Intracellular parasites.

• Spirochetes: Helical bacteria, some cause disease (e.g., Lyme disease).

• Cyanobacteria: Photosynthetic, oxygen-producing.

• Gram-positive bacteria: Diverse, includes Actinomycetes, Mycoplasma.

Diversity of Archaea

• Many are extremophiles:

  • Halophiles: Thrive in high salt concentrations.

  • Thermophiles: Thrive in high temperatures.

  • Methanogens: Produce methane, obligate anaerobes.

Roles in Nutrient Cycling

Prokaryotes are essential for ecosystem function:

• Decomposers: Release carbon and nitrogen to the environment.

• Photosynthesis: Cyanobacteria produce oxygen.

• Nitrogen fixation: Convert atmospheric nitrogen ($N_2$) into ammonia ($NH_3$).

Other Interactions

Prokaryotes often form symbiotic relationships with other organisms:

• Mutualism: Both partners benefit (e.g., gut bacteria aid digestion).

• Commensalism: One benefits, the other is unaffected.

• Parasitism: Pathogens cause disease (e.g., cholera, tuberculosis).

Antibiotics and Prokaryote Differences

Antibiotics target unique prokaryotic features:

• Peptidoglycan: Targeted by penicillin.

• Ribosomes: Use different proteins than eukaryotes; targeted by tetracycline.

• Resistance can arise via mutations or R plasmids (genes for antibiotic-destroying enzymes).

Beneficial Uses of Bacteria

Bacteria have many practical applications:

• Food production: Cheese, yogurt.

• Bioremediation: Breakdown of sewage, chemical spills.

• Genetic engineering: Production of vitamins, antibiotics, and other chemicals.

Additional info: Bacteria and Archaea are foundational to understanding biological diversity, evolution, and ecosystem function. Their study is essential for medicine, biotechnology, and environmental science.