Bacteria and Archaea: Diversity, Structure, and Ecological Roles

Introduction

Bacteria and Archaea are two of the three domains of life, representing the prokaryotic organisms. They are highly diverse, abundant, and play essential roles in Earth's ecosystems. This guide explores their differences, evolutionary relationships, ecological functions, genetic exchange mechanisms, structural diversity, and nutritional strategies.

Domains of Life and Evolutionary Relationships

The Three Domains

• Bacteria: Also called Eubacteria, these are single-celled prokaryotes with diverse forms and metabolisms.

• Archaea: Prokaryotes more closely related to Eukarya than to Bacteria, often found in extreme environments.

• Eukarya: Organisms with a true nucleus and membrane-bound organelles.

Prokaryotes (Bacteria and Archaea) lack a nucleus and membrane-bound organelles. Despite their similarities, Archaea share several features with Eukarya, such as certain ribosomal proteins and the presence of histones.

Evolutionary Relationships

• Archaea and Eukarya share a more recent common ancestor with each other than with Bacteria.

• Endosymbiotic theory: Mitochondria and plastids in eukaryotes originated from bacteria through endosymbiosis.

Structural Features of Prokaryotes

Cell Size and Shape

• Prokaryotes are typically 1–5 μm in diameter.

• Major shapes include:

  • Coccus: Spherical

  • Bacillus: Rod-shaped

  • Spiral: Includes flexible spirochaetes and rigid spirilli

Cell Wall Structure

• Most prokaryotes have a rigid cell wall outside the plasma membrane, maintaining shape and protecting against lysis.

• Bacteria:

  • Gram-positive: Thick peptidoglycan wall, stains purple, sensitive to penicillin.

  • Gram-negative: Thin peptidoglycan wall, outer membrane, more resistant to penicillin.

• Archaea: Cell walls lack peptidoglycan; have unique membrane lipids (ether linkages) that confer resistance to extreme conditions.

Internal Structure

• Prokaryotes lack membrane-bound organelles.

• Archaea have histone proteins and nucleosome-like structures, similar to eukaryotes.

Ecological Roles of Prokaryotes

Decomposers

• Break down dead organisms, releasing minerals and nutrients for reuse in ecosystems.

Producers

• Photosynthetic bacteria (e.g., Cyanobacteria) generate oxygen and organic carbon.

• Prochlorococcus species contribute to about 20% of atmospheric oxygen.

Nitrogen Fixers and Nitrifiers

• Nitrogen fixation: Conversion of atmospheric nitrogen () into ammonia (), making nitrogen available to plants.

• Nitrifying bacteria: Convert ammonia to nitrite () and nitrate (), further supporting plant nutrition.

Pathogens

• Some bacteria cause diseases such as cholera, leprosy, tetanus, pneumonia, and tooth decay.

Symbioses

• Mutualism: Both partners benefit (e.g., bioluminescent bacteria in squid, gut bacteria in humans).

• Commensalism: One benefits, the other is unaffected.

• Parasitism: One benefits at the expense of the other (e.g., pathogenic bacteria).

Mechanisms of Horizontal Gene Transfer

Horizontal gene transfer (HGT) is the movement of genetic material between organisms other than by descent. It is widespread in prokaryotes and has played a major role in their evolution.

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA via bacteriophages (viruses that infect bacteria).

• Conjugation: Direct transfer of DNA between cells through a pilus.

HGT allows rapid adaptation and the spread of traits such as antibiotic resistance.

Classification by Nutrition and Oxygen Response

Modes of Nutrition

• Photoautotroph: Use light as energy source and CO2 as carbon source (e.g., cyanobacteria).

• Photoheterotroph: Use light as energy source and organic compounds as carbon source.

• Chemoautotroph: Use inorganic chemicals as energy source and CO2 as carbon source.

• Chemoheterotroph: Use organic compounds for both energy and carbon (most bacteria).

Oxygen Response Types

• Obligate aerobes: Require oxygen for survival.

• Facultative aerobes: Can use oxygen but can also survive without it.

• Obligate anaerobes: Cannot tolerate oxygen.

• Aerotolerant anaerobes: Do not use oxygen but are not harmed by it.

Specialized Prokaryotes: Extremophiles

• Extremophiles: Organisms that thrive in extreme environments.

• Halophiles: Live in high-salt environments.

• Thermophiles: Live in high-temperature environments (e.g., hot springs).

• Methanogens: Produce methane as a metabolic byproduct, often found in anaerobic environments.

Prokaryote Reproduction

• Reproduce asexually by binary fission, a process of cell division resulting in two identical cells.

• Rapid cell division allows for quick population growth and adaptation.

Key Terms and Definitions

Applications and Importance

• Prokaryotes are essential for nutrient cycling, food webs, and ecosystem functioning.

• Some bacteria are used in biotechnology, such as plastic degradation and oil spill cleanup.

• Understanding prokaryotes helps address health, environmental, and industrial challenges.

Additional info:

• Prokaryotes are estimated to be 10 times more abundant than human cells in the body.

• Horizontal gene transfer is estimated to have affected up to 80% of prokaryotic genes at some point in their history.