Bacteria and Archaea: Diversity, Structure, and Ecological Roles

Overview of the Three Domains of Life

The three domains of life—Bacteria, Archaea, and Eukarya—represent the major evolutionary lineages on Earth. Bacteria and Archaea are both prokaryotic, lacking a nucleus, while Eukarya includes all eukaryotic organisms.

• Bacteria: Single-celled prokaryotes, highly diverse, also called Eubacteria.

• Archaea: Single-celled prokaryotes, more closely related to Eukarya than to Bacteria.

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

• Evolutionary Relationships: Archaea share several molecular features with Eukarya, such as histones and certain ribosomal proteins.

Prokaryotic Cell Structure and Diversity

Prokaryotes are characterized by their small size (1–5 μm), simple cell structure, and rapid cell division. They exhibit a wide variety of shapes and arrangements.

• Major Shapes:

  • Coccus: Spherical

  • Bacillus: Rod-shaped

  • Spiral: Includes spirochaetes (flexible) and spirilli (rigid)

• Cell Wall Types:

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

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

• Membrane Linkages: Archaea have ether linkages, which are more resistant to extreme conditions than the ester linkages found in Bacteria and Eukarya.

Extremophiles and Specialized Prokaryotes

Many Archaea and some Bacteria are extremophiles, thriving in environments with extreme temperature, salinity, or acidity.

• Halophiles: Prefer high-salt environments.

• Thermophiles: Thrive in high-temperature environments, such as hot springs.

• Methanogens: Produce methane as a metabolic byproduct in anaerobic conditions.

Ecological Roles of Prokaryotes

Prokaryotes play essential roles in ecosystems as decomposers, producers, nitrogen fixers, and pathogens, and through various symbiotic relationships.

• Decomposers: Break down dead organic matter, releasing minerals for reuse.

• Producers: Cyanobacteria and other photosynthetic bacteria generate organic compounds and oxygen.

• Nitrogen Fixers: Convert atmospheric nitrogen () into ammonia (), making nitrogen available to plants.

• Pathogens: Some bacteria cause diseases such as cholera, leprosy, tetanus, and pneumonia.

• Symbiosis: Includes mutualism, commensalism, and parasitism.

Symbiotic Relationships

Prokaryotes often form close associations with other organisms, influencing health, development, and nutrient cycling.

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

• Commensalism: One organism benefits, the other is unaffected.

• Parasitism: One organism benefits at the expense of the other.

Horizontal Gene Transfer in Prokaryotes

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.

• Conjugation: Direct transfer of DNA between cells via cell-to-cell contact.

• Transduction: Transfer of DNA via viral vectors (bacteriophages).

• Significance: HGT contributes to genetic diversity and adaptation, including the spread of antibiotic resistance.

Classification by Nutritional Type

Prokaryotes are classified based on their energy and carbon sources.

• Photoautotrophs: Use light energy and CO2 as a carbon source.

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

• Chemoautotrophs: Use inorganic chemicals for energy and CO2 as a carbon source.

• Chemoheterotrophs: Use organic compounds for both energy and carbon.

Classification by Oxygen Response

Prokaryotes differ in their requirements and tolerance for oxygen.

• 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 its presence.

Major Bacterial Groups

Bacteria are classified into many phyla, with Proteobacteria and Cyanobacteria being especially important.

• Cyanobacteria: Photosynthetic bacteria that generate oxygen and play a key role in carbon and nitrogen cycles.

• Proteobacteria: Diverse group including many important nitrogen fixers and pathogens.

Prokaryotes in the Global Carbon Cycle

Prokaryotes are crucial for the cycling of carbon and other elements in ecosystems.

• Producers: Cyanobacteria contribute significantly to atmospheric oxygen.

• Decomposers: Release minerals from dead organisms for reuse.

• Heterotrophic Bacteria: Increase trophic efficiency in food webs.

Prokaryote Reproduction: Binary Fission

Prokaryotes reproduce asexually by binary fission, a process that allows for rapid population growth.

• Binary Fission: One cell divides into two genetically identical daughter cells.

• Colony Formation: Used in laboratory methods to detect and count bacteria.

Definitions of Key Terms

• Halophile: Organism that thrives in high-salt environments.

• Extremophile: Organism adapted to extreme environmental conditions.

• Thermophile: Organism that thrives at high temperatures.

• Methanogen: Archaea that produce methane in anaerobic conditions.

• Decomposer: Organism that breaks down dead organic matter.

• Nitrogen Fixation: Conversion of atmospheric nitrogen () into ammonia ().

• Aerobic: Requires oxygen.

• Anaerobic: Does not require oxygen.

• Photoautotroph: Uses light energy and CO2 as a carbon source.

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

• Chemoautotroph: Uses inorganic chemicals for energy and CO2 as a carbon source.

• Chemoheterotroph: Uses organic compounds for both energy and carbon.

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA via viral vectors.

• Conjugation: Direct transfer of DNA between cells.

• Mutualism: Both partners benefit.

• Commensalism: One benefits, the other is unaffected.

• Parasitism: One benefits at the expense of the other.

Table: Comparison of Bacteria, Archaea, and Eukarya

Applications and Importance

• Biotechnology: Bacteria are used in genetic engineering, bioremediation, and industrial processes.

• Environmental Solutions: Some bacteria can degrade plastics and oil spills.

• Human Health: The human microbiome is essential for digestion, immunity, and overall health.

Example: Nitrogen Fixation Equation

The process of nitrogen fixation can be summarized as:

Additional info: Some context and definitions were expanded for clarity and completeness.