Bacteria, Archaea, and Protists: Structure, Diversity, and Evolution

Chapter 27 – Bacteria and Archaea

Structural Adaptations of Prokaryotes

Prokaryotes, which include Bacteria and Archaea, exhibit a variety of structural adaptations that contribute to their survival in diverse environments.

• Cell Walls: Most prokaryotes have a cell wall that provides shape and protection. In bacteria, the cell wall contains peptidoglycan, a polymer of sugars and amino acids. Archaea lack peptidoglycan and instead have unique polysaccharides and proteins.

• Capsules: Many prokaryotes secrete a sticky outer layer called a capsule, which aids in adherence to surfaces and evasion of host immune responses.

• Fimbriae: These are hair-like appendages that allow prokaryotes to attach to surfaces or other cells.

• Flagella: Long, whip-like structures used for movement. Prokaryotic flagella differ structurally from eukaryotic flagella.

Bacteria vs. Archaea: Cell Wall and Membrane Differences

Bacteria and Archaea are both prokaryotes but differ in several key structural features:

• Cell Wall Composition: Bacterial cell walls contain peptidoglycan; archaeal cell walls do not.

• Membrane Lipids: Bacteria have unbranched fatty acid chains in their membrane lipids, while Archaea have branched hydrocarbons and ether linkages, contributing to their ability to survive extreme environments.

Gram-Positive vs. Gram-Negative Bacteria

The Gram stain differentiates bacteria based on cell wall structure:

• Gram-Positive: Thick peptidoglycan layer; stains purple. More susceptible to certain antibiotics.

• Gram-Negative: Thin peptidoglycan layer and an outer membrane; stains pink. Often more resistant to antibiotics due to the outer membrane acting as a barrier.

Genetic Diversity Mechanisms in Prokaryotes

Prokaryotes generate genetic diversity through several mechanisms:

• Mutation: Random changes in DNA sequence; a major source of genetic variation.

• Transformation: Uptake of foreign DNA from the environment.

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

• Conjugation: Direct transfer of DNA between two prokaryotic cells via a pilus.

Metabolic Diversity in Prokaryotes

Prokaryotes display remarkable metabolic diversity, classified by energy and carbon sources:

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

• Chemoautotrophs: Obtain energy from inorganic chemicals and use CO2 as a carbon source.

• Heterotrophs: Require organic compounds for both energy and carbon.

• Oxygen Requirements: Prokaryotes may be obligate aerobes (require O2), obligate anaerobes (poisoned by O2), or facultative anaerobes (can survive with or without O2).

Major Clades of Bacteria and Archaea

Prokaryotes are classified into major clades, some of which include extremophiles and nitrogen-fixing species:

• Extremophiles: Archaea that thrive in extreme environments (e.g., thermophiles, halophiles).

• Nitrogen-Fixing Species: Convert atmospheric nitrogen (N2) into ammonia (NH3), making nitrogen available to plants.

Ecological Roles of Prokaryotes

Prokaryotes play essential roles in ecosystems:

• Decomposers: Break down dead organic matter, recycling nutrients.

• Symbionts: Live in close association with other organisms, often providing mutual benefits (e.g., gut bacteria in humans).

• Pathogens: Cause diseases in plants, animals, and humans.

• Chemical Recyclers: Participate in biogeochemical cycles, such as the nitrogen and carbon cycles.

Chapter 28 – Protists

Overview of Protists

Protists are a diverse group of mostly unicellular eukaryotes. They exhibit a wide range of structural and functional diversity and are found in nearly all environments where life exists.

• Most eukaryotes are protists.

• Protists can be autotrophic, heterotrophic, or mixotrophic.

Endosymbiosis and the Origin of Mitochondria and Plastids

The endosymbiotic theory explains the origin of mitochondria and plastids (e.g., chloroplasts) in eukaryotic cells:

• Primary Endosymbiosis: A eukaryotic cell engulfed a prokaryotic cell, which became a mitochondrion or plastid.

• Secondary Endosymbiosis: A eukaryotic cell engulfed another eukaryotic cell that already contained a plastid, leading to further diversification (e.g., in red and green algae).

Major Supergroups of Eukaryotes

Protists are classified into five major supergroups based on molecular and morphological evidence:

• Excavata: Includes diplomonads and euglenozoans, often with modified mitochondria.

• SAR: Includes Stramenopiles (e.g., diatoms, brown algae), Alveolates (e.g., apicomplexans), and Rhizaria.

• Archaeplastida: Includes red algae, green algae, and land plants.

• Unikonta: Includes amoebozoans and opisthokonts (animals and fungi).

• Rhizaria: Sometimes grouped within SAR; includes many amoeboid protists.

Key Protist Clades

• Excavata: Characterized by a feeding groove; includes diplomonads (e.g., Giardia) and euglenozoans (e.g., Euglena).

• SAR:

  • Diatoms: Unicellular algae with silica cell walls.

  • Brown Algae: Multicellular, marine, includes kelps.

  • Apicomplexans: Parasitic, includes Plasmodium (malaria agent).

• Archaeplastida: Red and green algae; green algae are closest relatives to land plants.

• Unikonta: Includes amoebozoans (e.g., Amoeba) and opisthokonts (animals, fungi).

Protist Nutrition

Protists display diverse nutritional strategies:

• Photoautotrophs: Use light to synthesize organic molecules.

• Heterotrophs: Ingest or absorb organic molecules.

• Mixotrophs: Combine photosynthesis and heterotrophic nutrition.

Protist Life Cycles

Protists exhibit a variety of life cycles, including:

• Alternation of Generations: Alternating multicellular haploid and diploid forms (e.g., in some algae).

• Sexual and Asexual Reproduction: Many protists can reproduce both sexually and asexually, increasing their adaptability.

Example: The malaria parasite (Plasmodium) has a complex life cycle involving both sexual and asexual stages in different hosts.