Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 27.3 Diverse Nutritional and Metabolic Adaptations Have Evolved in Prokaryotes
Prokaryotes display an extraordinary range of nutritional and metabolic strategies, enabling them to inhabit virtually every environment on Earth. This metabolic diversity surpasses that of eukaryotes and reflects billions of years of evolution.
Four Major Nutritional Modes
Organisms are classified based on:
- Energy source:
- Phototrophs use light.
- Chemotrophs use chemical compounds.
- Carbon source:
- Autotrophs use CO₂ or related compounds.
- Heterotrophs require organic nutrients.
These yield four combinations:
- Photoautotrophs
- Energy: Light
- Carbon: CO₂
- Examples: Cyanobacteria, algae, plants
- Chemoautotrophs
- Energy: Inorganic chemicals (e.g., H₂S, NH₃, Fe²⁺)
- Carbon: CO₂
- Unique to some prokaryotes (e.g., Sulfolobus)
- Photoheterotrophs
- Energy: Light
- Carbon: Organic compounds
- Unique to some prokaryotes (e.g., Rhodobacter)
- Chemoheterotrophs
- Energy & Carbon: Organic compounds
- Includes many prokaryotes, fungi, animals, some protists and plants
Oxygen and Metabolism
- Obligate aerobes: Require O₂ for cellular respiration.
- Obligate anaerobes: Poisoned by O₂; use fermentation or anaerobic respiration (e.g., using NO₃⁻ or SO₄²⁻ as electron acceptors).
- Facultative anaerobes: Use O₂ when available, but can switch to anaerobic methods if necessary.
Nitrogen Metabolism
- Nitrogen is essential for amino acids and nucleic acids.
- Prokaryotes metabolize nitrogen in multiple forms:
- Nitrogen fixation: Conversion of N₂ to NH₃ (ammonia).
- Performed by cyanobacteria and some methanogens (archaea).
- Enables organisms to synthesize organic nitrogen compounds.
- Nitrogen fixation: Conversion of N₂ to NH₃ (ammonia).
- These processes enrich ecosystems by supplying usable nitrogen to other organisms (e.g., plants).
Metabolic Cooperation
- Some prokaryotes cooperate to exploit resources they couldn’t access alone:
- Anabaena (a cyanobacterium) forms filaments with:
- Photosynthetic cells: Produce O₂ and carbohydrates.
- Heterocysts: Specialized for nitrogen fixation, with thickened walls that exclude O₂.
- Cells exchange nutrients via intercellular connections.
- Anabaena (a cyanobacterium) forms filaments with:
- Biofilms:
- Surface-coating colonies of cooperating prokaryotes.
- Cells communicate, stick together, and form protective layers.
- Common in nature and human-made systems (e.g., pipes, medical devices).
- Difficult to remove and often resistant to antibiotics, posing serious health risks.
In a Nutshell
Prokaryotes exhibit diverse metabolic pathways unmatched by any other group of organisms. Their flexibility in energy and carbon sources, oxygen tolerance, nitrogen fixation, and cooperative behaviors like biofilm formation enables them to dominate diverse and extreme environments.