BIOL_108_Wi25_10_Prokaryotes

Overview of Prokaryotes

Prokaryotes are microscopic organisms that constitute approximately 14% of global biomass and are predominantly small, unicellular cells lacking a nucleus and membrane-bounded organelles. Estimated at 10^11 prokaryotes in a handful of soil, they are highly adaptable, found in diverse ecosystems, including the human gut and extreme environments.

Definition and Classification

• Earliest Life Forms: Prokaryotes evolved into two domains: Archaea and Bacteria.

• Characteristics:

  • Small size (0.5–5 µm), typically unicellular.

  • Lack of nucleus; circular genome free in the cell.

  • No membrane-bounded organelles.

  • Various shapes: spheres, rods, spirals.

Prokaryotic Adaptations

Prokaryotes exhibit remarkable structural and functional adaptations that contribute to their survival and success in various environments:

• Cell-Surface Structures: These structures play a critical role in protection, adherence, and interaction with the surrounding environment. The presence of a cell wall composed of peptidoglycan in bacteria helps maintain their shape and prevent lysis in hypotonic conditions, whereas archaea have cell walls composed of distinct compounds without peptidoglycan.

• Gram Staining: This method differentiates bacteria based on their cell wall structure. Gram-positive bacteria have a thick peptidoglycan layer, retaining the crystal violet stain, while Gram-negative bacteria possess a thinner peptidoglycan layer and an outer lipopolysaccharide membrane, which renders them more resistant to certain antibiotics.

• Capsules: These viscous layers surrounding some bacteria provide additional protection against phagocytosis by immune cells and enhance their ability to adhere to surfaces, critical for colonization and infection.

• Fimbriae and Pili: These hair-like structures enable bacteria to attach to host cells and surfaces, facilitating colonization and horizontal gene transfer.

• Endospores: Specialized structures formed by some bacteria that allow them to withstand extreme environmental conditions, including heat, desiccation, and nutrient depletion, ensuring their survival until favorable conditions return.

Prokaryotic Metabolic Diversity

Prokaryotes demonstrate significant metabolic diversity, which enables them to exploit a wide range of energy and carbon sources:

• Phototrophs: Organisms that capture light energy to produce organic compounds through photosynthesis. They thrive in environments rich in sunlight.

• Chemotrophs: Organisms that obtain energy by oxidizing chemical compounds. They can be further divided into:

  • Chemoautotrophs: Using inorganic molecules (like hydrogen sulfide) for energy and carbon fixation.

  • Chemoheterotrophs: Relying on organic compounds for both energy and carbon.

• Autotrophs: Organisms that synthesize organic compounds from inorganic sources, essential for maintaining ecosystem nutrient cycles.

• Heterotrophs: Depend on organic molecules from other organisms; they play crucial roles in decomposing organic matter and recycling nutrients in ecosystems.

• Notably, some prokaryotes can fix nitrogen, transforming atmospheric nitrogen into forms usable by living organisms, thereby playing a crucial role in the nitrogen cycle.

Prokaryotic Genetic Variation

Prokaryotes exhibit a high degree of genetic variation, which enables them to adapt rapidly to changing environments:

• Asexual Reproduction: Prokaryotes primarily reproduce by binary fission, a process that allows rapid population growth under favorable conditions, leading to generation times as short as 20 minutes.

• Mutations: Despite relatively low mutation rates, the rapid pace of reproduction means that mutations can accumulate quickly, providing a source of genetic diversity that may result in new traits beneficial for survival and adaptation.

• Genetic Recombination: Prokaryotes can exchange genetic material through:

  • Transformation: Uptake of free DNA from the environment.

  • Transduction: Transfer of DNA between bacteria via bacteriophages.

  • Conjugation: Direct transfer of DNA between two bacterial cells through a pilus, allowing the exchange of plasmids or chromosomal fragments.

Major Lineages and Groups

• Domain Bacteria:

  • Diverse; includes pathogenic and beneficial varieties.

  • Five major groups include:

    • Proteobacteria

    • Chlamydias

    • Spirochetes

    • Cyanobacteria

    • Gram-Positive Bacteria

• Domain Archaea:

  • Extremophiles found in harsh environments and resistant to certain antibiotics. Notable groups include:

    • Methanogens

    • Halophiles

    • Thermophiles

Ecological Roles and Symbiosis

Prokaryotes play vital roles in oxygen production, nutrient recycling, and as decomposers. They engage in symbiotic relationships such as:

• Mutualism (both benefit)

• Commensalism (one benefits, other unaffected)

• Parasitism (one benefits at the expense of another)

Applications in Research and Technology

Prokaryotes are utilized in food production, bioremediation, sewage treatment, and genetic engineering; an example is Taq polymerase from Thermus aquaticus used in PCR.

Definitions

• Peptidoglycan: A polymer that forms the cell wall of bacteria.

• Gram Staining: A method for classifying bacteria based on cell wall composition.

• Capsule: A layer surrounding some bacteria aiding in protection and adherence.

• Fimbriae/Pili: Appendages for adhesion and genetic exchange.

• Endospores: Dormant, tough forms that survive extreme conditions.

• Plasmids: Circular DNA molecules within prokaryotes granting genetic advantages.

• Transformation: Uptake of DNA from the environment.

• Transduction: DNA transfer between bacteria via bacteriophages.

• Conjugation: Direct transfer of DNA between two bacteria.

• Photoautotrophy: Energy derived from light to synthesize organic compounds.

• Chemoautotrophy: Energy derived from chemical reactions involving inorganic substances.

• Photoheterotrophy: Organisms utilizing light for energy but requiring organic compounds for growth.

• Chemoheterotrophy: Organisms obtaining energy and carbon from organic compounds.

• Symbiosis: Interaction between two different organisms.

• Mutualism: A symbiotic relationship where both organisms benefit.

• Commensalism: A symbiotic relationship where one organism benefits while the other is unaffected.

• Parasitism: A symbiotic relationship where one organism benefits at the other's expense.