Prokaryotes- topic 10

Prokaryotes Overview

  • Prokaryotes are microscopic organisms that contribute significantly to global biomass.

    • They account for approximately 14% (around 77 Gt C) of global biomass, second only to plants which account for approximately 82%.

    • Prokaryotes are incredibly numerous despite their small size.

    • A handful of soil can contain around 101110^{11} prokaryotes, more than ten times Earth’s human population.

    • It is estimated that there are about 101010^{10} strains of prokaryotes on Earth, vastly outnumbering eukaryotic species.

    • Prokaryotes are highly adaptable and thrive in diverse environments, ranging from the human gut to extreme conditions such as deep underground, acidic lakes, hypersaline waters, and hot springs.

Definition of Prokaryotes

  • Earth’s earliest life forms were prokaryotes, which eventually evolved into two distinct domains: Archaea and Bacteria.

    • Both domains share a common ancestor and possess a simple prokaryotic cell structure.

    • Differences between prokaryotic and eukaryotic cells:

    • Prokaryotes are typically small (0.5–5 µm) and unicellular, although some species can form colonies.

    • Eukaryotic cells are generally larger (10–100 µm).

    • The small size of prokaryotes allows for rapid metabolism but limits their potential size.

    • Prokaryotes exhibit various shapes, including spheres, rods, and spirals.

    • Prokaryotes lack a nucleus; their circular genome is free within the cell (the term pro means first/before, and karyon means kernel, i.e., nucleus).

    • Prokaryotic cells lack membrane-bounded organelles.

Structural and Functional Adaptations

  1. Cell-Surface Structures

    • Most prokaryotes possess a cell wall that:

      • Maintains cell shape (providing structural support) and prevents bursting in hypotonic environments (lower solute concentration outside the cell).

      • Provides protection.

    • Bacterial cell walls contain peptidoglycan, a structural polymer consisting of modified sugars cross-linked by short polypeptides.

      • Peptidoglycan is unique to bacterial cell walls.

      • In contrast, Archaea cell walls are made of structural polysaccharides and proteins but lack peptidoglycan.

      • Eukaryotic cell walls (algae, plants, fungi) are comprised of cellulose or chitin.

    • Gram Staining:

      • Used to differentiate bacteria by cell wall composition:

      • Gram-positive bacteria have a simple cell wall with a thick peptidoglycan layer that traps crystal violet dye.

      • Gram-negative bacteria have a thinner peptidoglycan layer and an outer lipopolysaccharide membrane, which serves as a protective barrier, can contain toxins, and enhances resistance to certain antibiotics, making them often more pathogenic than Gram-positive bacteria.

    • Many prokaryotes have a capsule, a sticky layer of polysaccharides or proteins surrounding the cell wall.

      • Capsules help prokaryotes adhere to surfaces and each other, forming adhesive biofilms (e.g., dental plaque).

      • Capsules protect against desiccation and help some pathogenic bacteria evade the immune system.

    • Certain prokaryotes possess fimbriae, which are short, hair-like appendages that help cells adhere to surfaces.

    • Distinct from fimbriae, pili (or sex pili) are longer and facilitate the exchange of DNA between prokaryotic cells, enhancing genetic diversity.

  2. Endospores

    • Many prokaryotes form metabolically inactive endospores, which can survive extreme conditions for long periods (up to centuries).

      • Endospores are formed when environmental conditions become unfavorable, such as during extreme temperatures or nutrient deprivation.

      • They have tough, protective coatings that shield cells from extreme heat, desiccation, and chemical stress, allowing prokaryotes to remain dormant until conditions improve.

      • This strategy ensures the survival of prokaryotic organisms through periods of adversity, contributing to their long-term persistence in diverse and changing environments.

  3. Motility

    • About half of all prokaryotes are motile (capable of active movement) often using flagella for locomotion.

      • Flagella are long, whip-like appendages distributed across the cell surface or concentrated at one or both ends.

      • Flagella can enable movement up to 50 body lengths per second.

      • Bacterial flagella differ structurally from those of archaea and eukaryotes.

    • Prokaryotes respond to environmental stimuli through a behavior known as taxis:

      • For example, positive phototaxis refers to movement toward light.

  4. Simple Internal Organization and DNA

    • Prokaryotes have uncomplicated internal structures without complex compartmentalization.

    • They lack membrane-enclosed organelles (e.g., distinct nuclei, mitochondria, chloroplasts).

    • Some prokaryotes have specialized membranes for specific metabolic functions, which are typically infoldings of the plasma membrane; for example, cyanobacteria contain thylakoid membranes used for photosynthesis.

    • Genetic Structure:

      • Prokaryotes possess small genomes characterized by a single circular chromosome that is not membrane-contained; instead, it is enclosed in a nucleoid.

      • Prokaryotes also have plasmids, additional small circular DNA molecules that carry extra genes, providing greater genetic versatility.

Prokaryotic Reproduction and Genetic Diversity

  • Prokaryotes reproduce rapidly via binary fission, a form of asexual reproduction where a single prokaryotic cell divides into two identical cells.

    • Binary fission allows for rapid population growth in favorable environments.

    • For example, under optimal conditions, bacteria such as Escherichia coli can divide once every 20 minutes.

    • The rapid reproduction is facilitated by their small cell size, simple genomes, and the efficiency of binary fission, which is less complex than eukaryotic cell division (mitosis).

Factors Contributing to Prokaryotic Genetic Diversity

  1. Rapid Reproduction

    • Results in many generations in a short time, contributing to genetically diverse populations.

  2. Mutations

    • Mutations accumulate quickly in prokaryotic populations.

      • The per-cell-division mutation rates in prokaryotes are similar to or lower than those in multicellular eukaryotes, but due to quick generation times and large population sizes, prokaryotes generate more mutations per unit time.

      • Combined effects facilitate rapid evolution, showcasing that prokaryotes are highly evolved organisms, well-adapted to diverse settings.

  3. Genetic Recombination

    • Prokaryotes undergo genetic recombination, incorporating genetic material from one cell into another.

      • Unlike eukaryotes, prokaryotic genetic recombination does not occur through meiosis.

      • Genetic recombination in prokaryotes typically requires horizontal gene transfer (HGT), as binary fission involves only vertical inheritance without recombination.

    • Horizontal Gene Transfer (HGT):

      • The movement of genetic material between prokaryotic cells through mechanisms other than parent-to-offspring inheritance, including transformation, transduction, and conjugation.

      • Transferred DNA may recombine with the recipient genome or exist as an independent genetic element (like a plasmid).

Modes of Prokaryotic Genetic Recombination

  1. Transformation

    • The process where a prokaryotic cell takes up and integrates foreign DNA from its environment, often from dead prokaryotes.

    • Involves specific cell-surface proteins that help recognize and transport DNA.

  2. Transduction

    • The transfer of DNA via bacteriophages that infect prokaryotes.

  3. Conjugation

    • The direct transfer of DNA (usually a plasmid) between cells via a pilus.

      • During conjugation, the donor cell attaches to the recipient cell using a pilus to facilitate DNA transfer.

      • This process increases genetic diversity since the transferred DNA can recombine with the recipient's chromosome, enhancing the population's genetic variation.

      • Plasmid transfer can also spread antibiotic resistance genes among bacterial populations.

Prokaryotic metabolic diversity

  • Prokaryotes exhibit remarkable metabolic adaptations that enable them to survive across various environments:

    • Metabolism is crucial for the chemical pathways that organisms use to build (anabolism) or break down (catabolism) molecules to release energy.

    • Prokaryotes can be categorized by how they obtain energy and carbon:

    • Energy Source:

      • Phototrophs: Obtain energy from light (photo = light).

      • Chemotrophs: Obtain energy from chemicals.

    • Carbon Source:

      • Autotrophs: Use inorganic molecules (e.g., CO₂) to produce organic compounds (auto = self).

      • Heterotrophs: Acquire carbon by consuming organic matter (hetero = other).

    • Enzymes in prokaryotes digest organic molecules externally, with nutrients absorbed through the cell membrane.

Prokaryote Nutritional Modes

  • Energy and carbon sources combine to define four major nutritional modes:

    • Photoautotrophy: Light as energy source and CO₂ as carbon source; includes photosynthetic prokaryotes like cyanobacteria.

    • Chemoautotrophy: Inorganic chemicals (e.g. H₂S, NH₃) as energy and carbon sources.

    • Photoheterotrophy: Light as energy source and organic compounds as carbon source unique to some aquatic, extremophile prokaryotes.

    • Chemoheterotrophy: Organic compounds both for energy and carbon; includes many prokaryotes, protists, fungi, and animals.

Prokaryotic oxygen and nitrogen metabolism

  • Oxygen Metabolism:

    • Obligate Aerobes: Require O₂ for cellular respiration.

    • Obligate Anaerobes: Inhibited by O₂, rely on fermentation or anaerobic respiration.

    • Facultative Anaerobes: Can switch between using O₂ and not, based on availability.

    • Eukaryotes predominantly depend on aerobic respiration.

  • Nitrogen Metabolism:

    • Certain prokaryotes, including some bacteria and methanogens (archaea), can convert atmospheric nitrogen (N₂) into ammonia (NH₃) through biological nitrogen fixation.

    • This significantly contributes to nitrogen cycling within ecosystems, making nitrogen bioavailable to other organisms.

    • Eukaryotes cannot directly use atmospheric nitrogen (N₂) and rely on various nitrogen-containing compounds derived from other organisms for nitrogen acquisition.

Prokaryotic Diversity

  • Prokaryotes inhabit all environments capable of sustaining life. Advances in genomics reveal extensive prokaryotic diversity:

    • Molecular Phylogeny: Identifies two discrete domains of prokaryotes:

    • Domain Bacteria (Eubacteria): Includes most well-known prokaryotic species.

    • Domain Archaea: Share characteristics with both bacteria and eukaryotes.

  • Challenges to reconstructing the Tree of Life due to horizontal gene transfer complicate the identification of the evolutionary roots.

  • Archaea recognize characteristics of both bacteria and eukaryotes:

    • Like bacteria, they share a simple prokaryotic structure inherited from a common ancestor.

    • Unlike bacteria, Archaea cell walls do not contain peptidoglycan.

    • Archaea thrive in extreme environments (extremophiles) such as low pH, high salinity, or high temperature, but some are found in less extreme habitats like soils and oceans.

Bacterial Classification

  • Traditional classification was based on morphological, physiological, and staining methods including Gram staining, which has revealed hidden relationships.

  • Molecular systematics has offered insights leading to the identification of at least five significant bacterial groups, characterized by varied nutritional strategies, including:

    • Photoautotrophs

    • Chemoautotrophs

    • Heterotrophs

    • Categories including both anaerobic and aerobic bacteria.

    • Traditional classifications suggest there may be between 40 to 80 recognized bacterial phyla, with bacteria found in nearly every habitat.

    • Pathogenic bacteria account for approximately 50% of human diseases.

    • Many bacteria (500-1,000 species) contribute to human health through digestion support and nutritional interactions.

Major Groups of Bacteria

  • Proteobacteria: A large and metabolically diverse group of gram-negative bacteria divided into five sub-lineages (alpha to epsilon).

    • Some, like Rhizobium, are known for their symbiotic relationships with eukaryotes (e.g., legumes fixing nitrogen).

    • Hypothesized involvement of alpha-proteobacteria in the evolutionary origin of mitochondria through endosymbiosis.

  • Chlamydias: Obligate intra-cellular parasites lacking peptidoglycan in cell walls.

  • Spirochetes: Helical heterotrophs known for corkscrew-like movement, some being pathogens like Treponema pallidum (syphilis) and Borrelia burgdorferi (Lyme disease).

  • Cyanobacteria: Unique prokaryotes capable of oxygen-producing photosynthesis; thought to be involved in the Great Oxidation Event.

  • Gram-positive Bacteria: A diverse group that presents various roles, including decomposers (e.g., actinomycetes) and pathogens (e.g., Bacillus anthracis, Clostridium botulinum).

Domain Archaea

  • Archaea share traits with both bacteria and eukaryotes:

    • Archaea cell organization stems from a common ancestor with the same simplicity as bacteria but lack peptidoglycan.

    • Common in extreme environments where conventional energy sources are scarce:

    • Methanogens: Found in anoxic environments.

    • Extreme Halophiles: Thrive in saline environments.

    • Extreme Thermophiles: Adapted to high-temperature environments; many are chemoautotrophs.

  • Recent recognitions of Archaea diversification provide insights into early evolutionary life developments.

Roles of Prokaryotes in Ecosystems

  • Essential for ecological functions including:

    • Oxygen production

    • Nitrogen fixation

    • Recycling of chemical elements as decomposers (chemoheterotrophs)

    • Enhance essential nutrient availability (nitrogen, phosphorus, potassium) benefiting plants.

Prokaryotic Symbiosis

  • Prokaryotes frequently engage in symbiotic relationships, which can be classified as:

    • Mutualism: Both species benefit (e.g., methanogens in cow stomachs, intestinal bacteria in humans).

    • Commensalism: One organism benefits without affecting the other.

    • Parasitism: One organism benefits at the host's expense, with some parasites known as pathogens.

  • Notably, prokaryotes are involved in beneficial interactions supporting digestion among numerous human gut microbiome species.

Prokaryotes in Research and Technology

  • Utilized across various industries:

    • Food production (e.g., cheeses, yogurt).

    • Bioremediation: Employing prokaryotes to remove pollutants.

    • Sewage treatment for breaking down organic matter in wastewater.

    • Genetic Engineering: Utilizing bacteria for vitamin, antibiotic, hormone, biofuel, and bioplastic production.

    • Research on bacteria, such as E. coli, serves advancements in gene cloning.

    • Taq polymerase from thermophiles like Thermus aquaticus is used in PCR (Polymerase Chain Reaction) for DNA analysis procedures.