Bac/Archaea

Prokaryotic Overview

• Prokaryotes are unicellular organisms that lack a nucleus and membrane-bound organelles. They are divided into two primary domains: Bacteria and Archaea.

• Prokaryotes are incredibly versatile, thriving in a wide variety of environments, including extreme conditions that would be inhospitable to most life forms, such as highly acidic environments, extremely salty solutions, and high-temperature geothermal sites.

• Most prokaryotes are microscopic, typically ranging in size from 0.5 to 5 µm, significantly smaller than eukaryotic cells, allowing them to occupy ecological niches that larger organisms cannot.

• Their structural adaptations, such as the presence of a rigid cell wall, unique membrane lipids, and specialized metabolic pathways, aid in their survival across diverse environments.

Characteristics of Prokaryotes

• Typical size: 0.5–5 µm; their small size contributes to high surface area-to-volume ratios, facilitating nutrient uptake and waste elimination.

• Shapes: Prokaryotes come in various shapes, including cocci (spheres), bacilli (rods), and spirals (helices), which can influence their ecological roles and methods of reproduction.

• Most bacteria possess cell walls mainly made of peptidoglycan, providing structural support and protection. However, Archaea have cell walls that lack peptidoglycan and are composed of unique compounds, such as pseudomurein or polysaccharides, that contribute to their resilience in extreme environments.

Classification of Prokaryotes

• Gram stain classification helps differentiate bacteria based on their cell wall structure:

  • Gram-positive: Characterized by a thick peptidoglycan layer that retains the crystal violet stain, appearing purple under a microscope. These bacteria often contain teichoic acids that contribute to cell wall integrity.

  • Gram-negative: Have a much thinner peptidoglycan layer between an inner cell membrane and an outer lipid membrane, which can protect against certain antibiotics. They take up counterstain and appear pink under the microscope.

Structure & Movement

• Structures such as capsules (which aid in virulence) and fimbriae (which help bacteria adhere to surfaces) enhance bacteria’s ability to survive and colonize various environments.

• Prokaryotes demonstrate taxis, the ability to intentionally move toward or away from environmental stimuli, with chemotaxis being the movement in response to chemical gradients.

• Flagella are whip-like appendages that provide motility, which have evolved independently in different prokaryotic lineages, showcasing convergent evolution in response to similar challenges in mobility.

Genetic Information

• Prokaryotic genome is typically a single, circular chromosome located in a region called the nucleoid. This arrangement allows for efficient replication and transcription.

• Plasmids are additional small, circular DNA strands that can carry genes beneficial for survival, such as antibiotic resistance. They can be exchanged between bacteria through processes like conjugation.

Reproduction & Genetic Diversity

• Prokaryotes reproduce primarily through binary fission, a form of asexual reproduction that allows rapid population increase, often with generation times as brief as 1-3 hours.

• They exhibit genetic variation through:

  1. Rapid reproduction: leading to mutations being passed on frequently.

  2. Mutations: random changes in DNA that can introduce new traits.

  3. Genetic recombination: involving three key mechanisms: transformation (uptake of foreign DNA), transduction (DNA transfer via viruses), and conjugation (direct transfer through cell-to-cell contact).

Metabolic Diversity

• Energy sources can be classified as follows:

  • Phototrophs: Utilize light as their energy source, performing photosynthesis.

  • Chemotrophs: Obtain energy from chemical compounds, which can be organic or inorganic.

  • Autotrophs: Use carbon dioxide as their carbon source, crucial for the carbon cycle.

  • Heterotrophs: Require organic compounds for their carbon and energy needs.

• Oxygen requirements further classify prokaryotes:

  • Obligate aerobes: Require oxygen for survival.

  • Obligate anaerobes: Are poisoned by oxygen, often found in deep soil or aquatic environments devoid of oxygen.

  • Facultative anaerobes: Can survive in both the presence and absence of oxygen, showcasing metabolic flexibility.

Major Groups of Bacteria

• Proteobacteria: A diverse group including both beneficial and pathogenic species; significant members include Rhizobium, which fixes nitrogen in symbiosis with legumes.

• Chlamydias: Intracellular parasites; known for causing various human diseases, including chlamydia.

• Spirochetes: Characterized by their helical shape; include pathogens like Treponema pallidum, which causes syphilis.

• Cyanobacteria: Also known as blue-green algae, these photoautotrophs are important for oxygen production through photosynthesis and can contribute to harmful algal blooms.

• Gram-positive bacteria: This group includes important microorganisms like Actinomycetes, which are sources of many antibiotics, and pathogens like Bacillus anthracis, which causes anthrax.

Importance in Ecosystem

• Chemical Recycling: Prokaryotes act as decomposers, facilitating the breakdown of organic matter and restoring nutrients to the soil, ultimately supporting plant growth.

• Pathogens: Some bacteria are harmful, such as Mycobacterium tuberculosis, responsible for tuberculosis, illustrating how prokaryotes can impact human health significantly.

• Mutualistic Relationships: Many prokaryotes form beneficial symbiotic relationships with larger organisms, such as the gut microbiota in humans, aiding in digestion and synthesizing essential vitamins.

Summary

• Prokaryotes play crucial roles in various ecosystems, ranging from nutrient cycling and environmental resilience to being significant agents of disease and contributors to symbiotic relationships with complex organisms. Their remarkable adaptability allows them to survive and thrive in a plethora of environments, making them fundamental to life on Earth.