Bacteria and Archaea: Diversity, Structure, and Ecological Roles

Bacteria and Archaea

Introduction

Bacteria and Archaea are two of the three domains of life, alongside Eukarya. Though they are both prokaryotic (lacking a nucleus), they differ significantly in their genetic, biochemical, and structural characteristics. These organisms are the most abundant, diverse, and ecologically significant life forms on Earth.

Similarities and Differences Between Bacteria and Archaea

Key Similarities

• Both are prokaryotic (lack a membrane-bound nucleus and organelles).

• Both reproduce asexually, primarily by binary fission.

• Both have cell walls (though composition differs).

• Both can be found in a wide range of environments.

Fundamental Differences

• Cell wall composition: Bacterial cell walls contain peptidoglycan; archaeal cell walls do not.

• Membrane lipids: Bacteria have ester-linked lipids; Archaea have ether-linked lipids.

• Genetic machinery: Archaea have some genes and metabolic pathways more similar to eukaryotes.

• Response to antibiotics: Many antibiotics that affect bacteria do not affect archaea.

Importance, Diversity, and Abundance

Why Are Bacteria and Archaea So Important?

• They are the most abundant organisms on Earth, found in every environment, including extreme habitats (extremophiles).

• They play essential roles in ecosystem functioning, such as nutrient cycling (carbon, nitrogen).

• They are diverse in morphology, metabolism, and genetics, with new phyla still being discovered.

Extremophiles

• Extremophiles are organisms that thrive in extreme environments (e.g., high temperature, salinity, acidity).

• Studying extremophiles helps us understand the limits of life and the potential for life on other planets.

Medical Importance

• Some bacteria are pathogenic, causing diseases in humans, animals, and plants.

• Koch's postulates are criteria to establish a causative relationship between a microbe and a disease.

• Germ theory of disease states that many diseases are caused by microorganisms.

• Virulence refers to the degree of pathogenicity of a microorganism.

• Antibiotics are molecules that kill or inhibit bacteria, often by targeting cell wall synthesis, protein synthesis, or DNA replication.

Role in Bioremediation

• Bacteria and archaea can degrade pollutants, making them useful in cleaning up contaminated environments (bioremediation).

Methods for Studying Bacteria and Archaea

Enrichment Cultures

• Enrichment cultures are laboratory techniques that encourage the growth of specific microbes by providing favorable conditions.

Metagenomics

• Metagenomics involves sequencing DNA from environmental samples to study microbial communities without culturing them.

• PCR (Polymerase Chain Reaction) amplifies specific DNA sequences, enabling detection and analysis of microbes.

Molecular Phylogenies

• Comparing genetic sequences (e.g., rRNA genes) allows construction of evolutionary relationships (phylogenies) among prokaryotes.

Themes in the Diversification of Bacteria and Archaea

Morphological Diversity

• Size: Range from 0.2 μm to over 700 μm in diameter.

• Shape: Common shapes include cocci (spherical), bacilli (rod-shaped), and spirilla (spiral-shaped).

• Motility: Many move using flagella, pili, or gliding mechanisms.

Cell Wall Composition and Gram Stain

• Gram-positive bacteria: Thick peptidoglycan layer; stain purple; generally more sensitive to antibiotics targeting cell wall synthesis.

• Gram-negative bacteria: Thin peptidoglycan layer plus outer membrane; stain pink; often more resistant to antibiotics.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present
Stain Color	Purple	Pink
Antibiotic Sensitivity	Higher	Lower

Metabolic Diversity

• Bacteria and archaea display remarkable diversity in energy and carbon acquisition.

• Three ways to acquire energy for ATP production:

  • Phototrophs: Use light energy (photophosphorylation).

  • Chemoorganotrophs: Oxidize organic molecules (e.g., sugars).

  • Chemolithotrophs: Oxidize inorganic molecules (e.g., ammonia, methane).

• Two ways to acquire carbon:

  • Autotrophs: Use CO2 or methane to build organic compounds.

  • Heterotrophs: Obtain organic compounds from other organisms.

• Six major "feeding strategies" result from combinations of energy and carbon sources.

Energy Source	Carbon Source	Feeding Strategy
Light	CO2	Photoautotroph
Light	Organic compounds	Photoheterotroph
Organic molecules	CO2	Chemoorganoautotroph
Organic molecules	Organic compounds	Chemoorganoheterotroph
Inorganic molecules	CO2	Chemolithoautotroph
Inorganic molecules	Organic compounds	Chemolithoheterotroph

ATP Production Pathways

• Cellular respiration: Involves electron donors and acceptors; eukaryotes are typically chemoorganotrophs, while many prokaryotes are chemolithotrophs.

• Fermentation: Anaerobic process; less efficient than respiration because it does not use an electron transport chain.

• Photosynthesis: Three forms exist among prokaryotes:

  • Light-activated pigment drives ATP synthesis via chemiosmosis.

  • Geothermal radiation can substitute for light in some cases.

  • Electron transport chains powered by light-activated pigments; requires electron donors (e.g., water in oxygenic photosynthesis, hydrogen sulfide in anoxygenic photosynthesis).

Carbon Fixation Pathways

• Autotrophs: Fix CO2 using the Calvin cycle (plants, cyanobacteria) or alternative pathways (some bacteria).

• Some bacteria use methane, carbon monoxide, or methanol as starting points for carbon fixation.

• Heterotrophs: Obtain organic carbon from other organisms (animals, fungi, many bacteria and archaea).

Ecological Diversity and Global Impacts

• Bacteria and archaea have shaped Earth's chemistry for billions of years.

• The Oxygen Revolution:

  • No free O2 in the atmosphere for the first 2.3 billion years.

  • Cyanobacteria were the first to perform oxygenic photosynthesis, leading to the accumulation of atmospheric oxygen.

  • Oxygen enabled the evolution of aerobic respiration, a more efficient energy-generating process.

• Nitrogen Fixation and the Nitrogen Cycle:

  • Plants require nitrogen but cannot use atmospheric N2.

  • Certain bacteria convert N2 to ammonia (NH3), which plants can use.

  • Other bacteria and archaea convert ammonia to nitrates and nitrites, completing the nitrogen cycle.

• Nitrate Pollution: Excess nitrates from agriculture can disrupt ecosystems.

Key Lineages of Bacteria and Archaea

Bacteria (Selected Phyla)

• Firmicutes: Includes many Gram-positive bacteria; some are important in human health and industry.

• Cyanobacteria: Also known as "blue-green algae"; perform oxygenic photosynthesis.

• Actinobacteria: Gram-positive; important decomposers and antibiotic producers.

• Spirochaetes: Spiral-shaped; some are pathogens (e.g., Borrelia causes Lyme disease).

• Chlamydiae: Obligate intracellular parasites; some cause human diseases.

• Proteobacteria: Large, diverse group; includes Escherichia coli and many pathogens.

Archaea (Selected Phyla)

• Thaumarchaeota: Important in nitrogen cycling.

• Crenarchaeota: Many are extremophiles (e.g., thermophiles, acidophiles).

• Euryarchaeota: Includes methanogens and halophiles.

General Questions and Key Concepts

• Ecological roles: Decomposers, nitrogen fixers, primary producers, pathogens, symbionts.

• Population size: High reproductive rates, adaptability, and metabolic diversity allow large populations.

• Adaptations for extreme environments: Unique membrane lipids, enzymes stable at high temperatures or salinities.

• Monophyly of Bacteria: Supported by genetic and molecular data.

• Phylogeny construction: Uses molecular data (e.g., 16S rRNA gene sequences).

• Microbiome: The community of microorganisms living in a particular environment (e.g., human gut microbiome).

• Taxonomy: Based on morphology, metabolism, genetics, and molecular data.

• Locomotion: Flagella, pili, gliding, or non-motile.

• Genetic variation: Mutation, transformation, transduction, conjugation.

• Biofilms: Communities of microorganisms attached to surfaces and embedded in a self-produced matrix.

• Antibiotic resistance: Evolves via natural selection; bacteria with resistance genes survive and proliferate.

Summary Table: Gram-Positive vs. Gram-Negative Bacteria

Characteristic	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present
Stain Color	Purple	Pink
Antibiotic Sensitivity	Higher	Lower

Key Definitions

• Pathogen: An organism that causes disease.

• Virulence: The degree of pathogenicity.

• Antibiotic: A substance that kills or inhibits bacteria.

• Biofilm: A structured community of microorganisms attached to a surface.

• Microbiome: The collection of microorganisms in a particular environment.

Important Equations and Pathways

• Photosynthesis (Oxygenic):

  • $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

• Cellular Respiration (Aerobic):

  • $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$

• Nitrogen Fixation:

  • $N_2 + 8H^+ + 8e^- \rightarrow 2NH_3 + H_2$

Examples and Applications

• Human health: Gut bacteria aid digestion and immunity; some cause disease.

• Bioremediation: Bacteria degrade oil spills and toxic waste.

• Global cycles: Cyanobacteria produce oxygen; nitrogen-fixing bacteria support plant growth.

Additional info: Some details, such as the full list of phyla or specific mechanisms of antibiotic action, were expanded for academic completeness.