Prokaryotes: Domain Bacteria and Cyanobacteria – Structure, Function, and Laboratory Identification Lab 11

Prokaryotes: Domain Bacteria

Introduction to Prokaryotes

Prokaryotes are unicellular organisms lacking a membrane-bound nucleus and organelles. The majority of prokaryotes belong to the Domain Bacteria, often called "true bacteria." They are found in diverse environments and play essential roles in ecological processes.

• Definition: Prokaryotes are organisms whose cells do not contain a nucleus or other membrane-bound organelles.

• Size: Most bacteria range from 1 to 10 micrometers in size.

• Cell Envelope: Bacteria have cell walls and may possess an outer membrane.

• Habitat Diversity: Bacteria are found in soil, water, air, and as symbionts or pathogens in other organisms.

Heterotrophic Bacteria

Heterotrophic bacteria obtain energy by consuming organic matter. They include decomposers, pathogens, and bacteria involved in fermentation.

• Decomposers: Break down dead organic material, recycling nutrients in ecosystems.

• Pathogens: Cause diseases in plants, animals, and humans (e.g., Streptococcus pyogenes).

• Fermenters: Used in food production, such as yogurt and sauerkraut (e.g., Lactobacillus).

Cell Wall Composition

Most bacteria possess a cell wall made of peptidoglycan, a polymer that provides structural support and shape.

• Peptidoglycan: Composed of sugars and amino acids, forming a mesh-like layer outside the plasma membrane.

• Gram Staining: Differentiates bacteria based on cell wall structure (Gram-positive vs. Gram-negative).

Bacterial Shapes and Arrangements

Bacteria exhibit several characteristic shapes and arrangements, which aid in identification.

• Coccus: Spherical-shaped bacteria (e.g., Streptococcus).

• Bacillus: Rod-shaped bacteria (e.g., Bacillus subtilis).

• Spirillum: Spiral-shaped bacteria (e.g., Spirillum volutans).

• Arrangements: Chains, clusters, or pairs depending on species and division patterns.

Laboratory Identification of Bacteria

Microscopy is essential for observing bacterial morphology. High-power objectives (400X, 1000X) are used to view stained specimens.

• Procedure: Examine prepared slides at 40X, 400X, and 1000X magnification. Use oil immersion for highest resolution.

• Staining: Stains such as crystal violet or methylene blue enhance visibility of cell structures.

• Identification: Use shape, arrangement, and staining properties to identify bacteria.

Examples of Heterotrophic Bacteria

• Streptococcus pyogenes: Pathogen causing strep throat and skin infections.

• Lactobacillus: Used in yogurt and sauerkraut production; ferments sugars to lactic acid.

• Escherichia coli (E. coli): Common gut bacterium; some strains are pathogenic.

Cyanobacteria

Introduction to Cyanobacteria

Cyanobacteria, also known as "blue-green algae," are photosynthetic prokaryotes found in aquatic and terrestrial environments. They are important primary producers and contribute to oxygen production.

• Definition: Cyanobacteria are prokaryotes capable of oxygenic photosynthesis.

• Cell Structure: Unicellular or colonial; lack membrane-bound organelles.

• Ecological Role: Major component of pond scum and aquatic ecosystems.

Photosynthetic Pigments

Cyanobacteria possess pigments such as chlorophyll a and phycobilins that enable photosynthesis.

• Chlorophyll a: Main pigment for photosynthesis, also found in plants.

• Phycobilins: Accessory pigments that capture light energy.

Common Cyanobacteria Genera

• Gloeocapsa: Found in stagnant or slow-moving water; cells surrounded by jelly-like sheath.

• Oscillatoria: Filamentous colonies; move by oscillating motion.

• Nostoc: Forms large colonies; contains heterocysts for nitrogen fixation.

• Anabaena: Responsible for algal blooms; contains heterocysts and akinetes.

Nitrogen Fixation in Cyanobacteria

Some cyanobacteria can fix atmospheric nitrogen into ammonia using specialized cells called heterocysts.

• Heterocyst Function: Provide anaerobic environment for nitrogenase enzyme.

• Equation:

$N_2 + 8H^+ + 8e^- + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16P_i$

• Importance: Supplies nitrogen to aquatic ecosystems and supports growth of other organisms.

Laboratory Observation of Cyanobacteria

Prepared slides of cyanobacteria are viewed at 100X, 400X, and 1000X magnification. Identification is based on colony shape, presence of heterocysts, and filament structure.

• Staining: Not always necessary; natural pigments may be visible.

• Colony Types: Unicellular, filamentous, or colonial.

Table: Comparison of Heterotrophic Bacteria and Cyanobacteria

Feature	Heterotrophic Bacteria	Cyanobacteria
Nutrition	Heterotrophic (consume organic matter)	Autotrophic (photosynthetic)
Cell Wall	Peptidoglycan	Peptidoglycan
Pigments	None or limited	Chlorophyll a, phycobilins
Specialized Cells	None	Heterocysts, akinetes
Ecological Role	Decomposers, pathogens, fermenters	Primary producers, nitrogen fixers

Additional info:

• Some context and definitions were expanded for clarity and completeness.

• Examples and laboratory procedures were inferred and elaborated based on standard General Biology curricula.