6.3.2 CYANOBACTERIA - The Most Prominent Photosynthetic Bacteria

Introduction

Cyanobacteria, often referred to as blue-green algae, are a type of Gram-negative bacteria. They are the most prominent group of photosynthetic bacteria because they perform oxygenic photosynthesis, similar to plants. They inhabit a wide range of environments and play a crucial role in the global carbon and nitrogen cycles. Cyanobacteria are believed to be among the first life forms to colonize land and are responsible for introducing oxygen into the primitive atmosphere.

Definition

Cyanobacteria: Cyanobacteria (Gk. kyanoa, blue and bacterium, rod), often referred to as blue-green algae, are a type of Gram-negative bacteria that perform oxygenic photosynthesis.

In simple words: Cyanobacteria are photosynthetic bacteria that produce oxygen and are often called blue-green algae.

Core Concept

• Cyanobacteria are Gram-negative bacteria.
• They inhabit various environments:
• Damp places
• Saltwater
• Freshwater
• Moist soil
• Rocks in flowing streams
• Tree trunks
• Hot springs with temperatures reaching up to 85°C
• Cyanobacteria can lead an epiphytic or symbiotic lifestyle:
• Form partnerships with organisms like liverworts, ferns, and even invertebrates like corals.
• In association with fungi, they form lichens.
• It is believed that cyanobacteria were among the first life forms to colonize land during the course of evolution.
• Cyanobacteria exhibit diverse forms of life:
• Unicellular and solitary
• Colonies
• Filaments attached end to end

Detailed Explanation

Prokaryotic Features of Cyanobacteria

• Cyanobacteria are prokaryotic.
• They lack a nuclear membrane.
• Their chromosomes do not have protein combined with DNA.
• They lack membrane-bound organelles.
• Their cell wall contains muramic acid.
• The cell wall is often surrounded by a mucilaginous sheath.
• The genetic material is a circular strand of DNA.
• Many ribosomes are present in the cytoplasm.

Photosynthesis in Cyanobacteria

• Photosynthesis takes place in the extensive system of membranes located in the outer zone of the cytoplasm.
• Oxygen is released during photosynthesis.
• Cyanobacteria use phycobilins as accessory pigments.
• Phycocyanin, a blue pigment, is their predominant phycobilin.
• They are believed to be responsible for first introducing oxygen into the primitive atmosphere.

Reproduction in Cyanobacteria

Sexual reproduction is absent in cyanobacteria.

Asexual reproduction takes place by:

Nitrogen Fixation

• Approximately one-third of cyanobacteria have the capability to fix atmospheric nitrogen.
• Nitrogen fixation often occurs within heterocysts.
• Heterocysts are specialized cells with:
• Thick walls
• No nuclei
• In regions like Pakistan, cyanobacteria like Nostoc and Anabaena are intentionally cultivated to enhance soil fertility due to their nitrogen-fixing abilities.

Summary Table of Cyanobacteria

Intermediate Level

Prokaryotic Features in Detail

Photosynthesis in Detail

Reproduction in Detail

Nitrogen Fixation in Detail

Comparison of Cyanobacteria with Other Photosynthetic Bacteria

Important Points

• 1. Cyanobacteria (Gk. kyanoa, blue and bacterium, rod), often referred to as blue-green algae, are a type of Gram-negative bacteria.
• 2. They inhabit various environments, including:
• - Damp places
• - Saltwater
• - Freshwater
• - Moist soil
• - Rocks in flowing streams
• - Tree trunks
• - Hot springs with temperatures reaching up to 85°C
• 3. Cyanobacteria can lead an epiphytic or symbiotic lifestyle, forming partnerships with organisms like liverworts, ferns, and even at times with invertebrates like corals.
• 4. In association with fungi, they form lichens.
• 5. It is believed that cyanobacteria were among the first life forms to colonize land during the course of evolution.
• 6. Cyanobacteria exhibit diverse forms of life, which include being:
• - Unicellular and solitary
• - Colonies
• - Filaments attached end to end
• 7. The prokaryotic features of cyanobacteria are:
• - Nuclear membrane is absent
• - The chromosomes do not have protein combined with DNA
• - Membrane-bound organelles are absent
• - Cell wall contains muranic acid, which is found only in prokaryotes
• - The cell wall is often surrounded by mucilaginous sheath
• - The genetic material is a circular strand of DNA
• - Many ribosomes are present in the cytoplasm
• 8. Photosynthesis takes place in the extensive system of membrane, which is located in the outer zone of the cytoplasm.
• 9. Oxygen is released during photosynthesis.
• 10. Cyanobacteria use phycobilins as accessory pigment.
• 11. Phycocyanin, a blue pigment, is their predominant phycobilin.
• 12. They are believed to be responsible for first introducing oxygen into the primitive atmosphere.
• 13. Sexual reproduction is absent in cyanobacteria.
• 14. Asexual reproduction takes place by:
• - Cell division (e.g., unicellular form)
• - Fragmentation (breaking of the body into fragments; takes place at weak points next to heterocyst forming homogonia)
• - Formation of akinetes (certain cells of the filament may become enlarged; walls become thick; contain reserved food and DNA; resting stages; after resting stage, the wall of the akinete ruptures and a short filament of cells is released)
• - Spore formation (not common; sometimes the heterocyst forms endospores; the nuclear material divides and then the cytoplasm of the heterocyst divides within the parent cell wall and many spores called endospores are formed; each spore forms a new Nostoc filament)
• 15. Approximately one-third of cyanobacteria have the capability to fix atmospheric nitrogen, often occurring within heterocysts, specialized cells with thick walls and no nuclei.
• 16. In regions like Pakistan, cyanobacteria like Nostoc and Anabaena are intentionally cultivated to enhance soil fertility due to their nitrogen-fixing abilities.

Remember

• - Cyanobacteria = blue-green algae
• - Gram-negative bacteria
• - Oxygenic photosynthesis
• - Accessory pigment = phycobilins (phycocyanin)
• - No sexual reproduction
• - Asexual reproduction = cell division, fragmentation, akinetes, spores
• - Nitrogen fixation = in heterocysts
• - Examples: Nostoc, Anabaena, Oscillatoria
• - **Habitat** = diverse; up to 85°C
• - Role in evolution = first introduced oxygen
• - Symbiotic associations = liverworts, ferns, corals, fungi (lichens)

Common Mistakes

• 1. Confusing cyanobacteria with algae – Cyanobacteria are prokaryotes; algae are eukaryotes.
• 2. Thinking cyanobacteria are not bacteria – Cyanobacteria are Gram-negative bacteria.
• 3. Thinking cyanobacteria have a nucleus – Cyanobacteria lack a nuclear membrane.
• 4. Thinking cyanobacteria have membrane-bound organelles – Cyanobacteria lack membrane-bound organelles.
• 5. Confusing phycocyanin and phycoerythrin – Phycocyanin is blue; phycoerythrin is red.
• 6. Thinking cyanobacteria reproduce sexually – Sexual reproduction is absent in cyanobacteria.
• 7. Forgetting the role of heterocysts – Heterocysts are the site of nitrogen fixation.
• 8. Thinking all cyanobacteria fix nitrogen – Only one-third of cyanobacteria fix nitrogen.
• 9. Confusing akinetes and endospores – Akinetes are resting stages; endospores are reproductive spores.
• 10. Thinking cyanobacteria are harmful – Cyanobacteria are beneficial (oxygen production, nitrogen fixation).
• 11. Forgetting the examples – Nostoc, Anabaena, Oscillatoria.
• 12. Thinking cyanobacteria only live in water – Cyanobacteria live in diverse environments.

Quick Revision

• Cyanobacteria (Gk. kyanoa, blue and bacterium, rod), often referred to as blue-green algae, are a type of Gram-negative bacteria.
• They inhabit various environments, including damp places, saltwater, freshwater, moist soil, rocks in flowing streams, tree trunks, and even hot springs with temperatures reaching up to 85°C.
• Cyanobacteria can lead an epiphytic or symbiotic lifestyle, forming partnerships with organisms like liverworts, ferns, and even at times with invertebrates like corals.
• In association with fungi, they form lichens.
• It is believed that cyanobacteria were among the first life forms to colonize land during the course of evolution.
• Cyanobacteria exhibit diverse forms of life, which include being (a) unicellular and solitary (b) in the form of colonies (c) in the form of filaments attached end to end.
• The prokaryotic features of cyanobacteria are:
• (a) nuclear membrane is absent
• (b) the chromosomes do not have protein combined with DNA
• (c) membrane bound organelles are absent
• Cell wall contains muranic acid, which is found only in prokaryotes.
• The cell wall is often surrounded by mucilaginous sheath.
• The genetic material is a circular strand of DNA.
• Many ribosomes are present in the cytoplasm.
• Photosynthesis takes place in the extensive system of membrane, which is located in the outer zone of the cytoplasm.
• Oxygen is released during photosynthesis.
• Cyanobacteria use phycobilins as accessory pigment.
• Phycocyanin, a blue pigment is their predominant phycobilins.
• They are believed to be responsible for first introducing oxygen into the primitive atmosphere.
• Sexual reproduction is absent in cyanobacteria.
• Asexual reproduction takes place by:
• (a) Cell division e.g., unicellular form
• (b) Fragmentation is the breaking of the body of the organism into small pieces of fragments. It takes place at weak points next to heterocyst forming homogonia.
• (c) formation of akinetes where certain cells of the filament may become enlarged. The walls become thick. They contain reserved food and DNA. These are the resting stages. After resting stage, the wall of the akinete ruptures and a short filament of cells is released.
• (d) Spore formation is not common. Sometimes the heterocyst forms endospores. The nuclear material divides and then the cytoplasm of the heterocyst divides within the parent cell wall and many spores called endospores are formed. Each spore forms a new Nostoc filament.
• Approximately one-third of cyanobacteria have the capability to fix atmospheric nitrogen, often occurring within heterocysts, specialized cells with thick walls and no nuclei.
• In regions like Pakistan, cyanobacteria like Nostoc and Anabaena are intentionally cultivated to enhance soil fertility due to their nitrogen-fixing abilities.

60-Second Revision

• Cyanobacteria = blue-green algae
• Gram-negative bacteria
• Oxygenic photosynthesis
• Accessory pigment = phycobilins (phycocyanin)
• No sexual reproduction
• Asexual reproduction = cell division, fragmentation, akinetes, spores
• Nitrogen fixation = in heterocysts
• Examples: Nostoc, Anabaena, Oscillatoria
• **Habitat** = diverse; up to 85°C
• Role in evolution = first introduced oxygen
• Symbiotic associations = liverworts, ferns, corals, fungi (lichens)
• Warning: Cyanobacteria are prokaryotes, not algae. They have no nucleus or membrane-bound organelles. They perform oxygenic photosynthesis. They lack sexual reproduction. Only one-third fix nitrogen. Heterocysts are the site of nitrogen fixation. Phycocyanin is blue; phycoerythrin is red. Cyanobacteria are beneficial, not harmful. They are among the first life forms on land. They form lichens with fungi.