Cyanobacteria (Blue – Green Algae)

Cyanobacteria (Blue – Green Algae)

·       Cyanobacteria, commonly known as Blue Green Algae, are ancient Photosynthetic Prokaryotes with significant Ecological and Evolutionary roles.

·       Example - Anabaena, Nostoc, Oscillatoria, Spirulina, Gloeocapsa, Microcystis, Synechococcus and Prochlorococcus.

·       Number of Species – Approximately 8,000 species are estimated, though taxonomic debates persist. Around 5,185 species have been formally categorized into seven orders, including Chroococcales and Synechococcales.

·       Habitat

ü  Freshwater: Lakes, ponds, rivers.

ü  Marine: Oceans, seas, intertidal zones.

ü  Terrestrial: Moist soils, rocks, tree bark.

ü  Extreme Environments: Hot springs, hypersaline environments, deserts, polar regions.

ü  Symbiotic Relationships: With Plants (e.g., Azolla), Fungi (Lichens), and some animals.

·       Mode of Nutrition – Photoautotrophic (primary mode, using sunlight, CO₂, and water). Some species fix Atmospheric nitrogen (N₂) and convert into Ammonia via Heterocysts (e.g., Anabaena). A few show Mixotrophy (combining Photosynthesis and Organic carbon uptake).

·       Mode of Reproduction – Asexual reproduction only (no sexual reproduction):

a)     Binary fission - Simple cell division form the Mother cell.

b)     Fragmentation - Breaking of filaments or colonies into smaller pieces that grow into new individuals.

c)      Akinetes - Specialized, thick-walled, dormant cells formed under harsh environmental conditions. They serve as a survival mechanism, allowing the organisms to withstand unfavorable conditions and re-emerge when conditions improve.

d)     Hormogonia - Specialized, short filaments released from cyanobacteria for vegetative reproduction. They are motile and can move via gliding motility, often requiring a wet surface or viscous substrate like agar. Hormogonia are formed when cyanobacterial filaments break into smaller fragments between necridia (dead cells). These fragments then detach and become motile, eventually growing into new filaments. 

·       Mode of Respiration

ü  Cyanobacteria are generally Aerobic, meaning they require oxygen for respiration (Oxygenic Photosynthesis). They have a unique system where Photosynthesis and Respiration can occur in the same Thylakoid membranes, but these processes are often separated temporally (Photosynthesis during the day or in the light and Respiration at night or in the dark).

ü  Some can also perform Anaerobic respiration or Anoxygenic Photosynthesis under Sulfur-rich, low-oxygen conditions.

·       Type of Photosynthesis – Oxygenic photosynthesis (like plants).

·       Pigments

a)     Chlorophyll a - Green pigment for photosynthesis.

b)     Phycobilins - Water-soluble pigments located in Phycobilisomes attached to the Thylakoid membranes. These include:

ü  Phycocyanin: Blue pigment, giving many Cyanobacteria their characteristic Blue-green color.

ü  Phycoerythrin: Red pigment. The relative amounts of Phycocyanin and Phycoerythrin can vary depending on light conditions, leading to different colors (greenish, bluish, reddish-brown).

c)      Allophycocyanin - Another type of Blue pigment involved in Energy Transfer to chlorophyll a.

d)     Carotenoids (β-carotene & Xanthophylls) - Yellow and Orange pigments that also participate in light absorption and photoprotection.

·       Electron Donor – Water (H₂O) (in Oxygenic photosynthesis). In Anoxygenic photosynthesis (rare), Hydrogen sulfide (H₂S) may be used.

·       By-products

a)     Oxygen (O₂) (major by-product of oxygenic photosynthesis).

b)     Organic compounds (e.g., sugars).

c)     Some produce Toxins (e.g., Microcystins, Cylindrospermopsin) in Harmful Algal Blooms.

·       Photosystem – Both Photosystem I and Photosystem II.

·       Ecological role

ü  Primary producers: Contribute significantly to global carbon fixation.

ü  Nitrogen fixation: Enrich soil/water with bioavailable nitrogen (Example – Anabaena sp.).

ü  Oxygen production: Early Earth oxygenation (Great Oxygenation Event).

ü  Bioindicators: Indicate water quality and environmental conditions.

ü  Harmful blooms: Toxin production and Eutrophication disrupts ecosystems and human health.

ü  Symbiosis: Partner with fungi (Lichens), plants (Azolla), and Corals to enhance host Nutrient uptake.

ü  Carbon sequestration: Contribute to global carbon cycling in oceans and soils.

ü  Formation of Microbial Mats and Stromatolites: They can form extensive microbial mats in various environments and contribute to the formation of stromatolites, ancient layered structures.