The Cyanobacteria

The structure of a cyanobacterium (right):
1. cytoplasmic membrane
2. cell wall - gram negative
3. capsule
4. mucoid sheath
5. paired thylakoid membranes studded with phycobilosomes
6. cyanophycin granules
7. nuclear material
8. carboxysomes (polyhedral structures that resemble bacteriophage heads. Carboxysomes comprise 5-6 proteins forming a shell around the ribulose bisphosphate carboxylase. Carboxysomes are believed useful in situations of low carbon dioxide concentration because they concentrate CO2 inside the structure, increasing the efficiency of ribulose bisphosphate carboxylase.
9. 70s ribosomes
10. cytoplasm

The cyanobacteria were formerly called "blue-green algae" because of their ecology and their resemblance to the algae. However, cyanobacteria are prokaryotes, lacking a nuclear membrane and membrane-bound organelles, though they do have internal membranes. The algae are eukaryotes, possessing both a nuclear membrane and membrane-bound organelles.

A filamentous Cyanobacterium, Lyngbya sp. (left), compared to the filamentous algae, Spirogyra (right). Click on image for larger photomicrograph.


The serial endosymbiosis theory (SET) of the prokaryotic origin of eukaryotic chloroplasts and mitochondria is widely accepted. There is little doubt that the chloroplasts of green plants are derived from Cyanobacteria.

Although the Cyanobacteria were not the first cells to evolve on the planet, the "self-fossilizing" activity of Cyanobacterial mats has left us with the oldest fossils.

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Photochemistry

Cyanobacteria contain chlorophyll a, which is also found in plants and algae. Other photosynthetic organisms contain chlorophyll b (green algae and the plants) or chlorophyll c (dinoflagellates and photosynthetic Chromista).

Chlorophylls a and b differ only in the character of a group attached to the Mg-porphyrin ring moiety of the molecule: -CH3 for chlorophyll a, and -COH for chlorophyll b. Chlorophylls are embedded in the thylakoid membrane of the Cyanobacteria. (Because Cyanobacteria gave rise, through serial endosymbiotic transfers, to chloroplasts, chlorophylls are associated with the thylakoid membrane within chloroplasts in plants and Protoctista.)

Phycobilins are water-soluble pigments, so they are found in the cytoplasm of Cyanobacteria, or in the stroma of the chloroplast. Phycolibins occur only in Cyanobacteria (phycocyanin and phycoerythrin) and the "red algae", the Rhodophyta (phycoerythrin).

The overall reaction of oxygenic photosynthesis is:

6H2O + 6CO2 → C6H12O6 + 6O2

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