Cyanobacteria: Characteristics, Structure and Reproduction

In this article we will discuss about Cyanobacteria:- 1.Occurrence and distribution, 2.Characteristic features, 3.Morphology, 4. Cell structure, 5.Heterocyst, 6.Reproduction and 7.Economic importance

1. Occurrence and Distribution

• Cyanobacteria are ubiquitous and found in all kinds of habitats. E.g.
• Marine forms: Prochlorococcus, Crocosphaera, Trichodesmium, Synechoccoccus
• Fresh water forms: Oscillatoria, Microcystis, Nostoc
• Terrestrial forms: Porphyrosiphon, Scytonema
• Hot springs: Phormidium, Mastigocladus.
• Polar regions: Nostoc, Schizothrix, Microcoleus.
• Symbiotic forms: Nostoc colonies are found inside the tissues of Anthoceros and Notothylas. Anabaena cycadeae found inside the coralloid roots of Cycas and Anabaena azollae found in the tissues of Azolla.

2. Characteristic Features of Cyanobacteria

• Cyanobacteria commonly known as blue green algae.
• They were the first organisms that produce O2 on our earth.
• The organisms that can do nitrogen fixation .
• Photosynthetic pigments present in cyanobacteria are chlorophyl a, carotenoids and phycobilin. Due to phycobilin they are blue green in colour. (exception: Trichodesmium is a red coloured BGA).
• They may be unicellular (Chrococcus, Gloeocapsa), colonial (Nostoc, Aphanotheca) and filamentous (Oscillatoria ).
• The storage products are cyanophycean starch and proteins.
• The flagella are absent and sexual reproduction is absent.
• Gas vacuoles are present.

3. Morphology of Cyanobacteria

• Cyanobacteria may be unicellular (Prochlorococcus), colonial (Gloeocapsa) or filamentous (Oscillatoria).
• In colonial forms, the cells of the colony occur in a common gelatinous matrix.
• Filamentous form may be branched or unbranched.
• Each filament consists of a sheath of mucilage and one or more chains of individual cells called trichomes.
• Trichome filamentous two types:
  1. Homocystous- Undifferentiated e.g. oscillatoria.
  2. Hetrocystous- Differentiated and having hetrocyst. E.g. Nostoc.
• Flagella are absent but gliding movement is used by number of known cyanobacteria

4. Cell Structure of Cyanobacteria

• They have distinct cell wall, covered by a mucilaginous sheath.
• The complete cell is filled with cytoplasm.
• The protoplasm is differentiated into two regions:
  1. Chromoplasm: Outer peripheral region which is pigmented.
  2. Centroplasm: Central region which is not pigmented.
• Pigments of cyanobacteria: phycocyanin, phycoerythrin, carotenes, xanthophylls and chlorophylls.
• Gas vacuoles are filled with gas and bounded by single membrane. The gas vacuoles maintain buoyancy.
• Oil droplets and glycogen granules are found in the cytoplasm.
• Membrane-bound organelles are also absent. Ribosomes are present, 70S type.

5. Heterocyst of Cyanobacteria

• Specialized nitrogen fixing cells formed during nitrogen starvation by some filamentous cyanobacteria. E.g. Nostoc, Anabaena, Cylindrospermum (except : Oscillatoriaceae).
• These cells fix atmospheric nitrogen using the enzyme nitrogenase.
• Nitrogenase is inactivated by oxygen, so the heterocyst creates a microanaerobic environment.
• Heterocysts are thick walled, pale yellow, oval or spherical shaped structures in intercalary or terminal position.
• At the point of contact to the filament there is a pore in the heterocyst. The intercalary heterocyst is bipored (E.g. Nostoc, Anabaena) and the terminal one is monopored (E.g. Gloetricha).
• Heterocysts are the only cells that express nif genes and synthesize nitrogenase in heterocyst forming cyanobacteria.
• The HetR gene, which is activated by the deficiency in N2, help to initiation of heterocyst development.
• The following sequences take place in formation of heterocysts:
  1. The cell enlarges
  2. Deposition of additional three cell walls layers (barrier to O2)
  3. Degrade photosystem II (no oxygen is liberated)
  4. Only Photosyetem I is functional to generate ATP by cyclic photophosphorylation.
  5. Formation of polar plugs occurred, composed of cyanophycin, which slows down cell to cell diffusion

6. Reproduction

• Sexual reproduction is completely absent in cyanobacteria.
• Reproduction takes place by vegetative and asexual methods.
  1. Vegetative reproduction occurs by cell division, fragmentation and hormogonia formation.
  2. Asexual reproduction is facilitated through akinetes, endospores, exospores and heterocysts.

7. Economic Importance

• Release oxygen
• Helpful in the field of agriculture because they have the ability to fix the atmospheric nitrogen and used as a green manure.
• Certain cyanobacteria are used in the reclamation of usar soil.
• Used as food . Eg. Spirulina rich source of protein and used as single cell protein, Nostoc commune are collected, boiled and consumed as food by Chinese and South Americans.
• Used as model organism in the field of Biotechnology.
• The unicellular cyanobacterium Synechocystis sp. PCC 6803 was the third prokaryote and first photosynthetic organism whose genome was completely sequenced.
• Various species of cyanobacteria used as pollution indicators.