Cyanobacteria: occurrence, morphology, structure, reproduction

Introduction

The cyanobacteria are the most important and most diverse group of photosynthetic bacteria.
Cyanobacteria are Gram negative bacteria.
They are obligate photolithoautotrophs and oxygenic photosynthetic.
The only organism that can do nitrogen fixation and are able to perform oxygenic photosynthesis.
Cyanobacteria also referred as blue green algae.
Photosynthetic pigments present in cyanobacteria are chl a (in very few cases chl b and chl d), carotenoids and phycobilins (phycocyanin and phycoerythrin).

Cyanobacteria are ubiquitous, and are often found in all types of environment-freshwater, salt marshes, hot springes, tree trunks, sea water, moist rock, frozen water, moist soil.

Cyanobacteria are varying greatly in shape and appearance.
They range in diameter from 1 to 10 micrometres.
Both unicellular and filamentous (termed trichomes), forms of cyanobacteria occur in nature.
Filamentous form may be branched or unbranched.
Trichomes is a row of bacterial cells that are in close contact with one another over a large area.

Single trichome filamentous may be of two types:

- Homocystous- Undifferentiated e.g. oscillatoria.
- Hetrocystous- Differentiated and having hetrocyst. E.g. Nostoc.
Spirulina has a spirally coiled filament.
Gliding movement is used by number of known cyanobacteria because flagella are absent.

All cyanobacteria except Gleobacter violaceus, have an internal system of thylakoid membrane in which the light reaction of photosynthesis and respiration occur.
Besides the thylakoid membrane the cyanobacteria cytosol contains components such as carboxysomes, glycogen granules, cyanophycin granules, lipid bodies, polyphosphate bodies.
Cyanobacterial cells are more elaborate and larger than bacteria.

Cyanobacteria have typically prokaryotic cell structure naked DNA, 70s ribosome, one envelope organization with peptidoglycan.
Membrane bound structure like endoplasmic reticulum, mitochondria, Golgi bodies, SAP vacuoles, plastids are absent in cyanobacteria.
Cyanobacteria cell wall is four layered with peptidoglycan is present in the second layer.

Cyanobacteria employee’s variety of mechanism with respect to reproduction:

Fragmentation of filamentous cyanobacteria form small, motile filaments called hormogonia.
Some cyanobacterial species develop akinetes, specialised, thick- walled dormant resting cells that protect the organism in unfavourable conditions.
Cyanobacteria lack the enzyme alpha ketoglutarate dehydrogenase, thus they do not have a fully functional citric acid cycle.

Many nitrogen fixing, filamentous cyanobacteria have specialised cells in which nitrogen fixation occurs known as heterocyst.
It has enzyme nitrogenase.
Heterocyst depends on adjacent vegetative cells for its nourishment.
Photosystem II is absent.
Thick walls present which is impermeable to oxygen but permeable to nitrogen.

Nitrogen fixing cyanobacteria are often used for reclaiming user soils. E.g. Nostoc, Anabena.
Blue-green algae used as food for several aquatic animals.
In Africa, spirulina is regularly collected for human consumption. Nostoc is similarly used in China as a food.
From extract of, Lyngbia several antibiotic can be manufactured.
Several species of cyanobacteria are used to prevent the growth of mosquitoes Larvae Nearby E.g. Anabena, Aulosira.
Some cyanobacteria produces some toxins which may be very harmful to most of the aquatic animals. The important toxins producing cyanobacteria are: Microcytic aeruginosa, Anabena flosaquae.

1% – https://www.biologydiscussion.com/bacteria/cyanobacteria/cyanobacteria-classification-reproduction-and-parasexuality/54803
1% – https://www.researchgate.net/publication/11666410 _Proteins_ of_the_cyanobacterial_ photosystem_I
2% – https://www.biologydiscussion.com/bacteria/cyanobacteria/cyanobacteria-occurrence-morphology-and-cell-structure/52036