Reproduction in Cyanobacteria:
It reproduces by simple and primitive methods of reproduction, which are vegetative and asexual. The sexual reproduction is totally absent.
1. Vegetative Reproduction:
It occurs by following methods:
(i) Fission: Unicellular cyanobacterial cells divide and reproduce by fission.
(ii) Fragmentation: in this method, filaments break down into small pieces and each piece develops into a new colony.
(iii) Hormogonia: in this method, filaments break into pieces or trichomes, which are called hormogonia and develop into new filaments.
2. A sexual Reproduction:
Many non-motile Cyanobacteria reproduce by spores, which are of the following types:
(i) By Endospore : In this condition one or more cells increase in size and their protoplasm divides into many parts to form endospores, e.g., Dermocarpa.
(ii) By Exospore : These structures are formed in the distal part of the protoplasm, e.g., Chaemosiphon.
(iii) By Akinetes : These are formed close to the heterocysts. In this case, cells increase in size and a thick layer is formed around it. Under favourable conditions, they give rise to new filaments, e.g., Cylindrospermum.
(iv) By Nannocytes : In some of the filamentous Cyanobacteria, cells may divide into many parts without any change in shape. In this way so many bodies are formed which are known as nanocysts, e.g., Microcystis.
Some filamentous Cyanobacteria form special types of cells known as the heterocysts. Usually their position is definite in the filaments. In some species, they are found at approximately same distance from each other. But in some species they are found at one or both the ends. Their function is not known until now.
Cell wall of the heterocyst is made up of two layers. Outer layer is made up of pectin or cellulose and inner layer is of cellulose. At both ends of intercalary heterocyst and one end of apical heterocyst (which are in contact with other cells), there is present a pore and a polar nodule. Through polar pores, they remain attached with other cells.
Due to the presence of carotene, usually their color is yellow. They also contain lamellae, which help in photosynthesis. Following are the hypotheses proposed about the functions of the heterocysts:
(i) Their work is mechanical. Trichome may break easily at the site of heterocyst.
(ii) They are rudimentary reproductive organs.
(iii) They help in production of akinete.
(iv) Enzyme present in the heterocyst, forms gas vacuoles.
(i) Heterocyst secretes some material, which is essential for growth and metabolism.
Prokaryotic structure and asexual reproduction are the features of Cyanobacteria or blue-green algae. These organisms are important for us because they fix nitrogen and increase the soil fertility.
In sub-tropical countries blue-green algae such as Microcystis, Aphanizomenon and Anabaena cause harm to animals and are sometimes responsible for their death. In human beings, also they cause stomach diseases.
Examples of Cyanobacteria:
Qscillatoria, Nostoc, Anabaena, Gleocapsa, Merismopedia, Cylindrospermum, Aulosira, Scytonema, Tolypothrix etc.
In short, blue-green algae are the other name of Cyanobacteria.