Anabaena is a planktonic blue-green alga of the class Cyanophyceae. It is often found in freshwater habitats.
Anabaena is known for its nitrogen fixing abilities.
Salient Features of Anabaena
The salient features of Anabaena are as follows:
- The plant body is of the unbranched filamentous form. The filaments usually occur singly and never form a colony like that of Nostoc.
- Each filament is composed of a straight or twisted trichome.
- The trichome is surrounded by a watery, hyaline, mucilaginous sheath. In some species, the mucilage sheath is absent.
- The cells are spherical or barrel-shaped and arranged in a single row, like beads on a string.
- Reproduction takes place only by vegetative methods, such as hormogonia and akinetes formation. Akinetes are much more elongated than those in Nostoc.
- The filaments of Anabaena can be motile if free from adhesive mucilage.
- Some species of Anabaena (e.g., A. flosaquae) can produce neurotoxins that are harmful to domestic animals.
Occurrence of Anabaena
Anabaena is a genus in the family Nostocaceae. It is represented by about 25 species.
Most of the species are plankton and are commonly found in freshwater ponds, pools, ditches, etc. Some species occur as endophytes within the root tubercles of Cycas and the leaves of Azolla.
Anabaena flosaquae grows in large numbers in stagnant brackish water.
Species of Anabaena also occur abundantly in rice fields. They can fix atmospheric nitrogen and tend to maintain the fertility of rice fields.
- Some Anabaena species are: A. aequalis, A. variabilis, A. flosaquae, A. sphaerica, A. azollae, A. oscillariodes, A. circinalis, etc.
Thallus structure of Anabaena
The plant body of Anabaena is a filamentous form. The filament is similar to Nostoc’s filament and often very difficult to differentiate from each other. But the only difference is that the filaments of Nostoc are aggregated together within a mucilaginous mass and form a colony. The colony does not form in Anabaena.
The filament is unbranched and consists of a single trichome. The trichomes are usually straight but may be circinate or irregularly contorted in some species.
Each trichome is enveloped by an extremely watery, hyaline, gelatinous sheath. If free from the gelatinous sheath, it becomes motile (Kautz and Bold, 1969).
The cells of the trichome are cylindrical, spherical, or barrel-shaped, arranged like beads on a string. The terminal cells may be rounded, tapered, or conical in shape.
Many intercalary, thick-walled empty cells called heterocysts are present in each trichome. The heterocysts usually occur singly and are of the same shape as vegetative cells.
Cell Structure of Anabaena
The structure of vegetative cells in the trichome is typically cyanophycean and prokaryotic in nature.
Each cell is differentiated into a cell wall and protoplasm. The cell wall is thick and consists of three layers: the outer mucilage layer, the middle pectic layer, and the inner cellulose layer.
The protoplast is differentiated into an inner, colourless centroplasm and an outer, pigmented cytoplasmic region called the chromoplasm.
Many pigments, cyanophycean granules, and pseudovacuoles are present in the chromoplasm. However, pseudovacuoles or gas vacuoles occur mainly in planktonic species.
The centroplasm represents only the incipient nucleus, or nuclear body. It consists of numerous randomly oriented DNA fibrils.
Reproduction in Anabaena
The Reproduction of Anabaena is similar to that of Nostoc. It reproduces only vegetatively by the following methods:
The older trichome of Anabaena becomes very large and irregular, due to which it breaks up into short fragments.
Each of these short fragments is capable of developing into a new trichome.
Hormogonium or hormogone formation is very common in Anabaena. It is a short segment of the trichome, which consists of a few cells.
The hormogonia are formed in the region of the heterocysts. Each hormogonium slips out of the enclosing mucilaginous matrix and divides into vegetative cells. Some cells become heterocysts, and ultimately a new trichome is developed.
During unfavourable conditions, the vegetative cells of the trichome become enlarged and secrete a highly resistant, thick wall around them. Such modified vegetative cells are called akinetes or resting spores or arthospores.
Akinetes may develop singly or in chains, on either one or both sides of the heterocyst or far away from them.
With the onset of suitable environmental conditions, akinete may germinate into a new filament.
In some species, heterocysts germinate to form new trichomes.
Taxonomic Position of Anabaena
According to Desikachary, 1959;