Table of Contents
Blue green algae (cyanobacteria) has been given the status of the class by some workers and they call it Myxophyceae (Gr. myxa means slime; phyton, a plant) or Cyanophyceae (Gr. kyanas, a dark blue substance; phyton, a plant) while other workers believed that they should be placed at the level of Division, and should be called Myxophyta or Cyanophyta.
It comprises the most primitive and simplest of plants. It consists of 150 genera and 1500 species. They are known as blue-green because their cell contains blue-green pigments that dominate other pigments and embark specific color to them.
The pigments unlike those of other algae are not localized in definite chromatophores but are distributed throughout the entire peripheral portion of the protoplast i.e. they are Prokaryotic in nature.
Blue-Green Algae Classification
The division Cyanophyta includes only one class Cyanophyceae. It was considered in the earlier literature that there are only two orders e.g.
Division By Smith And Fritsch
Smith Divided It Into 3 Orders
Fritsch Divided It Into 5 Orders
It consists of a unicellular or colonial but never a filamentous body. Reproduction is by vegetative cell division. Endospore production is not reported. (Anacystis, Gloecapsa, Chroococcus).
It also consists of unicellular or colonial forms Reproduction is by endospore formation (Dermocarpa, Stichosiphon, Chamaesiphon).
The plant body is filamentous without the formation of hormogonia. (Pleurocapsa).
The plant body is filamentous and unbranched with the formation of hormogonia. (Spirulina, Lyngbya, Nostoc)
The plant body is filamentous and branched with the formation of hormogonia.
The Recent Division
Recently taxonomists divide the class into two tribes with seven orders
A). Tribe Coccogoneae (Unicellular And Non-Filamentous)
- Order 1. Chroococcales
Family: a). Chiococcaeae e.g. Chroococcus (b). Entophysalidaceae
- Order 2. Chamaesiphonales
Family: a). Pleurocapsaceae (b). Dermocarpaceae (c). Chamaesiphonaceae
B) Tribe Hormogoneae (Filamentous Blue-Green Algae)
- Order 3. Oscillatoriales
Family: Oscillatoriaceae e.g. Oscillatoria
- Order 4. Nostocales
Family: Nostocacae e.g. Nostoc
- Order 5. Scytonematales
Family: Scytonemataceae e.g. Scytonema
- Order 6. Stigonematales
Family: Stigonemataceae e.g. Stigonema
- Order 7. Rivulariales
Family: Rivulariaceae e.g. Rivularia
Phylogeny Of Cyanobacteria
Several fossil genera and species are reported from the Pre Cambrian to the present. The oldest fossil algae are considered to be Cyanophyta.
Most of the records from the Pre-Cambrian are extremely fragmentary and present no evidence of apparent phylogenetic significance.
No heterocysts have been reported in the fossil condition. Many specimens exhibit a tubular structure and have been considered to be sheaths of blue green algae.
Some phycologists consider many of the described blue green algal fossils invalid. In addition to these Pre-Cambrian records, specific fossils are known from the Mesozoic eras.
The significance of the Cyanophyta in the geologic past is probably great, judging from the ability of certain contemporary species to form calcium and magnesium carbonate deposits.
The importance of some lies in reef formation today and the ability of certain to fix nitrogen. Very little evolution from the basic unicellular, little-differentiated type appears to have taken place within the Cyanophyta.
The purported age, constancy in the form, and absence of sexual reproduction are no doubt responsible for the degree of uniformity present in the Cyanophyta (Cyanobacteria).
Following are the cyanobacteria characteristics or features of class Cyanophyta:
1. The dominant pigments responsible for the characteristic blue-green color of the cell are chlorophyll a, carotene, xanthophyll, c-Phycocyanin, and c-Phycoerythrin.
2. Pigments are not localized in definite chromatophores.
3. The plants are extremely simple in structure.
4. The plant may be unicellular, multicellular, or filamentous.
5. The condition of a cell wall is more or less unstable.
6. Flagella are not formed at any stage of the life cycle.
7. The photosynthetic product is Glycogen (Cynophycean starch).
8. Cell shows the primitive structure, i.e. absence of true nucleus, definite plastids, mitochondria, and endoplasmic reticulum i.e., the cell is Prokaryotic.
9. Protoplast secretes an abundant amount of pectin.
10. The cells are often enclosed in slime or gelatinous sheath.
11. Sexual reproduction is invariably absent. However genetic combination has been observed but the mechanism is unknown.
12. Their morphological characters associate them more closely with bacteria than with algal groups which they resemble mainly in the possession of chlorophyll.
Blue-Green Algae Examples
Below are a few examples of blue green algae:
Blue Green Algae Reproduction
The chief method of blue green algae reproduction is fragmentation. But also, various types of non-flagellated spores or spore-like bodies are produced.
Reproduction by fragmentation may occur by a simple breaking apart of a thallus into two or more units.
Each fragment is then capable of continuing growth to produce a new colony. Fragmentation is the sole mean of an increase in the number of individuals in the great majority of non-filamentous forms.
In filamentous species, hormogones are common in some genera, such as Oscillatoria. (Fig.2.15 C). A hormogone is not a simple fragment of a filament; rather the filaments break into short multicellular segments, which may be capable of gliding movement.
Hormogones are liberated from the sheath under optimum growing conditions and develop into a new plant. The break of a filament to form a hormogonium may result from the death of an intercalary cell.
Such a place in a filament is known as a separating disc, and the biconcave shape is the result of a decrease in pressure in the adjacent cells (Fig., 2.15 C).
There is no conspicuous differentiation in the cells of the hormogonium in any way, except that the terminal cell is rounded? Hormogonia are sometimes dormant and act as a resting stage.
They may form under unfavorable conditions and are frequently associated with rapid changes in temperature and desiccation such as when a pond or stream becomes dry.
Another type of reproductive Structure formed in certain filamentous genera is spore-like akinete (Fig., 2.15 B) a single cell that develops directly from the metamorphosis of a vegetative cell.
Usually, the akinete is considerably large than the ordinary cell and has a thickened wall outside the regular cell wall. The akinete contains a rich accumulation of food reserves such as cynophycin granules and is resistant to adverse conditions.
On germination, the akinete develops into a new filament within the old cell wall. Some akinetes germinate immediately after their formation but they may remain dormant for some time and can be extremely resistant to desiccation and high temperature.
Another type of spore-like structure is the heterocyst, which is common in certain filamentous genera. (Fig., 2.15 A). There is some question about whether all heterocysts behave in the same manner.
The heterocyst, too, is a metamorphosed vegetative cell, which in some instances functions to produce a new filament; in other instances, it is a degenerated tell.
In the latter case, the heterocyst lost its ability to act as a reproductive body. The contents of heterocysts are uniformly dense and generally yellowish. However, sometimes the protoplast remains a colorfully blue green.
Heterocyst may be intercalary or terminal in position. The wall becomes thickened, and an inner swelling or papilla develops at the end attached to the rest of the filament.
A median pore is present in each papilla and a delicate protoplasmic connection passes through it to an adjacent vegetative cell. Prominent, highly refractive granules called polar granules are often present in the heterocyst near these pores.
A filament may fragment at the point of an intercalary heterocyst; thus, this structure is regarded as contributing to vegetative reproduction by fragmentation.
In some filaments, the heterocysts persist after the adjacent vegetative cells die. Informs with both heterocysts and akinetes, the position of one is relative to that of others.
Finally, the true spores’ formation (aplanospores) within a cell is present in certain genera. One type of true spore is endospore (Fig., 2.15 E). In certain forms such as Dermocarpa, the protoplast divides endogenously into many units.
The wall of the parent cell eventually breaks down and liberates these units as individual thin-walled spores, which are then capable of germinating and growing immediately into new plants.
Heterocysts have been reported to germinate by the production of endospores. Sometimes another type of aplanospore, the exospore, may be distinguished, although it the not a striking’ one (Fig., 2.15 D).
Exospore is formed basipetally at the distal end of attached unicellular algae as in Chamaesiphon, by transverse division of the protoplast.
Sexual fusion, as known for most other algae groups, has not been observed in the blue green algae. However, there are recent reports of an exchange of genetic material. It is likely, that any sexuality in the Cyanophyta will be found to be similar to that reported in bacteria.
Economic Importance Of Blue Green Algae
Blue green algae play an important role in the life of man. If properly understood and utilize it can be further utilized for the service to man. Some services so for study are;
Addition Of Nitrogen To Soil
The direct utilization of blue green by man is at present limited. The Japanese eat certain freshwater species as a confection after treating them with sugar. However, their chief importance is indirect.
As a result of their autotrophic activities, they play an important part as primary producers in all various environments in which they occur. One of their greatest contributions is a nitrogen-fixers in developing soil fertility; especially in areas where rice is cultivated.
The presence of blue green algae in the paddies reduces the need for fertilizer rich in nitrogen. In certain desert soils, nitrogen fixation is mainly due to the presence of blue green algae.
This ability to fix atmospheric nitrogen is widespread in the filamentous blue green algae, occurring in certain species of Nostoc, Anabaena, Tolypothrix, etc. Scarcely any of the non-filamentous forms are able to fix nitrogen.
Development Of Soil
The presence of photosynthetic algae in the water-clogged fields prevents deterioration in aeration, by increasing the available oxygen to the roots. This minimizes disease susceptibility of roots.
The filamentous blue-green algae have been used to reclaim lands exhausted from overuse and over-irrigation. In such fields the land. is flooded with water and the algae develop in extensive mats.
After several months, the flooded area dries and the algal material is plowed into the soil. The algae thus serve as the beginning of the humus layer.
Blue green algae also form extensive mats on the soil surface, which bind the soil, absorb water, and thus reduce soil erosion.
Effects Of Toxic Algae Blooms
They may be present in large quantities in water to color the water brilliant blue-green or even red. This dense concentration of blue green algae is known as blooms or algae blooms.
What Causes Algae Blooms In Ponds And lakes?
They are caused by certain species of the non-filamentous Microcystis and filamentous genera including Nostoc, Gloeotrichia, and Oscillatoria.
Optimum conditions for such conditions are the interaction of many factors, which are long hours of sunlight, shallow warm water, high nitrogen contents, and possibly phosphorus.
Especially troublesome blooms occur in ponds, lakes, fish aquariums, and domestic drinking water supplies such as reservoirs, where they clog filters.
In the extreme conditions of algal growth, the water may be unfit for human or livestock consumption and may be toxic to fish or other aquatic organisms.
If the concentration of algae is low, there is only a disagreeable taste or smell. Livestock, including cattle and sheep, have been known to succumb from drinking water contaminated with certain species of Microcystis, Anabaena, and Aphanizomenon.
No death among humans has been reported, although many instances of 24-hour (gastrointestinal) problems may be due to such toxic substances.
Formation Of Travertine
Several blue green algae are believed to be responsible for many calcareous concretions or, pebbles in standing freshwater, as well as in thermal areas. The Cyanophyta are also important in tropical marine reef formations.
The exact mechanism is not understood, but the algae secrete calcium and magnesium carbonates and build up extensive deposits of travertine (white or light-colored calcareous rock deposits) in a variety of forms.