The name Cyanophyta (Gr. kyanos=blue + phyta= plants) has recently been adopted for this division. The division was named Myxophyta (Gr. + phyton=a plant) previously. It includes a single class Cyanophyceae. Most of the characteristics possessed by the members of this division are similar to those present in the bacteria, therefore these are placed along with bacteria in Kingdom. Monera However, these are considered nearer to algae because of presence of chlorophyll-a and liberation of O2 similar to other algal groups. They are common called blue-green algae because of dominant blue photosynthetic pigment in their chromatophores.
General Characteristics of Cyanophyta
- The members of this class are commonly known as blue-green algae.
- They are found in fresh water, ponds, ditchers, drains and moist soils.
- The terrestrial species form layers on moist, shaded bare rocks or soils.
- A few species are endophytes, i. e., live in the cavities of other plants, usually bryophytes, for example Nostoc is found in the thallus of Anthoceros and Anabaena in the roots of certain cycads.
- They are algal partners of lichens Many grow in waters of hot springs end form rock-like strata.
- Only few forms are marine.
- They are also found in hot water streams.
- Only few forms are unicellular, while majority of them are multicellular and some are members forms colonies. The cells are embedded in gelatinous sheath.
- A single row of cell in each colony is called as trichome and trichome with mucilage form the filament.
- Plastids are absent. The pigments are found in the cytoplasm called as chromatoplasm.
- Cells are blue in color due to the presence of phycocyanine (a blue pigment) chlorophyll, carotene and phycoerytherin.
- The cells are without any mitochondria, endoplasmic reticulum and Golgi bodies.
- A definite nucleus is absent and nuclear material is present as central body which lacks central nucleus.
Reserve Food Material
- Reserve food is in the form of sugars and glycogens. Minute oil drops are present.
- A proteneious material cyanophcin is also found as reserve food materiel.
Some members such as Anabaena can fix nitrogen into nitrates.
- Hormogones: Small parts of filament detached either by direct breaking by the formation of special disk called as hormogones.
- Heterocyst: Special large cells with thick walls called as heterocyst is present the filament. The filament is detached at this point.
- Hormospores: These are apical portion of the trichome in which the cells become enlarged and thick walled. On separation, these hormospores produced new filaments,
- Akinetes: When the vegetative cell along with the original cell wall is transformed into a spore.
- Endospores: when the vegetative cell give rise to several spores after division of protoplast.
- Motile reproductive bodies are entirely absent. There is no sexual reproduction present in the members of cyanophyta
- Sexual reproduction is entirely absent in this class.
Anabaena, Nostoc, Oscillatoria etc.
Economic Importance of Blue-green Algae
The most important economic aspects of blue-green algae are their role in nitrogen fixation and soil building.
Role in Nitrogen Fixation
Generally, algae utilize nitrates, ammonium salts and organic nitrogen as main source of nitrogen. Some species utilize ammonium ions and others nitrites. However, the members of Cyanophyta are capable of absorbing and fixing atmospheric nitrogen. This is called nitrogen fixation. The nitrogen is incorporated into the metabolic pathways as ammonia which is utilized in the synthesis of amino acids and proteins. The majority of the blue-green algae capable of fixing nitrogen have heterocysts in their filaments belong to Chroococcales, Nostacales and Stigonematales. A few non-heterocyst species like Oscillatoria princeps, Chlorogloea fristchii and Gloeocapsa sp have also been reported to fix atmospheric nitrogen.
Role in Soil Building
- The members of Cyanophyceae play an important role in preventing erosion reclamation of saline soils, and improving aeration of the soil. Therefore, these play an important role in soil building.
- Some members of blue-green algae such as Chroococcus Phormidium, etc. Form thick mats on soil surface, therefore act as soil binding agents. They prevent erosion especially in disturbed or burnt soils. These also Increase water absorbing capacity of the soil.
- The members of Cyanophyceae, e.g., Nostoc, Scytonema and Anabaena, form thick stratum on the surface of saline soil during rainy seasons. They help to reduce the pH of saline soils and increase water holding capacity of such soils.
- The rice fields are inoculated with Nostoc and Anabaena species. This results in promoting the yield of rice crop. The protein contents of the plants growing in such fields increase.
Harmful Aspects of Blue-green Algae
In addition to beneficial roles, the blue-greens produce certain harmful roles as well. These include:
Death to Fishes: Microcystis aeruginosa found in fish ponds produce a toxic compound poisonous to fish. Similarly, blooming species cause suffocation to fishes by choking theic gills.
Death to Animals: A few blue-greens such as Microcystis, Anabaena, Nodularia, Gloeotrichia spp produce toxins causing deaths to animals like horses, cattle, sheep, etc.
Water Pollution: Blue-green algae especially Oscillatoria, Anabaena, Microcystis spp grow immensely in storage water and pollute it. Their decomposition produces bad odour and cause interference in water filtration.
Affinities of Cyanophyceae
The members of Cyanophyceae are more near the bacteria than algae. The resemble bacteria in their prokaryotic cell structure, physiology and reproduction. Vilhelm placed blue-green algae in a group Archaeophyta along with bacteria. Fott classified blue-green algae in Prokaryonta (prokaryotes). Chadefausd (1960) recognized two groups among prokaryotes: the Cyanoschizophyta (blue-green algae) and Bacertioschizophyta (bacteria), He suggested that these two groups are linked together by transitional forms and blue-green algae are more primitive to bacteria. But, since the word algae is used for non-vascular green cryptogams, therefore the blue-green algae are included in algae. They exhibit some common characters with algae especially Rhodophyceae.
Affinities with Bacteria
- The blue-green algae exhibit following characteristics common with bacteria. The presence of mucilaginous sheath around the cells
- The construction and chemical nature of cell wall is almost similar in both groups.
- Both lack well organized nucleus and plastids. The nuclear material occupies centre of the cell or distributed throughout the cell cytoplasm.
- The members of both bacteria and blue-green algae are capable of nitrogen fixation.
- The reproduction is by fission or resting cells (akinetes).
- The true sexual reproduction is absented in both groups, however Para-sexual phenomena such as transduction are reported in blue-green algae.
- The members of both groups are characterized by presence of specific pigments, for example phycobilins in blue-green algae and bacteriochlorophylls in bacteria.
- Both are pioneers to colonize lifeless regions of volcanic origins.
Affinities with Red Algae
- Blue-green algae exhibit the following common characters with red algae.
- The principal constituents of mucilaginous sheaths are galactose, uronic acid, glucose and xylose in both cases.
- The presence of dominant phycobilin pigments (phycocyanin and phycoerythrin) in the members of both the groups.
- The reserve food in blue-green algae (Cyanophycean starch) resemble Floridean starch founds in red algae.
- The thylakoids containing photosynthetic pigments occur singly and widely separated in both groups.
- The members of both groups are characterized by absence of motile cells. However, the red algae differ from blue-green algae in that:
- They possess true nuclei.
- The presence of cellulose and carbohydrates in cell walls.
- The presence of true sexual reproduction.
- The presence of eukaryotic organelle like mitochondria. Golgi complex, endoplasmic reticulum, etc.