Cyanophyceae and potential application

Introduction to Cyanophyceae:

It is a primitive group of algae, consists of 150 genera and about 2,500 species. In India, the division is represented by 98 genera and about 833 species. Members of the class Myxophyceae (Cyanophyceae) are commonly known as Blue Green Algae(BGA). The name blue green algae is given because of the presence of a domi­nant pigment c-phycocyanin, the blue green pigment.

In addition, other pigments like chloro­phyll a (green), c-phycoerythrin (red), β-carotene and different xanthophylls are also present. The members of this class are the simplest living autotrophic prokaryotes

Important Characteristics of Cyanophyceae:

The important characteristics of the division are as follows:

1. The individual cells are prokaryotic in nature. The nucleus is incipient type and they lack membrane bound organelles.

2. Both vegetative and reproductive cells are non-flagellate.

3. Cell wall is made up of microfibrils and is differentiated into four (4) layers. The cell wall composed of mucopeptide, along with carbohydrates, amino acids and fatty acids.

4. Locomotion is generally absent, but when occurs, it is of gliding or jerky type.

5. The principal pigments are chlorophylls a (green), c-phycocyanin (blue) and c-phyco- erythrin (red). In addition, other pigments like β-carotene and different xanthophylls like myxoxanthin and myxoxanthophyll are also present.

6. Membrane bound chromatophore are absent. Pigments are found embedded in thylakoids.

7. The reserve foods are cyanophycean starch and cyanophycean granules (protein).

8. Many filamentous members possess specia­lized cells of disputed function (supposed to be the centre of N₂ fixation) known as heterocysts.

9. Reproduction takes place by vegetative and asexual methods. Vegetative reproduction takes place by cell division, fragmentation etc. Asexual reproduction takes place by endospores, exospores, akinetes, nannospores etc.

10. Sexual reproduction is completely absent. Genetic recombination is reported.

Occurrence of Cyanophyceae:

Members of Cyanophyceae are available in different habitats. Most of the species are fresh water (e.g., Oscillatoria, Rivularia), a few are marine (e.g., Trichodesmium, Darmocarpa), and some species of Oscillatoria and Nostoc are grown on terrestrial habitat.

Species of some members like Anabaena grow as endophytes in thallus of Anthoceros (Bryophyta) and in leaves of Azolla (Pteridophyta) and Nostoc in the root of Cycas (Gymnosperm).

Species of Nostoc, Scytonema, Gloeocapsa, and Chroococcus grow symbiotically with different fungi and form lichen. Some members like Nostoc, Anabaena etc. can fix atmospheric nitrogen and increase soil fertility.

Thallus Organisation in Cyanophyceae:

Plants of this group show much variation in their thallus organisation.

The thallus may be of unicellular or colonial forms:

1. Unicellular Form: In unicellular form, the cells may be oval or spherical. Common members are Gloeocapsa (Fig. 3.23A), Chroococcus and Synechococcus.

2. Colonial Form: In most of the members the cells after division remain attached by their cell wall or remain together in a common gelatinous matrix, called a colony.

The colonies may be of two types:

a. Non- filamentous, and

b. Filamentous.

a. NonFilamentous Type: The cells of this type divide either alternately or in three planes, thereby they form spherical (Gomphosphaera, Coelosphaerum), cubical (Eucapsis alpine, Fig. 3.23C), squarish (Merismopedia) or irregular (Microcystis, Fig. 3.23B) colony.

b. Filamentous Type: By the repeated cell division in one plane, single row of cells are formed, known as trichome. e.g., Oscillatoria (Fig. 3.23D), Spirulina, Arthosporia etc. The trichome when covered by mucilaginous sheath is called a filament. The filament may contain single trichome (Oscillatoria, Lyngbya) or several trichomes (Hydrocoleus, Microcoleus, Fig. 3.23E).

The trichomes may be unbranched (Oscillatoria, Lyngbya), branched (Mastigocladus limilosus, Fig. 3.23J) and falsely branched (Scytonema, Fig. 3.23K and Tolypothrix).

The cyanophycean cells are prokaryotic in nature, which rarely exceeds 10p in diameter. Each cell consists of outer covering of cell envelop which surrounds the membrane covered protoplast (Fig. 3.24C).

1. Envelop:

a. Mucilagenous Sheath: Presence of mucilagi­nous sheath is common in all cyanophycean members. It consists of three layers of microfibrils arranged reticulately within an amorphous matrix.

b. Cell Wall: The cell wall consists of four (4) (Fig. 3.24A) layers (under E.M.) named as LI, LII, LIII and LIV by Carr and Whitton (1973). Each layer is about 10µ in thickness. The LI is the layer situated near cell membrane and LIV is the outer­most.

Cell wall is composed of mucopeptide together with carbohydrates, amino acids and fatty acids like Gram-positive bacteria. The LI and LIII layers are electron transparent, but the LII and LIV layers are electron opaque (impervious).

2. Cytoplasmic Membrane:

The cytoplasmic membrane is also known as plasmalemma pre­sent just inner to the cell wall. It consists of two electron opaque layers separated by a trans­lucent layer 

3. Protoplast:

Studies with Electron Micros­cope by Wilden and Mercer, Pankrats and Bowen and many others show that the protoplast consists of thylakoids, cytoplasmic inclusion and nucleoplasm.

a. Thylakoids: These are the complex lamellar system, which functions like the protoplasts of eukaryotes. Thylakoids are not bounded by membrane instead they appear as elongated and flattened sacs composed of two unit membranes.

b. Cytoplasmic Inclusions: The cytoplasmic inclusions present in the cyanophycean cell are ribosomes, cyanophycean granules, polyhedral bodies, polyphosphate bodies, polyglucoside bodies, α-granules, β-granules and gas vacuoles 

c. Nucleoplasm: The nucleoplasm is usually centrally located and contains numerous fine randomly oriented fibres of DNA.

Heterocysts

These are specialized cells of the filament distinguished from others by their thick wall, polar nodule(s) and homogenous contents.

Heterocysts are commonly found in the members of Stigonematales and Nostocales (except Oscillatoriaceae). They grow in the filament either terminally (Gloeotrichia) or inter­calary (Nostoc) or both terminal and intercalary (Anabaena desikacharyiensis).

Structure: Pale yellow, thick-walled specia­lized cells, larger than vegetative cells are called heterocysts. The wall of the heterocyst is diffe­rentiated into three regions, an outer fibrous, middle homogenous and inner lamellar layer.

It has pore(s) at the pole(s) where it remains attached with the vegetative cell. The pores are plugged with polar nodule(s) or polar granules. The wall of the heterocyst is thicker towards the polar regions.

The inner content is dense and appears to be homogeneous. The thylakoides i.e., the photo­synthetic lamellae are present, but the ribosomes are less in number. Other granular inclusions are appears to be absent. Due to absence of phycobillins and chlorophyll a, photosynthesis does not take place.

Function of Heterocyst:

1. Some considered it as the store-house of food materials.

2. Some others considered that they help in hormogonia formation.

3. According to Brand (1903), it is of spore-like structure.

4. Fritsch (1951) told that during vegetative period it secretes certain substances which promote growth and cell division.

6. Singh and Kumar (1970) pointed out that in addition to N₂ fixation; it also helps in growth and development of the filament.

Gaidukov phenomenon or complementary chromatic adaptation:

The efficiency to change the pigment com­position, to absorb maximum light for photosyn­thesis, with the variation of the incident light is called complementary chromatic adaptation.

Many members of Cyanophyceae have the capacity to change their colour in relation to the wave length of incident light. Due to variation of the wavelength of incident light they can change their pigment composition. It may appear blue green in yellow light, green in red light and reddish in green light. Gaidukov (1903) first invented the phenomenon and according to his name it is also known as “Gaidukov phenomenon”.

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Importance of Cyanophyceae

1. They are one of the early colonizers of bare and barren areas. They provide suitable conditions for the growth of other organisms even in the most hostile environment.

2. Blue green algae function as food to several aquatic animals. Spirulina is regularly collected for human consumption in parts of Africa. Nostoc is similarly used in China. In Rajasthan Anabaena and Spirulina are collected from Sambar Lake and used as fodder and manure. Spirulina is very easily cultivated in tanks and can be used as a palatable protein rich food supplement for humans and animals.

3. Several cyanobacteria have the ability of nitrogen fixation. The filamentous forms possess special large pale cells or heterocyst’s for this. Some of the fixed nitrogen comes out as excretion. After death of cyanobacteria the substratum becomes rich in nitro­gen. Such nitrogen fixing cyanobacteria are now regularly inoculated in the rice fields. This saves consumption of nitrogen fertilizers.

4. Nitrogen fixing cyanobacteria are often used for reclaiming usar soils, e.g., Nostoc, Anabaena. These cyanobacteria produce acidic chemicals for counteracting alkalinity of the soil and nitrogenous compounds which are generally deficient in these soils.

5. Antibiotic can be manufactured from extract of Lyngbia.

6. Species of Anabaena and Aulosira do not allow mosquito larvae to grow nearby. Such cyanobacteria can be inoculated in village ponds and rice fields to prevent the growth of mosquitoes.

7. Cyanobacteria can grow on the walls and roofs of buildings during the rainy seasons causing discolouration, corrosion and leakage.

8. They produce water blooms, imparting bad odour and colour to water bodies.

9. Some cyanobacteria produce toxins harmful to most aquatic animals. They may prove equally toxic to human beings drinking or bathing in such water. The important toxins producing cyanobacteria are Microcytic aeruginosa (= Anacystis cyanea), Anabaena flosaquae, Aphanizomenon flos-aquae.

Source: biology discussion/botanyvvc.blogspot.in/Sridhar Uddamari