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Algae

 

General Characteristics of Algae
Reproduction in Algae
Life Cycle

Economic Importance 

Algae in Different Uses

 

General Characteristics and Classification of Algae

1. Algae are cosmopolitan in distribution.

2. They occupy first place in the plant kingdom.

3. Their plant body ranges from a single cell to complex giant kelps.

4.  Different groups exhibit different colours due to differences in pigmen­tation.

5.  They reproduce in three ways: 1. vegetative 2. asexual and 3. sexual.

6. Different types of life-cycles are known in algae.

7. The algae are divided into  number of divisions.

8.  Pigmentation, storage products, flagella, cell wall and cellular organiza­tion are important criteria in the classification of algae.

The algae, commonly known as "pond scums", "water mosses", "sea weeds" etc., comprise a large heterogeneous assemblage of plants. Fritsch pointed out that unless certain limits are drawn, the designation of alga must include all those forms that fail to exhibit the characteristics of an archegoniate plant. That means, the algae can be distinguished by the following features:

1. The absence of archegonium like structure.

2. The reproductive bodies, either sporangia or sex organs, are unicellular or multicellular and in both all cells are fertile (except Chara)   without any sterile cells.

Distribution

The algae are cosmopolitan in distribution. They occur in a wide variety of habitats - fresh, brackish and marine waters, in or on the soils, moist walls, tree trunks, snow, hot springs etc. They adapt themselves well to a particular habitat. However, large majority of forms occur in aquatic environments. Aquatic algae may be floating or suspended i.e., planktonic or attached and living in the bottom i.e., benthic. Some algae live endozpically or epizoically in or on the animals like protc-zoans, coejenterates and molluscs. Similarly a few of them occur epiphytically or endophytically. Cephaleuros is a common parasitic alga on the leaves of a number of angiospennic trees.

Some algae develop in the form of blooms. They secrete a number of substances and some of them are toxic. Blue-green algae and dinoflagellates are common examples of this category.

The algae are transported from one pi ice to another through the agency of wind and birds. They are transported either in vegetative state or in spore rendition.

Position in the Plant Kingdom

In the plant kingdom, the algae are studied first because (1) the fossil record indicates that the ancient organisms with chlorophyll 'a' were probably blue-green algae, (2) the relative simplicity of the plant bodies and (3) the clear illustration of many important biological phenomena (e.g., sexual reproduction). So a number of Botanists view the algae (speci­ally the green algae) as likely progenitors for the other groups of Plant Kingdom, because of similarity in pigmentation (presence of chlorophylls 'a' and 'b') and reserve food as starch.

Range of Plant Body

The form of algae varies from a single cell to the complex giant kelps (seaweeds) and rockweeds. Species Chlorella (5-8) are in the range of bacterial size whereas some of the seaweeds may attain a length of 60 m. Unicellular, colonial, filamentous (unbranched, branched, heterotrichous), membranous or foliose and tubular types of plant body occur. Some have highly differentiated blade-like types and a few have root like organs, stems and leaves, although these organs lack vascular tissue but have conducting cells.

Growth of various algae may be diffuse or generalized or localized. It may be apical (Chard), basal (Bulbochaete) or intercalary (Oedogonium).

The diversity of algal organization can be evolved from ancestral unicellular flagellate algae. It has been postulated that two major series developed from such ancestors: 1.Colonial motile habit representing the Volvocine series and 2.nonmotile algae which include gelatinous, palraelloid colonies representing the Tetrasporine series.

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Reproduction in Algae

Reproduction in Algae

In algae reproduction takes place in 3 ways: I. vegetative 2. asexual and 3. sexual.

Vegetative Reproduction

In some unicellular organisms reproduction is by cell division. The divisions may be repeated in rapid succession. This is sometimes called binary fission (Fig.1A). In colonial and certain multicellular algae, cell division and subsequent enlargement occur. Many filamentous forms, non-colonial and other multicellular algae reproduce by fragmentation. The fragments have the capacity to continue growth and develop into new individuals. Among the filamentous blue-green algae this is a specialised process and the fragments which exhibit gliding movement are called hormogonia (Fig.1B). In coenobic algae the reproduction is by autocolony formation. Autocolony is a miniature colony produced by a cell of a parent colony (Fig.1C).

Algae produce a variety of spores out of which akinetes are very common in blue-green and green algae. An akinete is a vegetative cell with its wall thickened and can withstand the unfavourable conditions (Fig.1D).

Fig.1. Methods of reproduction (Diagrammatic) A. Bipartition or binary fission. B. Fragmentation or hormdgone formation. C. Autocolony formation D. Akinete formation. E. Zoospore formation F Aplanospore formation. G. Autospore formation, H. Isogamy. I. Anisogamy. J. Ooganiy.

Asexual Reproduction                                                                      

Asexual reproduction is achieved by the production of various types of spores. Except Cyanophyceae and Rhodophyceae, most of the groups produce zoospores which are mottle unicells (Fig.1E). In some cases the zoospores loose their motility and such spores are called aplanospores (Fig.1F). In some, the aplanospores appear identical to the parent cell and these are referred to as autospores (e.g., Chlorella, Fig.1G). Some­times the aplanospores thicken their walls and develop into hypnospores. The endospores and exospores of Cyanophyceae, monospores, tetraspores etc. of red algae are other types of asexual spores. These are described in detail under the individual groups. In reproduction the word 'swarmer' is commonly used for a motile spore which behaves either as a zoospore or as a gamete. In multicellular forms the spores may be formed in all cells or it may be restricted to well defined 'sporangia'. In Phaeophyceae two kinds of sporangia are known: 1. plurilocular and 2. unilocular. The former is multicellular and produce swarmers which can be either gametes or zoospores. The unilocular sporangia produce a number of swarmers which are always asexual in nature.

Sexual Reproduction

In sexual reproduction there is an opportunity for exchange of genetic material and formation of new combinations. Sexual reproduction is not observed in Cyanophyceae. It is not yet confirmed in Euglenophyceae. In all the other classes of algae it is present. This is effected by three basic methods: 1. Isogamy 2. Anisogamy and 3. Oogamy. In isogamy fusion occurs between two morphologically identical gametes. Anisogamy involves the pairing of two dissimilar gametes, i.e., one gamete is smaller than the other. In certain cases the morphologically identical gametes behave differently, thus exhibit the physiological anisogamy. In some algae the gametes may be highly dimorphic. The larger, non motile gamete is called an egg or ovum and the smaller, motile one is called the sperm or spermatozoid. A spermatozoid unites with an egg and this type of sexual reproduction is known as Oogamy (Fig.1H-J). After the fusion of the gametes a zygote is formed. The germination of zygote may vary in different algae but usually the contents divide to form zoospores. These zoospores germinate into the parent plant. In rare cases the zygote ger­minates directly into an adult plant. In Charophyceae the germination of the zygote is indirect and produces a protonema from which the adult plant develops.

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Life - Cycles

Life - Cycles

Different types of life-cycles have been recognised in the algae:

1.   Haplontic:    In this, the parent is haploid and the zygote represents thediploid phase with reduction division occuring at the time of the ger-     mination of the zygote (e.g., Volvox, Oedogonium).

2. Diplontic: The parent is a diplont and the  sexual   spores   (gametes) constitute the haploid  phase.   Reduction  division   takes  place  at  the time of gametogenesis (e.g., Diatoms).

3. Diplo-haplontic: There   will be an  alternation of diploid  sporophyte with the  haploid  gametophyte. Reduction  division is  effected at  the time of formation of spores by the sporophyte (e.g., Cladophora).

4. Haplo-biontic: Two haplod generations (gametophyte and carposporophyte) alternating with a  diploid one  represented by   the zygote (e.g., Batrachospermum).

5. Diplo-biontic:  Two diploid phases (carposporophyte and tetrasporo-phyte)   and  a haploid   phase  (gametophyte)   alternating    with   each other   (e.g , Polysiphonia).

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Economic Importance

Economic Importance

The algae are economically very useful and important in the fields of agriculture, industry and human consumption. They are also used as fodder and feed for animals, as source of minerals and as manure. Algae are highly important as primary producers of organic matter in aquatic habitats because of their photosynthetic activity.

Algae in Industry

The algae yield a number of products of commercial importance. The major products are 1. Agar agar, 2. Carrageenin, 3. Aliginic acid and 4. Diatomite.

Agar Agar

It is a slimy or mucilage substance obtained from the cell walls of certain red algae. It is extracted from the thalli of Gelidium, Gracilaria and Gigartina. Japan was the largest producer of agar agar until 1939. The extract is a gel containing galactose and a sulphate. It melts between 90 and 100F and solidifies at lower temperatures. Agar agar is used in the preparation of food stuffs such as salads etc. and taken as a dish along with the diet regularly. It is used as an emulsifier in dairy products (ice-cream preparation). It is largely used as a substrate in the culturing of various microorganisms such as bacteria, algae and algae in the laborato­ries. It is also used in cosmetics, leather and textile industries.

Alginic Acid and Alginates

These are extracted from the marine brown algae such as Ascophyllum, Laminaria, Macrocystis etc. Algin is a carbohydrate present in the middle lamellae and primary walls of the seaweeds. It is a colloidal substance present in the form of a Calcium salt which is soluble. The salts are known as the alginates. These are used as thickening agents in food industry, cosmetics, and in textiles as printing pastes. They are also used in the preparation of plastics and artificial fibres. The alginates have great value as emulsifiers, gellying agents, dental impression powder, paints etc. 

Carrageenin (or Carragheen)

It is a cell wall polysaccharide. It is mucilaginous in nature. It is obta­ined mainly from the red alga, Chondrus crispus (Irish Moss) and in lesser quantity from Gigartina. The mucilaginous extract is used in food, textile, pharmaceutical, leather and brewing industries. It is used as a component in tooth pastes, deodorants, cosmetics etc. It is also used to stabilize emulsions.

Iodine

Japan produces iodine regularly from the kelps (brown seaweeds) e.g., Laminaria, Fucus, Eisenia and Ecklonia. Similarly bromine can be obtained from the red algae-Rhodotmela and Polysiphonia.

Minerals

The kelps are also a source of soda and potash. The ash of the keps will be added to the soil to increase the mineral content of the soil.

Glue

Another important algal industry in Japan is glue manufacturing. For this purpose Gloeopeltis furcata, a red alge, is used. This glue is known as 'funori' and is used for sizing paper and cloth. It is also used as an adhesive.

Diatomite

It is a diatomaceous deposit formed due to the deposition of indestru­ctible, siliceous frustules of diatoms over a number of years over the sea floors. This diatomaceous earth has got several commercial uses. It is used as a filter for oils, in sugar industry and for clearing solvents. It is used in the insulation of refrigerators, boilers, hollow tile bricks for the construction of constant temperature rooms, sound proof rooms and in metal polishes. It is also a constituent of some tooth powders, bleaching powders and a reinforcing agent in concrete. It is also used as a base on automo­bile and silver polishes.

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Algae in Different Uses

Algae in Agriculture

Soil algae, specially the blue-green algae arc capable of fixing atmo­spheric nitrogen and increase the fertility of the soil. They also enhance the crop production. These algae are the chief agents of nitrogen fixation in rice fields. Hence they are known as the bio-fertilizers or algal fertilizers. Species of Anabaena, Nostoe, Tolypothrix and Aulosira are important in this field. These are cultivated on mass scale, dried and supplied in packets as seed material to be applied in the rice fields at the time of crop cultivation. The blue-green algae are also useful in the reclamation of barren alkaline soils i.e., 'Usar soils'. By growing the blue-green the alka­linity may be neutralized and fertility may be increased. Likewise such soils are reclaimed and brought under cultivation.

A number of seaweeds are used as fertilizers (as manure). Large brown and red algae are important in this. They are rich in 'Potassium' and poor in 'Nitrogen and 'Phosphorus' than the farm manure. They are applied to the field directly and ploughed. Such fields are used for growing vegetables in countries like France, Ireland and Sri Lanka. In Japan they are used in the rice fields and in China for growing groundnut and sweet potatoes. In India Turbinaria is used as a fertilizer for plam trees. The seaweeds are also used as compost. Sometimes the burnt ash is added to the farm lands. The concentrated liquid extracts of some seaweeds are sold as liquid fertilizers and also as insecticides.

Lithothamnion and Lichmophyllum which are encrusted with lime, are used in place of lime, after grinding the material. The freshwater alga, Chara can also be used similarly.

Algae as Food

Algae serve as a source of food for fishes, aquatic and terrestrial animals and human beings. More than 70 species of marine algae (red and brown seaweeds) have been used for food in oriental countries like China, Japan etc. The red, brown and green algae form a regular portion of human diet and are used prolifically. Spirogyra and Oedogonium in India and Viva in Europe are important. The mucilage balls or colonies of Nostoc are boiled and eaten in Brazil. The raw red and brown algae are chopped and added to other dishes. The young stipes of Laminaria and sporophylls of Alaria are also eaten in Japan, Durvillea Ulva are dried, salted and sold. Large quantities of these are consumed in Chile. Ulva lactuca was used in salad and soups in Scotland. Porphyra is a tasteful dish in Korea, Japan and China. It is rich in vitamins 'B' and 'C'. In Philip­pines Caulerpa is cultivated as a source of food.

Some algae are rich in proteins, fats and vitamins A,B,C & E. The diatom Nitzschia is rich in vitamin A. Vitamin 'B' is common in Ulva, Enteromorpha, Laminaria, Porphyra and Chondrus. Ulva, Enteromorpha, Alaria etc. also contain vitamin 'C'. 

Algae as Fodder  

Norway, France, Denmark, Newzealand and U.S.A. use marine algae as fodder for cattle. Ascophyllum, Fucus and Laminaria are processed into suitable cattle feed and given to the cattle, poultry and pigs. This enhances the milk yielding capacity of the cattle. Similarly butter and fat content of milk increases. The egg laying capacity of the poultry increases and egg-yolks will have increased iodine and carotene content. Rhodymenia and Sargassum are used as fodder in France and China respectively.

Algae in Medicine

Laminaria species have high iodine content. Codium contains a considerable amount of iodine. Gelidium and Grateloupia also contain iodine. This iodine is used in the preparation of various goiter medicines. Some algae are a source of antibiotics. Chlorellin from Chlorella is an antibiotic. But this has not been chemically characterized. The extracts of Cladophora and Lyngbya possess antiviral and antibacterial properties. The Charophytes possess larvicidal properties. So these plants are useful in destroying mosquito larvae. Because of antibiotic property certain algae were used in phycotherapy, healing of wounds in earlier days.

Agar agar is used in the manufacture of pills and ointments. It also forms a base for many medicines which are used as laxatives. Carrageenin acts as a blood coagulant. Alginic acid controls bleeding. The extracts of Digenea, Codium and Durvillea have vermifuge effect.

Algae in Oil and Gas

The organic compounds derived from the dead plants and animals constituting the plankton accumulate at the bottom and burried in the sediments of oceans. These compounds are decomposed and converted into oil (petroleum) and fuel gas (methane) by the action of methane produ­cing bacteria.

Algae in Sewage Treatment

Chlorella, Scenedesmus, and Euglena grow very well in domestic wastes and help in converting it into an odourless valuable fertilizer. The algae flourish on the nutrients present in sewage and liberate oxygen during their photosynthesis. The oxygen is used by the microorganisms for the decomposition of organic matter in sewage. Thus the algal-bacterial system helps in the purification and disposal of sewage. Besides this, the algae can be separated from sewage after a certain period of growth, dried and used as a feed for poultry.

Algae in Experimental Work

Algae are used extensively in biological research. The cultures of Chlorella, Scenedesmus, Anacystis and other microalgae have been widely used in the investigations of photosynthesis. The sexual reproduction at the cellular and molecular level has been thoroughly understood through the studies made in Chlamydomonas and other Volvocalcan algae.

Chlorella pyrenoidosa, Spirulina and Synechococcus can be used as a possible food source in space flights. These algae multiply rapidly, synthesize food by utilizing CO2 and liberate oxygen.

Algae in Water Supplies

In freshwater ponds, lakes and reservoirs/tanks certain algae (blue-greens, diatoms and euglenoid flagellates) grow in abundance and constitute the water blooms. Such algae impart colour and unpleasant odour to water, thus making it unlit for drinking. Certain algal growths may choke the pipes and interfere with water supplies. Blue-greens like Microcystis and Aphanizomenon produce toxic substances into the water which are poisonous to fish, cattle etc.

Some biologists have emphasized the role of air-borne algae as causative agents of allergies.

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