Algae

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (both. fresh water and marine) organisms.

They occur in a variety of other habitats: moist stones, soils and wood. Some of them also occur in association with fungi (lichen) and animals (e.g., on sloth bear).

Characteristics

Size

• The form and size of algae is highly variable.

• The size ranges from the microscopic unicellular forms like Chlamydomonas, to colonial forms like Volvox and to the filamentous forms like Ulothrixand Spirogtyra.

• A few of the marine forms such as kelps, form massive plant bodies.

Reproduction

The algae reproduce by vegetative, asexual and sexual methods.

(i) Vegetative reproduction:

It  is by fragmentation-Each fragment develops into a thallus. Asexual reproduction is by the production of different types of spores, the most common being the zoospores. They are flagellated (motile) and on germination gives rise to new plants.

(i) Sexual reproduction:

Sexual reproduction takes place through fusion of two gametes. These gametes can be flagellated and similar in size (as in Chlamydomonas) or non ­flagellated (non-motile) but similar in size (as in Spirogyra). Such reproduction is called isogamous. Fusion of two gametes dissimilar in size, as in some species of Chlamydomonas is termed as anisogamous. Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete is termed oogamous, e.g., Volvox, Fucus.

Occurence

• Plants growing on snow or ice are called as cryophytes. Different algal forms produce a specific colour effect while growing as cryophyte e.g., yellow-green by Chlamydomonas yellowstonensis, red by C. nivalis, black by Scoiiella nivalis.

• Plants growing in hot water are called as thermophytes. Some blue-green algae grow in hot water springs at about 70°C e.g., Oscillatoria brevis.Several algal forms grow on other plants (algae, angiosperms) as epiphytes. e.g., Oedogonium, Cladophora, Vaucheria 'etc.

• Some blue-green algae grows as endophytes inside other plants e.g., Anabaena growing inside the leaf of Azolla (fern), Nostoc inside the thallus of Anthoceros (hornwort) and Anabaena, Nostoc, Oscillatoria inside the coralloid roots of cycas.

• Algae growing on the bodies of animals are described as epizoic. e.g., Cladophora crispata grows on snail shell, Characium grows on the antennae of mosquito larvae, Cyanoderma (red alga) and Trichophilus (green alga) are grow on scales of sloth.

• Algae growing inside the body of animals. e.g., Chlorella grow within the tissue of Hydra. Some blue-green algae also grow in the respiratory tracts of animals. The blue-green algae which grow endozoically inside the protozoans are called as cyanellae.

• Some algae like Chlorella, Nostoc etc. growing in symbiotic relationship with members of Ascomycetes and Basidiomycetes (Fungi) constitute' the lichen.

• The alga Cephaleuros virescens grows a parasite on the tea leaves. In addition, Rhodochytrium, Phyllosiphon are other parasitic algal forms.

Thallus Organisation

The algae show a considerable variation in the organization of the thallus:

• Several members of algae are unicelled. They may be motile (Chiamydomonas) or non-motile (diatoms). Some forms have a thick wall and become sedentary for certain duration in their life history. They are called as coccoid e.g., Chlorella, Chlrococcus.

• A colony consists of independent organisms. While the colony of Volvox is motile, that of Hydrodictyon is fixed. A colony having fixed number of cells and division of labour is called as coenobium e.g., Volvox.

• Most of the algal groups (except blue-green and dinoflagellates) show eukaryotic cell structure. The cell wall is made up of cellulose. Some red algae (Corallina) have inpregnationor CaCO₃.

Pigmentation

Differences amongst red, brown and green algae

Economic Importance of Algae

• Nitrogen fixation : Some fifty species of blue-green algae are capable of fixing atmospheric nitrogen in the soil e.g., Anabaena, Aulosira, Cylindrospermum, Calothrix, Gleotrichia, Nostoc, Scytonema, Stigonema, Tolypothrix etc. Under aerobic conditions, nitrogen is fixed by heterocysts only. The fixation is brought about by the enzyme nitrogenase. Under anaerobic condition the vegetative cells also show nitrogenase activity.

• Algae as food : Many green algae such as Chlorella, Ulva, Caulerpa, Enteromorpha, etc. are used as food. Chlorella, a unicellular green alga, possesses a high quality of food value. It has about 50% protein and 20% of lipid and carbohydrates. The Chlorella protein contains all the amino acids essential for human nutrition. Besides, it contains vitamins A, B, C, K and various other essential elements. Ulva is collected and processed as food product. Ulva lactuca has formerly used in salad and soup in Scotland.

• Green algae in space research : In recent years biologists have realized that unicellular green algae (e.g., Chlorella) could be used to provide O₂ during space flight trips. The alga can reuse CO₂ during the process of photosynthesis and release O₂ for the use by Astronauts.

• Antibiotics : The genus Chlorella yields an antibiotic chlorellin, which is used against Gram +ve and Gram –ve bacteria, especially Escherichia coli, Shigella dysenteriae and Staphylococcus aureus. The genus Caulerpa also yields antibiotics.

• Alginates : Alginic acid is a polymer of carbohydrate. It occurs in the cell wall and middle lamella. The alginates particularly ammonium, Fe, Na and K, salts are water soluble. They are obtained from Laminaria, Ascophyllum, Fucuc, Nereocystis, Turbinaria etc. They are viscous, gel-forming and non-toxic. Hence they are used in pharmaceuticals as emulsifiers and stabilizers as well as for making pills, antibiotic capsules etc. They are also used in the preparation of soups, jellies, cosmetics, toothpastes, polishes, hair dyes, compact powders, lotions, shampoos etc.

• Carrageenin : It is a polysaccharide colloid (phycocolloid) obtained from the red algae Chondrus crispus and Gigartinia stellata. It is widely used in soups, sauces, milk shakes, cheese, jellies, cream and fruit juices. It is also used in painting and printing.

• Kieselguhr or Diatomite : The fossil deposits of unicelled alga, diatoms are formed due to their highly siliceous cell wall (frustules). This is called as diatomite or diatomaceous earth. It is used in making sound proof buildings, lining furnaces and boilers, as insulating material and also as a filter.

• Agar-agar : It is a non-nitrogenous carbohydrate consisting of two polysaccharides namely agarose and agaropectin. It is obtained from several red algae e.g., Gracilaria, Gelidium, Gigartinia, Pterocladia, Chondrus, Furcellaria, Phyllophora etc. It is insoluble in cold water but soluble in hot. It is used as a base for a variety of culture media.

Resemblances between Algae and Fungi

Classification of Algae

Chlorophyceae

• The members of chlorophyceae are commonly called green algae. They are usually grass green due to the dominance of pigments chlorophyll a and b. The pigments are localised in definite chloroplasts. The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species.

• Most of the members have one or more storage bodies called pyrenoids located in the chloroplasts. Pyrenoids contain protein besides starch.

• The plant body may be unicellular, colonial or filamentous.

• Some algae may store food in the form of oil droplets. Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.

• Vegetative reproduction usually takes place by fragmentation or by formation of different types of spores. Asexual reproduction is by flagellated zoospores produced in zoosporangia,

• The sexual reproduction shows considerable variation in the type and formation of sex cells and -it may be isogamous, anisogamous or oogamous.

• Some commonly found green algae are: Chlamydomonas, Volvox, Ulothrix, Spitogyra and Chara. 

(i) Spirogyra:

• It is an unbranched filamentous green alga of stagnant fresh waters which forms floating masses (supported by bubbles of oxygen) called pond scum. A sheath of mucilage occurs on the outside. It gives a silky touch. Hence Spirogyra is also called water silk or mermaid's tresses.

• Normally asexual reproduction is absent in Spirogyra. U occurs only occasionally by the formation of akinetes, aplanospores and azygospores (Parthenospores).

• The sexual reproduction in Spirogyra is called conjugation, It involves the fusion of two morphologically identical, but physiologically dissimilar gametes.

• Life cycle in Spirogyra is haplontic as dominant phase in life cycle is haploid (n) and diploid phase is represented by only zygospore and it undergose R.D. or meiosis (zygotic meiosis).

(ii) Ulothrix: 

It is a green filamentous algae found in slow running streams. The common species U. zonata occurs in cold water whereas U. flacca is marine. U. implexa occurs in esturies (where river meats the sea) as lithophytes.The Uloihrix reproduces vegetatively, asexually as well as sexually.

Differences between Isogamy Anisogamy

Phaeophyceae

• The members of phaeophyceae or brown algae are found primarily in marine habitats.

• They show great variation in size and form.

• They range from simple branched, filamentous forms(Ectocarpus) to profusely branched forms as represented by kelps, which may reach a height of 100 metres.

• They possess chlorophyll a, c, carotenoids and xanthophylls. They vary in colour from olive green to various shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin present in them.

• Food is stored as complex carbohydrates, which may be in the form of laminarin or mannitol.

• The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin.

• The protoplast contains, in addition to plastids, a centrally located vacuole and nucleus. The plant body is usually attached to the substratum by a holdfast, and has a stalk, the stipe and leaf like photosynthetic organ -the frond.

• Vegetative reproduction takes place by fragmentation.

• Asexual reproduction in most brown algae is by biflagellate zoospores that are pear-shaped and have two unequal laterally attached flagella.

• Sexual reproduction may be isogamous, anisogamous or oogamous.

• Union of gametes may take place in water or within the oogonium (oogarnous species). The gametes are pyriform (pear-shaped) and bear two laterally attached flagella.

• The common forms are Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus.

Rhodophyceae

• Rhodophyta are commonly called red algae because of the predominance of the red pigment, r-­phycoerythrin in their body. Majority of the red algae are marine with greater concentrations found in the warmer areas.

• They occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates.

• The red thalli of most of the red algae are multicellular. Some of them have complex body organisation. The food is stored as floridean starch which is very similar to amylopectin and glycogen in structure.

• The red algae usually reproduce vegetatively by fragmentation.

• They reproduce asexually by non-motile spores and sexually by non-motile gametes.

• Sexual reproduction is oogamous and accompanied by complex post fertilisation developments.The common members are: Polysiphonia, Porphyra Gracilaria and Gelidium.
   

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