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Algae

Introduction:

  • Algae are chlorophyllous, simple, thalloid, autotrophic and largely aquatic (both freshwater and marine) organisms.
  • The study of algae is known as phycology.
  • Algae belong to the subdivision thallophyta.
  • Thallophytes are the most basic seedless, non-vascular plants.
  • The plant body is a thallus, which is gametophytic (n) and lacks differentiation into roots, stems, and leaves.
  • Vascular and mechanical tissues are absent.
  • The sex organs, known as gametangia, are unicellular.
  • The embryo stage is absent.

In this concept, we will learn about the following subconcepts:

  • Algae- Salient features & Classification
  • Green algae/ Chlorophyceae
  • Brown algae/ Phaeophyceae
  • Red algae/ Rhodophyceae

Algae- Salient features & Classification
 

Introduction:

  • These are simple living organisms with a thallus-like plant body that possess chlorophyll.
  • They are the simplest forms of producers in a food chain.
  • They can be single-celled or multicellular.
  • Algae are mostly aquatic (freshwater, marine) but are also found in a variety of other habitats such as moist stones, wood, and even soil.
  • Some of them also show a mutual association with fungi, leading to an entirely new organism called the lichens.
  • Among the 70 species of marine algae which are used as food, many species of Porphyra, Laminaria and Sargassum.
  • Large amounts of hydrocolloids (water holding substances) which are used commercially are obtained from certain marine brown and red algae, e.g., algin from brown algae, carrageenan and agar from red algae.

Detailed explanation:
 

Characteristics Features of Algae:

  • They are usually found in water (either marine or freshwater). Some algae are found on moist stones, soils, wood, some of them also occur in association with fungi (lichen) and animals (e.g. on sloth bear).
  • Its plant body is covered by mucilage that provides protection from water currents and epiphytic growth.
  • The plant body is thallus-like that can be unicellular (Chlamydomonas), filamentous (Spirogyra & Ulothrix), colonial (Volvox), and composed of true parenchyma.
  • The cell wall consists of cellulose, galactans, mannans and calcium carbonate.
  • Pigment - Chlorophyll-a and -carotene are universal pigments (similar to higher plants).
  • Reserve food is mainly starch.
  • Reproduction:
    (1) Vegetative : By Fragmentation (Each fragment develops into a thallus) and by formation of different types of spores (hypnospores, akinete etc.).
    (2) Asexual : By different type of spores. The most common asexual spore is zoospore (Flagellated / Motile and produced new algal plant on germination).
    (3) Sexual : Sexual reproduction take place through fusion of two gametes.Sex organs - unicellular and non-jacketed gametangia. Male sex organ - antheridium
    Female sex organ - oogonium.
    Type of sexual reproduction:
    (a) Isogamy: Fusion of gametes of Similar size.
    Flagellated isogametes - Ulothrix.
    Non-flagellated isogametes e.g. Spirogyra.
    (b) Anisogamy: Fusion of gametes of dissimilar size,e.g., Udorina.
    (c) Oogamy: Fusion between one large, non-motile (static) female gamete and smaller, the motile male gamete, e.g., Volvox, Fucus.
  • Fusion of gamete produces zygote which immediately undergoes meiosis (zygotic meiosis) so embryo stage is absent.
  • Lifecycle type in algae can be
    i) Haplontic - Volvox, Spirogyra, some species of Chlamydomonas;
    ii) Diplontic - Sargassum and Fucus;
    iii) Haplodiplontic - Polysiphonia, Ectocarpus, Kelps.

Classification of algae-
 

  • Whittaker classified Algae on the basis of photosynthetic pigments into three classes – Green algae, brown algae and red algae.


    Classification of algae


Green Algae/ Chlorophyceae
 

Introduction:

  • Chlorophyceae is commonly known as green algae.
  • Scientists think them to be the ancestors of green plants as they show many similarities to the land plants such as:
    i) They store food in the form of starch.
    ii) Possess a similar chloroplast structure with pigments such as chlorophyll a and b, xanthophyll, and carotene.
    iii) Have cellulose cell walls

Detailed explanation:
 

  • Green algae are cosmopolitan (Mostly freshwater). Some species are marine e.g. Caulerpa, Acetabularia
  • Thallus may be in various forms-
    i) Unicellular - Acetabularia (largest unicellular algae/ umbrella plant), Chlorella (non-motile), Chlamydomonas (motile).
    ii) Colonial- Volvox (motile), Hydrodictyon (water net; non-motile).
    iii) Multicellular Filamentous- Ulothrix (pond wool, attached to substratum), Spirogyra (pond silk,free-floating).
    iv) Multicellular thalloid - Ulva (sea lettuce).
  • The shape of chloroplast in algae: Reticulate- Oedogonium, Cladophora; Stellate - Zygnema; Girdle shaped-Ulothrix; Cup-shaped- Chlamydomonas; Spiral or ribbon-shaped- Spirogyra; Discoid- Chara .
  • Pigments: chl a, b, carotenes(, and type), xanthophyll like lutein.
  • Starch is a reserve food material stored in one or many storage bodies called pyrenoids located in the chloroplast. Pyrenoids contain protein in the centre and starch in the periphery. Pyrenoids are subcellular microcompartments found in chloroplasts of many algae and hornworts. Its primary function is to act as a carbon dioxide fixing centre. It keeps photosynthetic enzymes in a carbon dioxide-rich environment.
  • The cell wall consists of an inner cellulose layer and a pectose outer layer. Hemicellulose is also found to be present.
  • Reproduction:
    i) Vegetative reproduction-fragmentation or by the formation of different types of spores.
    ii) Asexual reproduction - Flagellated zoospores produced in zoosporangia.
    iii) Sexual reproduction takes place through isogamy, anisogamy and oogamy.
  • Some common examples of green algae- Chlamydomonas, Volvox, Ulothrix, Spirogyra, and Chara.


    green algae

     
  • Economic importance:
    (i) Food : Chlorella pyrenoidosa (called space algae) is used by exobiologists for food. It is also used as SCP (single cell protein) due to its high nutritional value. Ulva (sea lettuce) and Codium are used as salad.
    (ii) Sewage oxidation: Chlamydomonas, Scenedesmus, and Chlorella are found in sewage oxidation tanks where they produce O2. The latter helps aerobic bacteria to decompose sewage.
    (iii) Antibiotic: Chlorellin is extracted from Chlorella. It is effective against bacteria.
    (iv) Larvicidal property: Chara, Nitella show the larvicidal property. They kill mosquito larvae.
    (v) Parasitism : Exceptionally, Cephaleuros virescens is a parasite. It causes the red rust disease of tea.

Brown algae/ Phaeophyceae
 

Introduction:

  • Brown algae are a type of algae that belongs to the Phaeophyceae family.
  • Their colour ranges from brown to olive green.
  • They are mostly seen in coastal areas.
  • Brown algae are made up of roughly 1500 different species that vary widely in size and appearance.
  • They are multicellular, and their colour is determined by the ratio of chlorophyll to fucoxanthin pigment.

Detailed explanation:
 

  • These are brown-colored marine eukaryotic algae.
  • They show great variation in size and form.
  • They range from simple branched, filamentous forms (Ectocarpus) to profusely branched forms shown by Kelps, which may reach a height of 100 meters, e.g., Macrocystis.
  • Pigments- They have chlorophyll a, c, carotenoids, and xanthophyll (fucoxanthin). They vary in color from olive green to different shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin, present in them.
  • Stored food is present in the form of laminarin or mannitol.
  • The vegetative cells have a cellulosic wall that is usually covered on the outside by a gelatinous coating of algin. This prevents the drying of the plant.
  • 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 (for fixation with substratum), and has a stalk, the stipe, and leaf-like photosynthetic structure called a frond.
  • Conducting tubes or trumpet hyphae are found in larger brown algae or kelps. It helps in food conduction (analogous to sieve tubes of higher plants).


    laminaria-fucus-dictyota

     
  • Reproduction:
    i) Vegetative reproduction takes place by fragmentation.
    ii) Asexual reproduction in most brown algae is by zoospores(biflagellate) that are pear-shaped and have two unequal laterally attached flagella.
    iii) Sexual reproduction may be isogamous, anisogamous, or oogamous. Fusion of gametes may take place within the water or within the oogonium (oogamous species). The gametes are pyriform (pear-shaped) that bear two laterally attached flagella.
  • Zygotic meiosis is absent but sporic meiosis occurs.
  • The life cycle is of the diplontic type.
  • Some common examples of brown algae are- Ectocarpus, Dictyota, Laminaria (devil’s apron), Fucus, and Sargassum.
     
  • Economic importance:
    i) Algin: It is non sulphated phycocolloid and obtained from Laminaria, Macrocystis, Fucus, Sargassum. It is used in flameproof plastics, security glass, gauze, surgical threads, shaving creams, toothpaste, cosmetic creams, shampoos, sauces, sizing textiles, etc.
    ii) Iodine: It is extracted from Laminaria and Fucus.
    iii) Mannitol: It is used as food and added to inks, plastic, paints, and varnishes. It is half as sweet as sugar and is a good substitute for sugar for diabetic patients.
    iv) Food: Some brown algae are used as food in some countries. Kombu is rich in carbohydrates and formed by Laminaria.
    v) Potash: It is extracted from Macrocystis and Nereocystis and used as fertilizer in Germany.

Red algae/ Rhodophyceae
 

Introduction:

  • Rhodophyceae is commonly known as red algae.
  • Majority of them are marine and abundantly found in the warmer areas.
  • They are red in colour due to the predominance of the red pigment, r-phycoerythrin.

Detailed explanation:
 

  • Red colour of algae is due to the presence of the red pigment, r-phycoerythrin that makes them unique among others.
  • The red algae are mostly marine except Batrachospermum which is the only freshwater red alga.
  • The red thalli of most of the red algae are multicellular. Some of them have complex body organisation.
  • Flagellated cells are completely absent in the life cycle.
  • Cell wall is mucilaginous and contains cellulose, pectin and abundant sulphated phycocolloids (like agar, carrageenan, funori).
  • Pigments – Chl-a, Chl-d carotenoid ( 𝛽-carotene) and phycobilins (r-phycoerythrin - red colour and r - phycocyanin - blue colour).
  • They are found in both well-lighted regions close to the surface of the water and also at great depths in oceans where only little light penetrates.
  • When growing deep, algae appears red due to the predominance of r-phycoerythrin that captures short-wavelength that is able to reach the maximum depth in water. These algae, therefore, reach the maximum depth (30-90 mt) in the sea where no other photosynthetic organism grows.
  • They appear bluish-green when growing near the surface of the sea because of the little production of phycoerythrin. This property of change in color with depth is called chromatic adaptation(Gaidukov's effect).
  • The stored food is Floridean starch very similar to amylopectin and glycogen in structure.
  • Reproduction:
    i) Vegetative reproduction – Usually by fragmentation.
    ii) Asexual reproduction – By many types of non-motile spores like monospores, carpospores, tetraspores.
    iii) Sexual reproduction – It is the most advanced (Oogamous) type followed by complex post fertilization development.
  • Some common examples of red algae are- Polysiphonia, Porphyra, Gracilaria and Gelidium.


    porphyra-polysiphonia

     
  • Economic importance:
    (i) Phycocolloids (Sulphated): 
        (a) Agar-agar is obtained from Gelidium, Gracilaria etc (these algae are called agarophytes). It is used to solidify a cultural                 medium. It is also used as a laxative stabilizer or thickener in preparing jams, jellies, creams, pudding, baby food, ice                     cream and bakery products.
         (b) Carrageenan is obtained from the cell wall of Chondrus crispus. It is used in confectionery, bakery, jelly, creams, as                      clearing agent in liquors (Beer) and leather finishing, as emulsifier in chocolates, icecreams, sauces, toothpastes paints and            cosmetics.
         (c) Funori is obtained from Gloiopeltis. It is a glue used as adhesive and in sizing textiles, papers etc.
    (ii) Food: Some red algae are edible e.g. Laver (Porphyra), Dulse (Rhodymenia), Irish moss (Chondrus).

Difference between Chlorophyceae, Phaeophyceae and Rhodophyceae
 

Characteristics Chlorophyceae Phaeophyceae Rhodophyceae
Pigments Chlorophyll a, b, and carotenoids Chlorophyll a,c and fucoxanthin(xanthophyll) Chlorophyll a, d and phycoerythrin
Habitat Found in Freshwater, brackish water or saltwater Found in Freshwater rarely, brackish water, salt water Found in Freshwater(some), brackish water, salt water(most)
Stored food Starch Mannitol and laminarin starch Floridean starch
Cell wall Cellulose Cellulose + Algin Cellulose, pectin, polysulphate esters.
Flagella Present; 2-8 apical and equal Present; 2 unequal and lateral Absent
Examples Chlorella, Volvox Sargassum, Fucus Gracilaria, Gelidium


Frequently asked questions (FAQ)

Q1. Who is the father of Indian phycology?
Ans:

M.O.P Iyengar is known as the father of Indian phycology.

Q2. What is the basis of the classification of algae?
Ans:

The presence or absence of pigments is the main basis of the classification of algae.They can be classified as-

  • Chlorophyceae: Chlorophyll a and b are present in them and impart the green colour. Chlorophyceae are also called green algae.
  • Phaeophyceae: Chlorophyll a and c and fucoxanthin are present. Fucoxanthin imparts the brown colour. Phaeophyceae are also called ‘brown algae’.
  • Rhodophyceae: Chlorophyll a and d and phycoerythrin are present. Phycoerythrin imparts red colour. Rhodophyceae are also called ‘red algae’.

Q3. What is algae? Write its important features and classification of algae?
Ans:
Features of Algae are as follows-

  • These are simple living organisms with a thallus-like plant body that possess chlorophyll.
  • They are the simplest forms of producers in a food chain.
  • They can be single-celled or multicellular.
  • Algae are mostly aquatic (freshwater, marine) but are also found in a variety of other habitats such as moist stones, wood, and even soil.

Classification of Algae: Whittaker classified Algae on the basis of photosynthetic pigments into three classes – Green algae, brown algae and red algae.

  • Chlorophyceae: Chlorophyll a and b are present in them and impart the green colour. Chlorophyceae are also called green algae.
  • Phaeophyceae: Chlorophyll a and c and fucoxanthin are present. Fucoxanthin imparts the brown colour. Phaeophyceae are also called ‘brown algae’.
  • Rhodophyceae: Chlorophyll a and d and phycoerythrin are present. Phycoerythrin imparts red colour. Rhodophyceae is also called ‘red algae’.

Q4. What are common examples of brown algae?
Ans:

Ectocarpus, Dictyota, Laminaria, Fucus, and Sargassum.

Q5. Why are Rhodophyceae called red algae?
Ans:

Rhodophyceae is commonly known as red algae. Majority of them are marine and abundantly found in the warmer areas.They are red    in colour due to the predominance of the red pigment, r-phycoerythrin.

Q6. What is the stored food material found in red algae?
Ans:

Floridean starch is the stored material found in red algae.

Q7. What is the function of trumpet hyphae in Fucus?
Ans:

Conducting tubes or trumpet hyphae are found in larger brown algae or kelps. It helps in food conduction (analogous to sieve tubes of  higher plants).

Q8. How does asexual reproduction occurs in brown algae?
Ans:

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

Q9. What is Gaidukov’s effect?
Ans:

  • Red algae are found in both well-lighted regions close to the surface of the water and also at great depths in oceans where only little light penetrates.
  • When growing deep, algae appears red due to the predominance of r-phycoerythrin that captures short-wavelength that is able to reach the maximum depth in water. These algae, therefore, reach the maximum depth (30-90 mt) in the sea where no other photosynthetic organism grows.
  • They appear bluish-green when growing near the surface of the sea because of the little production of phycoerythrin. This property of change in color with depth is called chromatic adaptation(Gaidukov's effect).

Q10. Name the stored food material, pigments and type of asexual spores found in brown algae.
Ans:

  • Stored food material in brown algae: Mannitol and laminarin
  • Pigments present in brown algae: Chlorophyll a,c and fucoxanthin.
  • Asexual spores present in brown algae: Zoospores(biflagellate) that are pear-shaped and have two unequal laterally attached flagella.

Q11. What are pyrenoids?
Ans:

  • Starch is a reserve food material stored in one or many storage bodies called pyrenoids located in the chloroplast. Pyrenoids contain protein in the centre and starch in the periphery.
  • Pyrenoids are subcellular microcompartments found in chloroplasts of many algae and hornworts. Its primary function is to act as a carbon dioxide fixing centre.
  • It keeps photosynthetic enzymes in a carbon dioxide-rich environment.

Q12. Which phycocolloids are found in the cell wall of red algae?
Ans:

Carrageenan and agar are the phycocolloids that are found in the cell walls of red algae.

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