11th Class Biology Plant Kingdom Algae

(Gk. Phycos = alga or sea weed)

The branch of botany dealing with the study of algae is called as phycology or algology. They are simple, thallose, autotrophic non-vascular plants having unicelled sex organs and no embryo formation. In Whittaker's classification, algae are grouped in three kingdom – Monera (blue green algae), Protista (dinoflagellates, diatoms, euglenoids) and Plantae (green algae, red algae and brown algae).

According to Fritsch, (1935) the designation alga must include all holophytic organisms, as well as their numerous colourless derivatives, that fail to reach the level of differentiation characteristic of archegoniatae plants.

Occurrence : The algae occur in a variety of habitats which are summarised here under :

Fresh water forms : They occurs in rivers, ponds, pools, lakes and ditches. Those forms which remain attached to bottom soil are called as epipelic. Several forms remain attached to bottom or at the bank or to submerged objects. They are described as benthos. Many forms remain attached to rocks or stones. They are described as epilithic or lithophytic.

Marine forms : Most of the members of brown algae, red algae some green and blue-green algae occur in sea. While some occur as phytoplanktons and benthos, others occur as lithophytes. The giant forms like Macrocystis (60 meters) and Nereocystis (50 meters) are also marine.

Terrestrial forms : Several members of green and blue-green algae and a few others occur on damps soil. While forms like Oscillatoria and Nostoc occur on alkaline and calcareous soil, Fritschiella grown on acidic soil. Xanthophyceae members like Vaucheria and Botrydium growing on damp shady soil or on shady walls, are often described as Sapophytes.

Specialized habitats

Cryophytes : 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 Scotiella nivalis and purple-brown by Ancylonema nordenskioldii.

Thermophytes : 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.

Epiphytes : Several algal forms grow on other plants (algae, angiosperms) as epiphytes. e.g., Oedogonium, Cladophora, Vaucheria etc.

Endophytes : 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.

Epizoic : 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.

Endozoic : Algae growing inside the body of animals. e.g., Chlorella grow with in 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.

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

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

Thallus organization

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

(1) Unicellular forms : Several members of algae are unicelled. They may be motile (Chlamydomonas) 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.

(2) Multicellular forms : Multicellular forms are following :

Colonial : 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.

Palmelloid : Here the vegetative cells of the alga get surrounded by a mucilagenous matrix e.g., Tetraspora.

Dendroid : Here the colony appears like a microscopic tree. There is secretion of mucilage from the polar end e.g., Ecballocystis.

Filamentous : Most of the algal forms are filamentous. The filaments may be uniseriate or multiseriate, free floating or attached, unbranched (Ulothrix) or branched (Cladophora). The branches may be monomorphic (Cladophora) or dimorphic (Batrachospermum). The branching may be lateral or dichotomous, true (Ectocarpus) or false (Scytonema). The filaments may be monosiphonous (Batrachospermum) or polysiphonous (Polysiphonia). In some filamentous forms there is distinction of a prostrate system and an erect system, thus constituting the heterotrichous habit. e.g., Stigeoclonium.

Siphonous : An aseptate, multinucleate (coenocytic) condition of a filament or thallus constitutes the siphonous habit e.g., Vaucheria.

Parenchymatous : Parenchymatous organization of the thallus has been observed in many members of brown algae (Sargassum, Laminaria), red algae (Gracillaria, Porphyra) and a few green algae (Chara, Ulva) etc.

Cell organization

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 inpregnation of CaCO₃. The cells possess a well organised nucleus. The minimum chromosome number in algae is n = 2 (Porphyra linearis) and the maximum number is n = 592 (Netrium digitalis). The cells possess distinct mitochondria, plastids, E.R., ribosomes and Golgi body. There may be a single thylakoid in the granum of Rhodophyceae, two in Cryptophyceae, three in Phaeophyceae and Bacillariophyceae but generally many.

The motile forms also possess flagella. They show the usual 9 + 2 structure. They are of two types – acronematic (whiplash type) and pleuronematic (tinsel type).

Reproduction

The algae reproduce vegetatively, asexually and sexually. Various method involved in reproduction are discussed in the following account.

(1) Vegetative reproduction : It occurs by following types.

Fragmentation : It occurs due to breakage of filament or thallus into fragments, each of which behaves as an independent  organism e.g., Ulothrix, Spirogyra etc.

Fission : The unicelled forms like diatoms, desmids multiply by fission i.e., simple cell division.

Budding : A bud arises as a papilla on the parent cell. It enlarges and finally separates e.g., Protosiphon.

Akinetes : Due to deposition of food material followed by thickening of the parent wall, a cell is transformed into an akinete. They may be formed in a chain. On the arrival of favourable conditions, they germinate to forms a new plant e.g., Cladophora, Ulothrix, Nostoc etc.

(2) Asexual reproduction : It occurs by the formation of various types of spores in sporangia. Except the zoospores, all other types of spores are non-motile.

Zoospores : These are thin walled motile spores. They are anteriorly biflagellate and the two flagella are similar in Cladophora. In Vaucheria and Ectocarpus they are laterally biflagellate and the two flagella are dissimilar. Multiflagellate zoospores are formed in Oedogonium and Vaucheria. In Vaucheria the flagella are present all over the surface in pairs and hence it is called as synzoospore.

Aplanospores : They are thin walled and non-motile spores commonly formed in Chlamydomonas, Ulothrix etc.

Autospores : They are also thin walled, non-motile spores which resemble the parent cell e.g., Chlorella.

Hypnospores : These are thick walled non-motile spores formed to tide over unfavourable condition. They germinate on the arrival of favourable conditions e.g., Chlamydomonas, Ulothrix.

Carpospores : In red algae, carposporangia are formed at the tip of gonimoblast filaments which produce a single haploid or diploid carpospore e.g., Batrachospermum, Polysiphonia.

Tetraspores : Four non-motile tetraspores are formed inside a tetrasporangium as a result of mitosis in brown algae (e.g., Dictyota) or by meiosis in red algae (e.g., Polysiphonia).

Monospore : The juvenile stage of Batrachospermum, a red alga, multiplies by forming a single monospore formed in side a monosporangium.

(3) Sexual reproduction : The sexual reproduction in algae is broadly of three types as under :

Isogamy : It involves fusion of gametes which are morphologically and physiologically similar. They are called as isogametes e.g., Chlamydomonas eugametos. In diatoms, there is simplification of isogamous reproduction.

Anisogamy : It involves fusion of two gametes which are dissimilar e.g., Chlamydomonas, Ectocarpus, Pandorina etc. When the two gametes are morphologically dissimilar, the anisogamy is said to be morphological e.g., Chlamydomonas braunii, Ectocarpus secundus. Here the smaller gamete may be called as male and the large one as female. When the two gametes are morphologically similar but differ in their behaviour, the anisogamy is said to be physiological e.g., Spirogyra, Ectocarpus siliculosus.

Oogamy : In this process there is formation of unicelled sex organs. The male sex organ is called as antheridium and the female as oogonium. The antheridium forms the male gametes called antherozoids which are generally flagellate. The oogonium forms a non-motile female gamete called egg. The oogamy involves fusion of antherozoids with egg. The simplest type of oogamy is seen in Chlamydomonas coccifera.

In Sargassum the sex organs are formed in special pitcher shaped depressions called conceptacles formed on receptacles. In red algae (Polysiphonia) the male gametes called spermatia are non-motile. The female sex organ called carpogonium is formed on a specialized filament. The highest degree of specialization is seen in Chara where the antheridia and oogonia appear to be surrounded by sterile cells. The structures so formed are called as globule and nucule, respectively. As a result of fertilization, the zygote is formed which secretes 1 – 2 thick walls and undergoes a period of rest. On the arrival of favourable conditions, it germinates. Usually it undergoes meiosis to forms meiospores. In some forms, it forms the diploid plant e.g., Cladophora, Ectocarpus.

Important features of some selected classes

Chlorophyceae : Plants are fresh water or marine and forms unicelled to parenchymatous. Chief photosynthetic pigments are chlorophyll a, b; a, b, g– carotenes, lycopene; lutein, violaxanthin. Reserve food is starch. Zoospore are formed and male gametes are flagellate. Flagella identical. Sexual reproduction – Isogamous, anisogamous or oogamous type.

Xanthophyceae : Plants are generally fresh water and forms unicelled to siphonous. Chief photosynthetic pigments are chlorophyll a, e; b–carotene, violaxanthin, neoxanthin. Reserve food is chrysolaminarin and oils. Zoospore are formed and male gametes flagellate. Flagella non-identical (unequal). Sexual reproduction – Isogamous, anisogamous or oogamous type.

Phaeophyceae : Plants are marine and forms unicelled to parenchymatous. Chief photosynthetic pigments are chlorophyll a, c; b–carotene, fucoxanthin, lutein, violaxanthin, diatoxanthin. Reserve food is laminarin, mannitol and oils. Zoospore are formed and male gametes flagellate. Flagella unequal. Sexual reproduction – Isogamous, anisogamous or oogamous type.

Rhodophyceae : Plants are generally marine and forms filamentous to parenchymatous. Chief photosynthetic pigments are chlorophyll a, d is present but chlorophyll c is absent; a, b–carotene, lutein, violaxanthin, fucoxanthin, myxoxanthin, g–phycoerythrin, g–phycocyanin and allophycocyanin. Reserve food is floridean starch, galactan –SO₄ polymers. Zoospores are not formed and male gametes are non-flagellate. Sexual reproduction by specialized type of oogamy. Life cycle haplobiontic or diplobiontic.

Myxophyceae (Cyanophyceae) : Plants are generally fresh water, a few forms marine and forms unicelled to filamentous. Cells showing prokaryotic organization. Chief photosynthetic pigments are chlorophyll a; b-carotene; lutein, myxoxanthin, oscillaxanthin, c-phycocyanin, c-phycoerythrin, allophycocyanin. Reserve food is cyanophycean starch (glycogen) and cyanophycin (protein). Zoospore are not formed and no flagellate bodies. Sexual reproduction is absent.

Economic importance

Useful aspects

Nitrogen fixation : Some fifty species of blue-green algae are capable of fixing atmospheric nitrogen in the soil e.g., Anabaena, Aulosira, Cylindrospermum, Nostoc and Tolypothrix etc. Under aerobic conditions, nitrogen is fixed by heterocysts only. 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 has about 50% protein and 20% of lipid and carbohydrates. The Chlorella protein contains all the amino acids essential for human nutrition. 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.

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. They are obtained from Laminaria, Ascophyllum, Fucus, Nereocystis, Turbinaria etc. 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 Gigartina stellata. It is widely used in soups, sauces, milk shakes, cheese, jellies, cream and fruit juices. It is also used in painting and printing.

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, Gigartina and Chondrus etc. It is insoluble in cold water but soluble in hot. It is used as a base for a variety of culture media.

Source of minerals and elements : The members of brown algae called 'kelps' have been the source for obtaining iodine e.g., Laminaria, Macrocystis, Fucus. About 25% of total iodine is extracted from kelps.

Sewage disposal : Green unicellular algae such as Chlorella and Chlamydomonas are used in sewage disposal ponds. They remove CO₂ and restore O₂ by the process of photosynthesis.

Medicines : Sodium lamining sulphate is blood anticoagulant and obtained form Laminaria and Durivillea has antiworm (vermifuge) properties.

Harmful aspects

Algal toxicity : Some dinoflagellates like Prymnesium, Gymnodinium are extremely poisonous to fishes. The blue-green alga Microcystis secretes hydroxylamine which not only kills aquatic life. While Lyngbya and Chlorella may cause skin allergies in human beings.

Algal parasitism : The red alga Cephaleuros virescens causes red rust of tea thus destroying the tea leaves. Similar disease are caused by the species of Cephaleuros to coffee plant, Piper and Citrus sp.

Spoilage of drinking water : Forms like Anabaena, Microcystis not only spoil the taste of drinking water but also produces toxic effect. The growth of algae is controlled by using algicides such as dichlorophen, sodium perborate, phygon XI, exalgae, delrad etc.

Water blooms : Algae grow abundantly in water reservoirs where excess of nutrients are available to them. This algal growth floats on the water surface and look like foam or soap lather. It is called water bloom. e.g., Microcystis, Anabaena, Oscillatoria etc.