The algae occur in a variety of habitats which are summarised hereunder :

# (i) Fresh water forms : They occurs in rivers, ponds, pools, lakes and ditches. Some forms e.g., diatoms occur as passively floating and drifting flora. They are called as phytoplanktons. 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). Some blue-green algae form water blooms on the surface of water.

# (ii) 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.

# (iii) Terrestrial forms : Several members of green and blue-green algae and a few others occur on damps oil. 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.

# (iv) Specialized habitats
- (a) 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.
- (b) 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.
- (c) Epiphytes : Several algal forms grow on other plants (algae, angiosperms) as epiphytes. e.g., Oedogonium, Cladophora, Vaucheria etc.
- (d) 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.
- (e) 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.
- (f) 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 cyanallae.
- (g) Symbiotic forms : Some algae like Chlorella, Nostoc etc. growing in symbiotic relationship with members of Ascomycetes and Basidiomycetes (Fungi) constitute the lichen.

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

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

(i) Unicellular forms : Several members of algae are unicelled. They may be motile (Chlamydomonas) or non-motile (diatoms). Forms showing movement by pseudopodia are called as rhizopodial. Some forms have a thick wall and become sedentary for certain duration in their life history. They are called as coccoid e.g., Chlorella, Chroococcus.

(ii) Multicellular forms : The multicelled algae show a considerable range in their organization.
- (a) 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.
- (b) Palmelloid : Here the vegetative cells of the alga get surrounded by a mucilagenous matrix e.g., Tetraspora.
- (c) Dendroid : Here the colony appears like a microscopic tree. There is secretion of mucilage from the polar end e.g., Ecballocystis.
- (d) 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.
- (e) Siphonous : An aseptate, multinucleate (coenocytic) condition of a filament or thallus constitutes the siphonous habit e.g., Vaucheria.
- (f) 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.

The algae show three types of cellular organization, prokaryotic, mesokaryotic and eukaryotic. The mesokaryotic organization is seen in Dinophyceae where a nuclear membrane and chromosomes are present but the basic proteins are absent. Here we shall describe only prokaryotic and eukaryotic organization in some details.

# (i) Prokaryotic : The blue-green algae, also called as Cyanobacteria, show prokaryotic cell structure. The cell wall may have a mucilagenous sheath. The inner wall layer is made up of mucopeptide. It is a polymer of two aminosugars. The cell wall is followed by a plasma membrane. Light microscope, the cell is distinguished into a central colourless centroplasm and a peripheral coloured chromoplasm. The cell lack a well organised nucleus, mitochondria, E.R., and Golgi. However, 70 s ribosomes are present. There are no plastids either but the cell does possess photosynthetic lamellae which are comparable to thylakoids. The cells have gas vacuoles and granules of several types such as α, β, polyhedral and structured granules. The α–granules are now called as polyglucoside granules whereas the structured granules are cyanophycin granules.
In some filamentous forms like Nostoc, Anabaena, certain large specialized cells also occur. They are called as heterocysts. They form polar nodules, they may be intercalary or terminal in position.

# (ii) Eukaryotic : Most of the algal groups (except blue-green and dinoflagellates) show eukaryotic cell structure. The cell wall is made up of cellulose. However, there may be additional layers of pectin and chitin. While silica is also present in the cell wall of diatoms whereas 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. In Micromonas, there is only one mitochondrion in a cell. The shape of chloroplasts is variable. It may be cup-shaped (Chlamydomonas, Volvox), spiral (Spirogyra), stellate (Zygnema), girdle-shaped (Ulothrix) reticulate (Oedogonium) but generally discoid. There may be a single thylakoid in the granum of Rhodophyceae, two in Cryptophyceae, three in Phaeophyceae and Bacillariophyceae but generally many. The cells may also possess simple or contractile vacuoles.
The motile forms and male reproductive cells also possess a photosensitive eye spot or stigma. In dinoflagellate the eye spot is distinguishable in a lens, photosensitive region and a pigmented cup.
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).

The grouping of algae based on pigments. Algae possess three classes of pigments namely chlorophylls, carotenoids and biliproteins.

# (i) Chlorophylls : A chlorophyll molecule consists of a head and a tail. The head is made up of porphyrin, a tetrapyrrole closed ring compound with a central Mg atom; the tail is made up of phytol. Some five types of chlorophylls occur in algae namely chl. a, b, c, d and e. The chlorophyll a occurs in the all algal groups, b in Chlorophyceae, c in Phaeophyceae and Bacillariophyceae, d in Rhodophyceae and e in Xanthophyceae.

# (ii) Carotenoids : These pigments are isoprene (C5H8) derivatives. They are of two types namely carotenes and xanthophylls. Some six types of carotenes have been located in algae which have been designated as α, β, γ, ε carotenes, lycopene and flavicine. Of these, β–carotene is found in all the algal group, α–carotene in Chlorophyceae and Rhodophyceae, γ–carotene and lycopene in Chlorophyceae, ε–carotene in Cryptophyceae and Bacillariophyceae and flavicine in Cyanophyceae. Some 20 xanthophylls have been identified from different algal groups such as lutein, fucoxanthin, violaxanthin, astaxanthin, zeaxanthin, myxoxanthin etc. Of these, lutein occurs in Chlorophyceae, Phaeophyceae, Rhodophyceae and Cyanophyceae, fucoxanthin in Phaeophyceae, Rhodophyceae and Bacillariophyceae. Myxoxanthin occurs in Cyanophyceae and astaxanthin in Chlorophyceae.

# (iii) Biliproteins or Phycobilins : This comprise a bile pigment conjugated to a protein moiety. They are of three types namely phycocyanin, phycoerythrin and allophycocyanin. While c–phycocyanin and c–phycoerythrin occurs in Cyanophyceae, r–phycoerythrin and r–phycocyanin are found in Rhodophyceae. The allophycocyanin is found in both the groups.

The food is reserved in different forms in various algal groups, some of which are summarised here under :
(i) Chlorophyceae : Starch and Oils.
(ii) Xanthophyceae : Chrysolaminarin (carbohydrate) and Oils.
(iii) Bacillariophyceae : Chrysolaminarin and Oils.
(iv) Phaeophyceae : Laminarin (carbohydrate), Mannitol (alcohol) and Oils.
(v) Rhodophyceae : Floridean Starch and Galactan –SO4 polymers.
(vi) Cyanophyceae : Cyanophycean Starch (glycogen) and Cyanophycin (protein).

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

# (i) Vegetative reproduction : It occurs by following types.
(a) 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.
(b) Fission : The unicelled forms like diatoms, desmids multiply by fission i.e., simple cell division.
(c) Budding : A bud arises as a papilla on the parent cell. It enlarges and finally separates e.g., Protosiphon.
(d) Propagules or Gemmae : They arise as modified branches. They are rich in food and germinate into new plant on detachment e.g., Sphacelaria.
(e) Hormogonia : In certain blue-green algae e.g., Oscillatoria, Nostoc, the filament breaks up into hormogonia due to the formation of separation disc (necridia) or at the junction of intercalary heterocysts.
(f) Tubers : In certain forms like Chara, tubers are formed on the lower nodes as also on the rhizoids. They germinate into new plants.
(g) 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.

# (ii) 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.
(a) 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.
(b) Aplanospores : They are thin walled and non-motile spores commonly formed in Chlamydomonas, Ulothrix etc.
(c) Autospores : They are also thin walled, non-motile spores which resemble the parent cell e.g., Chlorella.
(d) Hypnospores : These are thic walled non-motile spores formed to tide over unfavourable condition. They germinate on the arrival of favourable conditions e.g., Chlamydomonas, Ulothrix.
(e) Exospores : These are specialised type of aplanospores formed externally by pinching off the exposed protoplast e.g., Chamaesiphon.
(f) Endospores : In many blue-green algae, one or more endospores are formed inside an endosporangium by fragmentation of the protoplast e.g., Dermocapsa, Pleurocapsa.
(g) 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.
(h) 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).
(i) Monospore : The juvenile stage of Batrachospermum, a red alga, multiplies by forming a single monospore formed in side a monosporangium.

# (iii) Sexual reproduction : The sexual reproduction in algae is broadly of three types as under :
- (a) 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. Here two nuclei of opposite strains (+ and –) fuse and the phenomenon is called as autogamy. In Chlamydomonas sp. two vegetative cells may fuse to form a zygospore and the phenomenon is called as hologamy. As a result of fusion of two gametes, the zygospore is formed.
- (b) 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.
- (c) 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.

Linnaeus (1754) differentiated a group of plant and called as 'algae' where he included lichens and liver worts also. This group was delimited to its present form by de Jussieu (1789). Endlicher (1836) proposed the term Thallophyta where he included algae along with fungi. Fritsch (1935) classified algae into the eleven classes as under :
(i) Chlorophyceae (Green algae)
(ii) Xanthophyceae (Yellow-green algae)
(iii) Chrysophyceae
(iv) Bacillariophyceae (Diatoms)
(v) Cryptophyceae
(vi) Dinophyceae (Dinoflagellates)
(vii) Chloromonadineae
(viii) Euglenineae (Euglenoids)
(ix) Phaeophyceae (Brown algae)
(x) Rhodophyceae (Red algae)
(xi) Myxophyceae or Cyanophyceae (Blue green algae)

# (i) Chlorophyceae
(a) Plants fresh water or marine.
(b) Forms unicelled to parenchymatous.
(c) Cells showing eukaryotic orgnaization.
(d) Chief pigments – Chlorophyll a, b; α, β, γ– carotenes, lycopene; lutein, violaxanthin.
(e) Reserve food – Starch and oils.
(f) Zoospore formation occurs.
(g) Male gametes flagellate.
(h) Flagella identical.
(i) Sexual reproduction – Isogamous, anisogamous or oogamous.

# (ii) Xanthophyceae
(a) Plants generally fresh water.
(b) Forms unicelled to siphonous.
(c) Cells showing eukaryotic organization.
(d) Chief pigments – Chlorophyll a, e; β–carotene, violaxanthin, neoxanthin.
(e) Reserve food – Chrysolaminarin and oils.
(f) Zoospore formation occurs.
(g) Male gametes flagellate.
(h) Flagella non-identical (unequal).
(i) Sexual reproduction – Isogamous, anisogamous or oogamous.

# (iii) Phaeophyceae
(a) Plants marine.
(b) Forms unicelled to parenchymatous.
(c) Cells showing eukaryotic organization.
(d) Chief pigments – Chlorophyll a, c; β–carotene, fucoxanthin, lutein, violaxanthin, diatoxanthin.
(e) Reserve food – Laminarin, mannitol and oils.
(f) Zoospore formation occurs.
(g) Male gametes flagellate.
(h) Flagella unequal.
(i) Sexual reproduction – Isogamous , anisogamous or oogamous.

# (iv) Rhodophyceae
(a) Plants generally marine.
(b) Forms filamentous to parenchymatous.
(c) Cells showing eukaryotic organization.
(d) Chief pigments – Chlorophyll a, d is present but chlorophyll c is absent; α, β–carotene, lutein, violaxanthin, fucoxanthin, myxoxanthin, γ–phycoerythrin, γ–phycocyanin and allophycocyanin.
(e) Reserve food – Floridean starch, galactan –SO4 polymers.
(f) No zoospore formation.
(g) Male gametes non-flagellate.
(h) Sexual reproduction by specialized type of oogamy.
(i) Life cycle haplobiontic or diplobiontic.

# (v) Myxophyceae (Cyanophyceae)
(a) Plants generally fresh water, a few forms marine.
(b) Forms unicelled to filamentous.
(c) Cells showing prokaryotic organization.
(d) Chief pigments – Chlorophyll a; β-carotene; luteins, myxoxanthin, oscillaxanthin, c-phycocyanin, c-phycoerythrin, allophycocyanin.
(e) Reserve food – Cyanophycean starch (glycogen) and cyanophycin (protein).
(f) No zoospore formation.
(g) No flagellate bodies.
(h) No sexual reproduction.

There are sufficient evidences which suggest that the land plants have originated and evolved from algae. Morphological, cytological and biochemical similarities between the green algae and the land plants suggest that the ancestral algae was green, flat and fleshy with heterotrichous filamentous habit similar to Fritschiella (Smith). The various evidences which suggest algal origin of land plants are :
(i) Presence of similar type of photosynthetic pigments – Chlorophyll a, Chlorophyll b and carotenoids in both green algae and land plants.
(ii) Presence of cellulose and pectose as constituents of cell wall in both the groups.
(iii) Accumulation of starch as reserve food material.
(iv) Similar structure of flagella in green algae and land plants.

(i) USEFUL ASPECTS :
# (a) 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.

# (b) 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.

# (c) 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.

# (d) 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.

# (e) 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.

# (f) 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.

# (g) 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.

# (h) 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.

# (i) 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. Besides iodine, the kelp also contain Bromine, Boron, Copper, Cobalt, Chromium, Iron, Manganese, Molybdenum and Zinc.

# (j) 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 and makes the sewage water habitable for many fishes and aerobic bacteria.

(ii) HARMFUL ASPECTS
# (a) 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 but also the birds and cattles who care to drink that water. While Lyngbya and Chlorella may cause skin allergies in human beings.

# (b) 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.

# (c) Fouling of marine vessels : Some brown and red algae grow on the metallic and wooden submerged parts of naval vessels. As a result, their surfaces are corroded. This creates problems in their navigation.

# (d) Spoilage of drinking water : Forms like Anabaena, Microcystis not only spoil the taste of drinking water but also produces toxic effect. The water filters are blocked due to growth of diatoms, Spirogyra, Oscillatoria etc. Forms like Chaetophora, Anacystis grow inside the water pipes and boilers, and thus corrode their surface by their secretion. The growth of algae is controlled by using algicides such as dichlorophen, sodium perborate, phygon XI, exalgae, delrad, cuson etc. Besides, cyanophages (LPP-1) are also used for the destruction of Lyngbya, Phormidium and Plectonema.

# (e) 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., Member of cyanophyceae (Microcystis, Anabaena, Oscillatoria etc.) are common. Water bloom deplete oxygen of water reservoirs and therefore, aquatic animals die of deoxygenation.