Introduction :
The branch of botany dealing with the study of algae is called as phycology or algology. It is derived from the Greek word Phykos which means 'alga' or 'sea weed'. They are simple, thallose, autotrophic non-vascular plants having unicelled sex organs and no embryo formation.
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.
Father of Phycology - Fristch ⇒ Book ⇒ “Structure & Reproduction of algae”
Father of Indian phycology - M.O.P. Iyengar
Occurrence :
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.
Thallus organization :
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.
REPRODUCTION
The algae reproduce vegetatively, asexually and sexually. Various method involved in reproduction are discussed in the following account.
Vegetative reproduction :-
(i) Binary fission - Cell is divided into two parts and nucleus is also divided into two parts by mitosis. eg. Found only in unicellular algae
(ii) Fragmentation - Filaments break down into small pieces & from new filaments. e.g. All filamentous algae
(iii) Budding : A bud arises as a papilla on the parent cell. It enlarges and finally separates e.g., Protosiphon.
(iv) Propagules or Gemmae : They arise as modified branches. They are rich in food and germinate into new plant on detachment e.g., Sphacelaria.
(v)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.
(vi)Tubers : In certain forms like Chara, tubers are formed on the lower nodes as also on the rhizoids. They germinate into new plants.
(vii)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.
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.
(i) 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.
(ii) Aplanospores : They are thin walled and non-motile spores commonly formed in Chlamydomonas, Ulothrix etc.
(iii) Autospores : They are also thin walled, non-motile spores which resemble the parent cell e.g., Chlorella.
(iv) 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.
(v) Exospores : These are specialised type of aplanospores formed externally by pinching off the exposed protoplast e.g., Chamaesiphon.
(vi) Endospores : In many blue-green algae, one or more endospores are formed inside an endosporangium by fragmentation of the protoplast e.g., Dermocapsa, Pleurocapsa.
(vii) 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.
(viii) 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).
(ix) Monospore : The juvenile stage of Batrachospermum, a red alga, multiplies by forming a single monospore formed in side a monosporangium.
Sexual reproduction :
The sexual reproduction in algae is broadly of three types
(i) 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.
(ii) 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.
(iii) 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
Note :
(1) Chlamydomanas exhibits complete evolution of sexual reproduction.
Isogamous - Simplest
Oogamous - Advanced
(2) Ulothrix exhibits origin of sexual reproduction
Classification :
Linnaeus (1754) differentiated a group of plant and called as 'algae' where he included lichens and liver worts also.
Endlicher (1836) proposed the term Thallophyta where he included algae along with fungi.
The classification of algae is mainly based on the photosynthetic pigments. In addition to this cell wall composition and stores food are also the base of classification.
Algae is divided into following divisions
Chlorophyceae - GREEN ALGAE
Phaeophyceae - BROWN ALGAE
Xanthophyceae - YELLOW - GREEN ALGAE
Rhodophyceae - RED ALGAE
Chlorophyceae (Green Algae)
Green algae is the most advanced algae. It is believed that green algae are the ancestor of the higher plants.
Habitat : Green algae is comopolitan in nature.
Different forms of Green algae (Structure) : Green algae is found in many forms
(1) Unicellular :-
(i) Chlamydomonas : Motile unicellular algae. This algae moves with the help of flagella.
(ii) Chlorella - Non motile unicellular algae. Calving discovered “Calvin cycle” by experimenting or Chlorlela.
(iii) Acetabularia - Umbrella plant - It is the largest unicellular plant. The diameter of its cell is 10 cm.
Hammerling experimented on Acetabularia.
(2) Coenocytic - Some green algae are coenocytic i.e. multinucleated. e.g. Caulerpa
Note :- According to five kingdom system the algae described above should be placed in Protista but
exceptionally its life cycle is similar to green algae. Therefore it is placed in Plantae.
(3) Colonial - Some green algae are found in colonies. They form colony of cell. The number of cells in a colony is fixed. Colony with fixed number of cells called coenobium.
eg. Volvox - Motile colony
Hydrodictyon - Non motile colony (called as water net)
(4) Multicellular filamentous - Mostly algae are multicellular filamentous.
e.g. Ulothrix - Known as pond wool
Spirogyra - Known as pond sill
Note : Some green algae are heterotrichous i.e. two types of branches prostrate and erect – Fritscheilla, Stigeoclonium Coleochaeta (Fritscheilla tuberosa has approach to the early land plants)
(5) Multicellular thalloid or Parenchymatous - Some algae are multicellular in length & width. eg. Ulva - Also called as sea lettuce
Photosynthetic pigments :
Chlorophyll - Chl ‘a’ and Chl ‘b’
Carotene - \(\beta\)carotene
Xanthophyll - Luteaxanthin and Violoxanthin - Hellow coloured.
Note: The green colour of green algae is due to chlorophyll.
Stored food : In green algae the food is stored in the form of starch
Note :On the basis of pigments (Chl ‘a’, Chl ‘b’, Carotenoids) stored food (starch) & cell wall (made up of cellulose or pectin), green algae is considered similar to higher plants.
Reproduction :
Zoospore formation occurs.
Male gametes flagellate
Sexual reproduction – Isogamous, anisogamous or oogamous
Economic Importance. :
(1) Food -Chlorella is used as food, because after Spirulina, Chlorella has largest amount of protein.
(2) Antibiotics - Chlorellin antibiotic is obtained from Chlorella.
(3) Space research -In space, Chlorella is used as a source of food and O2.
(4) Parasitic algae - Cephaleuros algae remains parasitically in the leaves of tea plant and cause disease ‘red rust’.
Phaeophyceae (Brown algae or kelps or Sea weeds)
Brown algae are found in marine water.
Brown algae are multicellular filamentous.
Brown algae are the largest in size (up to 60 meter in length).
Largest brown algae - Macrocystis
Thallus of brown algae is divided into two parts :
(i) Lamina - Leafy part of photosynthetic part
(ii) Stipe - Elongated tubes called trumpet hyphae are present for food conduction in stipe. Trumpet hyphae are analogus to sieve tubes of vascular plants. Due to lamina and stipe brown algae look like leaf (leaf algae).
Pigments :
Chlorophyll - Chl ‘a’ , Chl ‘c’
Carotene - Only \(\beta\) carotene
Xanthophylls - Mainly Fucoxanthin
Note : The amount of Fucoxanthin is more in brown algae due to which this algae is brown in colour (Xanthophylls are mostly yellow but fucoxanthin is brown)
Stored food : Laminarin and mannitol - both are derivatices of carbhohyddrates.
Phycocolloids :
In the cells wall of brown algae some colloid substances like fucinic acid, alginic acid and fucoidin are present which are known as phycocolloids.
Phycocoloids protects brown-algae against desiccation and shocks. Phycocoloids are used in ice-cream to make them more viscous. Alginates, salts of alginic acid used for dentury measurement.
Reproduction :
Zoospore formation occurs.
Male gametes flagellate
Sexual reproduction – Isogamous, anisogamous or oogamous
Special name :
(1) Postelsia : It is known as Sea palm.
(2) Ancyclonema - Is is called Ice bloom because it grows on marine ice.
(3) Sargassum - It is known as Gulf weed because Sargssum is a free floating algae. It grows rapidly in North Atlantic ocean and covers thousands of hectares of area. Therefore this region is called as sargasso sea.
(4) Laminaria (Kelps)- It is called as Devil’s Aprin.
Economic Importance :
(1) Alginates or Alginic acid - Obtained from Laminaria, Fucus, Macrocystis.
Alginates is used in the manufacturing of soap, ice-cream, polish, cream and plastic.
(2) Iodine and Bromine - Obtained from Laminaria.
Xanthophyceae (Yellow Green Algae)
Habitat and Structure : All the algae in this group of acellular coenocytic and are found in moist soil.
Pigments :
Chlorophyll - Chl ‘a’ , Chl ‘e’
Carotene - \(\beta\) carotene
Xanthophylls - Many
Note : The yellow colour of these algae is due to the presence of xanthophyll.
Stored food : Leucosin and Oil (Leucosin is derivative of carbohydrate)
eg. Vaucheria, Botrydium, Tribonema.
Note : In Vacucheria algae some special type of spores are formed which are known as synzoospores.
Reproduction :
Zoospore formation occurs.
Male gametes flagellate
Sexual reproduction – Isogamous, anisogamous or oogamous
Rhodophyceae (Red algae)
Red algae is second most ancient algae after blue green algae i.e. they are developed after blue green algae.
eg. Polysiphonia
Red algae mainly found in marine water. But exceptionally Batrachospermum is found in fresh water (river) and Poryphyridium is found on land.
Red algae are multicellular but exceptionally Prophyridium is unicellular.
There is no motile stage found in life cycle of red algae and BGA i.e. cilia & flagella are absent.
Cell wall of red algae is complex and made up of cellulose & pectin.
The cell was of red algae is also complicated like blue green algae. Their cell wall has many different type of substances such as xylan, galactose, polyurinoc acid, polysulphate esters.
But in some algae calcium carbonate is also present in the cell wall. Due to which their thallus become stony. These algae form lime stone & coral reefs e.g. Corallina and Lithothamnion.
Pigments :
Chlorophyll - Chl ‘a’ and Chl ‘d’
Carotense - \(\beta\) Carotene
Phycobilins - R - phycoerythrin (red coloured) and R - phycocyanin (blue coloured)
On the basis of pigments red algae is similar to blue green algae.
Colour of red algae changes according to depth in sea this is called as Gaudikov’s effect.
When red algae are present on the surface of the sea then their colour is blue and when they are at the bottom, their colour is red. At the surface of sea the amount of R - phycocyanin is more while in depth the amount of R - phycoerythrin is more. Penetration power is maximum in ultra violet and violet light. R-phycoerythrin is only pigment to absorb ultra violet and violet light. Due to phycoerythrin red algae are deepest algae.
Gaudikov’s effect is also found in Blue - Green algae.
Red algae not always red, it may be clue coloured also. eg. Batrchospermum - This is blue coloured algae.
Stored Food :
Floridean starch - It is a primitive type of starch. Structurally floridean starch is similar to the cyanophycean starch of blue green algae. Starch of higher plants is less branched yet floridean starch in highly branched.Glycogen is also highly branched so that floridean & cynanophycean starch is structurally similar to glycogen.
Phycocolloid : Agar, carrageen and funori phycocolloids are found in the cell was of red algae.
Reproduction :
(1) Vegetative - By fragmentation
(2) Asexual - Non motile spores [By monospore, carpospores, tetraspore]
(3) Sexual reproduction –
(i) Sexual reproduction is oogamous type.
(ii) The female sex organs are called carpogonia. They are apparently similar to archegonia of bryophyta. Carpogonia is unicellular & jacketless but archegonia is multicellular & jacketed.
(iii) The male sex organs of red algae are known as spermatangia. Non motile spores like gametes are formed in spermatangia which are known as spermatia.
(iv) Exceptionally life cycle of Polysiphonia is diplontic
Special points and Economic importance.
(1) Harveyella - It is a colourless parasitic algae. It remains as parasite on other algae.
(2) Porphyra – It is an edible algae.
Gelidium and Gracilaria - Agar - Agar colloid is obtained from these. It is used to prepare culture medium
Chondrus crispus - It is also called Irish moss. Carrageenin colloid is obtained from this algae. It is used as gelating agent in food industries (i.e. to make the food item viscous)
Capsule of medicines is also prepared from carrageenin.
Haematococcus nivalis - ”Red snow” - It likes to grow on snow and impart red colour to snow.