Algae

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&aposs 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 $\alpha$, $\beta$, $\gamma$– 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 $\beta$�rotene, 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 $\beta$�rotene, 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 $\alpha$, $\beta$�rotene, lutein, violaxanthin, fucoxanthin, myxoxanthin, $\gamma$–phycoerythrin, $\gamma$–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 $\beta$-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 &aposkelps&apos 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.

Some representative alga

Spirogyra

Habitat

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 muscilage occurs on the outside. It gives a silky touch. Hence Spirogyra is also called water silk or mermaid&aposs tresses.

Structure

The thallus is an unbranched and uniseriate filament where cells are arranged in a single row. In some species hold fast is present (e.g., S. fluviatilis). The cells are elongated and cylindrical. The cell wall is two layered the outer is of pectic substance and the inner of cellulose. The outer part (pectin) dissolves in water to form a muscilaginous sheath.

Due to this reason Spirogyra filament&aposs are slippery. Transverse or septum can be plane, colligate (with H-shaped piece), replicate (ring like ingrowths) and unduliseptate (undulate). The protoplast is differentiated into plasma membrane, thin layer of cytoplasm, single nucleus, one (e.g. S.sahni and S. venkataramanni) or many (16 in S.rectispora) ribbon (spiral) shaped chloroplasts (wavy margin) with pyrenoids and a large central vacuole. Nucleus occurs inside the central vacuole where it is suspended by means of cytoplasmic strands.

Reproduction

Spirogyra reproduces by vegetative, asexual and sexual methods.

(1) Vegetative reproduction : It takes place by fragmentation of filament. Later on, each segment gives rise to new plant.

(2) Asexual reproduction : Normally asexual reproduction is absent in Spirogyra. It occurs only occasionally by the formation of akinetes, aplanospores and azygospores (Parthenospores).

Akinetes : Under unfavourable conditions, the cells of the filament develop into thick walled structures, which are known as akinetes. On the onset of favourable conditions, these give rise to new plants. Their wall is made up of cellulose and pectin e.g., S. farlowi.

Aplanospores : These nonmotile aplanospores are either round or oval. These later on under favourable condition give rise to the new individuals. Aplanospores are known to occur in S. aplanospora, S. articulate etc.

Azygospores or Parthenospores : If there is sudden change in the environment, the gametes fail to fuse and each functions as parthenospore.

(3) Sexual reproduction : The sexual reproduction in Spirogyra is called conjugation, It involves the fusion of two morphologically identical, but physiologically dissimilar gametes.

The conjugation is of two types :

(1) Scalariform conjugation : This is the most common and advanced type of conjugation. It involves two filaments of Spirogyra and takes place between two recently formed cells. The cells of one filament show the formation of papilla towards the other filament.

It stimulates the formation of similar papilla in cells lying opposite to them. The two papillae fuse by enzymatic dissolution of the wall thus forming a conjugation canal.

The conjugation tube between the two filaments looks like a ladder, through which gamete from one of the gametangia passes through to fuse with the passive gamete of another filament. The gametes are formed singly and both active and passive gametes are considered male and female gametes respectively. The fusion of both kinds of gametes with each other results into formation of zygospore.

The zygospore wall is differentiated into three layers, the outer exospore which is thin, the middle mesospore which is a thick layer of cellulose, chitinized and pale yellow to brown in colour, and inner endospore which is thin and cellulose in nature.

(2) Lateral conjugation : It takes place between two nearest cells of the same filament (homothallic). Both male and female gametes are found in same filament. It is of two types.

(i) Indirect lateral conjugation : Two outgrowths appear on both sides of a transverse septum of two adjacent cells which later on form a conjugation tube. Of the two cells, one behaves as male gametangium from which gamete passes through the tube into female gametangium. By fusion, zygospore is formed. Thus in each second cell of a filament zygospore is formed. It is commonly seen in S. affinis and S. tenuissima.

(ii) Direct lateral conjugation : In this type of conjugation, the male gamete after passing through an aperture in the transverse septum of adjoining gametangium enters the female gametangium and fuses to form a zygospore. The two gametes, though morphologically alike but differ in their behaviour. Hence this type of sexual reproduction corresponds physiologically anisogamy. It is commonly seen in S. jogensis.

Germination of zygospores : The zygospores on the arrival of favourable conditions germinate. The nucleus undergoes meiosis to form four haploid nuclei (tetra nucleate). Of these three nuclei degenerate and one functions.

 The exo and mesospores rupture and the endospore protudes out in the form of germling. The new cell undergoes transverse division continuously to form a new filament. Thus in the life cycle of Spirogyra, there is no flagellate phase.

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).

Ulothrix

Habitat

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.

Structure

An unbranched filament, consisting of numerous cylindrical or rectangular cells joined end to end. The filaments remain attached to some substratum by means of rhizoidal cell, i.e., showing distinction in base and apex. The basal cell is elongated and colourless known as holdfast while the uppermost cell is rounded. Cell wall consists of two layers. Inner layer is made up of cellulose and outer layer is mostly made up of protopectin which is insoluble in water.

Due to presence of protopectin, Ulothrix filaments appear as wet threads. In the centre of the cell is a nucleus and a girdle or ring shaped (U. zonata) or collar shaped chloroplast. One (U. rorida) or more pyrenoids are present in the chloroplast. A vacuole is present, surrounding it is a thin layer of cytoplasm (primordial utricle).

Reproduction

The Ulothrix reproduces vegetatively, asexually as well as sexually.

(1) Vegetative reproduction : It takes place by fragmentation. In this process a filament divides into small parts. These smaller fragments grow and give rise to new filaments of Ulothrix.

(2) Asexual reproduction : It takes place by the production of zoospores, aplanospores, hypnospores, akinetes and palmella stage.

Zoospore formation : In favourable conditions, each cell produces zoospores except holdfast. Cytoplasm collects in the centre and divides into 2,4,8,16 or sometimes 32 segments. Each segment develops into biflagellate or quadriflagellate zoospores. Each zoospore is uninucleate, pear-shaped with thin cell membrane (zoospores not having cell wall). The zoospores are of 3 types :

Quadriflagellate macrozoospores : Usually 4 per cell.

Quadriflagellate microzoospores : Usually 8 per cell.

Biflagellate microzoospores : Usually 16-32 per cell.

The zoospores come out into a vesicle, later on the wall disintegrates (forming a pore in lateral wall) and macro and microzoospores are liberated free and float in water. Microzoospores attach to substrate by their anterior ends while macrozoospores attach by their posterior ends. After some time zoospores give rise to new individuals.

In Ulothrix, aplanospores, hypnospores and akinetes are also formed under unfavourable conditions.

Palmella stage : It is produced in water deficiency or presence of toxic chemicals. A number of small green naked cells are formed in mucilage sheath. These cells can grow and divide. On the approach of favourable condition each cell now change into a quadriflagellate zoospore. Zoospore develop into new plants.

(3) Sexual reporduction : It occurs at the end of growing season. Ulothrix is heterothallic. Sexual reproduction is of isogamous type. The gametes are motile and biflagellate. Except holdfast each cell of the filament can give rise to 64 to 128 gametes.

These gametes are smaller than zoospores. On dehiscence of gametangium, the gametes come out in a bag like structure and float on water.

When two gametes of (+) and (–) strain come together, they fuse and a quadriflagellate zygospore is formed which after floating for sometime on water, rests on the bottom of the pond. At this time, its four flagella disintegrate and a wall is formed surrounding it from all sides. After taking a rest for long period it divides meiotically and gives rise to 4-16 aplanospores or zoospores. These come out of the sac and give rise to a new plant of Ulothrix.