In this article we will discuss about:- 1. Introduction to Chlorophyceae (Green Algae) 2. Occurrence of Chlorophyceae 3. Characteristics 4. Thallus Organisation 5. Classification 6. Economic Importance.
Introduction to Chlorophyceae (Green Algae):
Chlorophyceae (chloros, green; phyceae, algal organisation) is commonly known as green algae’. Fritsch (1935) considered to include the green algae under the class Chlorophyceae, which have been raised to the rank of division Chlorophyta by Smith (1938), Tippo (1942) and Bold (1950).
Later Prescott (1969) and Round (1973) considered it to the rank of phylum Chlorophyta. Papenfuss (1946) included the suffix ‘phyco’ to the divisions of algae and named chlorophyta as Chlorophycophyta. Later Bold and Wynne (1978) also followed the same suggestion. Considering more appropriate, the classification of Fritsch (1935) is followed in this book.
This class consists of 425 genera and about 6,500 species but, later Prescott (1969) reported that the number of species may be as many as 20,000; with more being discovered continuously. The name green alga is given because of the presence of dominant pigments like Chlorophylls a and b over the carotenoids and xanthophylls. They are all eukaryotes.
Occurrence of Chlorophyceae (Green Algae):
The members of Chlorophyceae generally grow in fresh water (about 90%) and the rest in saline water, terrestrial habitat etc. The fresh water members such as Volvox, Oedogonium, Spirogyra etc. grow in ponds, pools and lakes.
Members of conjugales (e.g., Spirogyra, Zygnema etc.) and Oedogoniales (e.g., Oedogonium etc.) are strictly fresh water, but the members of Ulvaceae and Siphonales are predominantly marine. Some members of Volvocales, Chaetophorales and Cladophorales grow both in fresh and saline water.
Some species of Ulothrix and Vaucheria are subaerial and grow on damp soil. Some members may be terrestrial and grow as epiphytes on tree trunk, leaves etc. (e.g., Trentepohlia); as epizoic i.e., (growing on animal bodies (species of Characium and Cladophora); as endophytes (e.g., Chlorella), as parasites (e.g., Cephaleuros, Rhodochytrium and Phyllosiphon) and also cause diseases.
They can also grow in further different habitats like hot springs (Chlorella), snow (Chlamydomonas yellowstonensis), saline water (C. ehrenbergi) and some remain as partners in lichen associations.
Important Characteristics of Chlorophyceae (Green Algae):
1. Members of Chlorophyceae grow mostly in fresh water, a few in brackish and saline water and a few are terrestrial.
2. They show wide range of variations in their thallus structures like unicellular motile (Chlamydomonas) and non-motile (Chlorella), coenobium (Volvox), palmelloid (Tetraspora), dendroid (Ecballocystis), filamentous branched (Cladophora) and unbranched (Spirogyra), heterotrichous (Coleochaete), siphonaceous (Vaucheria) and parenchymatous (Ulva).
3. Flagella are 1-many, equal in size and inserted either apically or sub-apically. The flagella show typical 9+2 arrangement when viewed under E.M.
4. The cells are eukaryotic in nature. Usually there is only one nucleus in each cell, but in Siphonales and Cladophorales many nuclei are present in their coenocytic body. Normally the number of nucleolus is one per nucleus, but several nucleoli are present in the members of Conjugales.
5. The cell wall is mainly made up of cellulose, which comprised of hydroxyproline glycosides or xylans and mannans. In Chara the cell wall is encrusted with calcium and magnesium carbonate.
6. Inner to the cell wall, semipermeable cell membrane is present which encircles the protoplast. The cytoplasm contains many small vacuoles which pushes the nucleus with cytoplasm towards the periphery and called primordial utricle.
7. The flagellate cells have eye-spot or stigma in the anterior portion, which remain inserted at one side of the chloroplast.
8. The pigments are located in the chloroplast. Chkiroplast generally contains pyrenoid(s).
9. The main pigment is chlorophylls a and b; those dominate over α-and β-carotenes and xanthophylls.
10. Phycobilins are absent.
11. The reserve food is starch, composed of amylose and amylopectin.
12. They reproduce by all the three means i.e., vegetative (cell division and fragmentation), asexual (zoospore, aplanospore, akinete etc.) and sexual (isogamy to oogamy). The sexual reproduction is absent in some members of Chlorococcales.
13. The zygote or oospore is the only diploid structure in their life cycle.
Thallus Organisation of Chlorophyceae (Green Algae):
The class Chlorophyceae shows a range of variation in the structure of plant body (thallus). It ranges from unicellular e.g., Chlamydomonas, Chlorella, Sphaerella etc. to multicellular structure. The multicellular forms may be of different types. They may have a number of cells arranged in colonies of definite shape, the coenobium.
The number of cells in a coenobium may be definite and motile as in Volvox, Pandorina, Pleodorina etc. or the number may be indefinite, arranged in net-like masses and are non-motile as in Hydrodictyon. The multicells may aggregate and form a non-motile palmelloid structure, where the cells remain embedded in an amorphous or gelatinous matrix as found in Tetraspora and Palmodictyon.
In multicellular forms the cells may be arranged in a single row to form the filament. The filament may be branched (e.g., Pithophora, Cladophora etc.) or unbranched (e.g., Oedogonium, Spirogyra, Ulothrixetc.). The multicells may aggregate and form an expanded sheet-like structure as found in Coleochaete.
It shows heterotrichous habit where the erect system is well-developed. In some members the plant body is like cylindrical tube i.e., coenocy- tic as in Vaucheria. In some algae like Ulva, the plant body is leaf-like. The highly organised plant body in Chlorophyceae is found in Chara, where the plant is very much complicated in structure with well protected sex organs.
Classification of Chlorophyceae (Green Algae):
1. Order. Volvocales:
The order Volvocales includes 60 genera and about 500 species.
i. They are commonly found in fresh water. Some are grown in brackish water, marine water and also on soil.
ii. The plant body is unicellular or multicellular and the multicellular ones are colonial in habit.
iii. Both unicellular and colonial members are motile, either throughout or some part of their life cycles.
iv. They reproduce both asexually and sexually. Asexual reproduction takes place by zoospores, aplanospores, hypnospores etc. and sexual reproduction by iso-, aniso-, and oogamy.
Fritsch (1935) divided the order Volvocales into 3 suborders and 7 families.
The outline of classification is given below:
I. Sub order. Chlamydomonadineae
Family. i. Chlamydomonadaceae
II. Sub order. Tetrasporineae
Family. i. Tetrasporaceae
III. Sub order. Chlorodendrineae
Family. i. Chlorodendraceae
i. Plant body is unicellular, uninucleate with definite cell wall.
ii. Cells have 2-4 flagella which a.re equal in length.
iii. Chloroplast is generally cup-shaped, but it may be H-shaped, reticulate, stellate etc.
iv. Asexual reproduction takes place mainly by zoospores.
v. Sexual reproduction takes place by all the three means iso-, aniso-, and oogamy.
2. Order. Ulotrichales
The order Ulotricales includes 80 genera and about 430 species. Most of them are fresh water, while a few are marine (e.g., Ulva).
The important characteristics of the order are:
i. They are commonly found in fresh water (e.g., Ulothrix) or on soil, but a few are marine (e.g., Ulva, Enteromorpha).
ii. Plant body is commonly an unbranched filament; but in Ulvaceae it is parenchymatous or foliaceous.
iii. Cells are uninucleate and contain chloroplast of different types like C-shaped, parietal, axial etc.
iv. Each chloroplast contains one or more pyrenoids.
v. Asexual reproduction takes place by means of bi- or quadriflagellate zoospores, aplanospore and akinetes.
vi. Sexual reproduction takes place by gametic union and may be iso-, aniso-, or oogamous type.
Fritsch (1935) divided the order Ulotrichales into 3 suborders and 6 families.
The outline of classification is given below:
I. Sub order. Ulotrichineae
Family. i. Ulotrichaceae
II. Sub order. Prasiolineae
Family. i. Prasiolaceae
III. Sub order. Sphaeropleineae
Family. i. Sphaeropleaceae
i. The plant body is an unbranched filament.
ii. Cells of the filament are uninucleate.
iii. Cells have single girdle-shaped, parietal chloroplasts.
iv. Sexual reproduction is isogamous and takes place by the union of biflagellated gametes.
3. Order. Chaetophorales:
Chaetophorales are the plants with hair or setae.
The important characteristics of the order are given below:
i. Members are generally found in fresh water.
ii. Plant body is filamentous and shows prominent heterotrichous habit; however, in Coleochaete, the prostrate system is well- developed and in Microthamnion the erect system is well-developed.
iii. Some members have setae (Coleochaete) or hairs (Stigeoclonium) of different types. The hairs may be in the form of single elongated cell or rows of fine and elongated cells.
iv. The cells contain a parietal chloroplast with many pyrenoids.
v. Erect system bears reproductive structures.
vi. Vegetative reproduction takes place by fragmentation.
vii. Asexual reproduction takes place by bi- or quadriflagellate zoospores, aplanospores or akinetes.
viii. Sexual reproduction is commonly isogamous (Fritschiella, Stigeoclonium), anisogamy (Aphanochaete) and oogamy (Coleochaete) are found occasionally.
Fritsch (1935) classified the order into 5 families.
Chaetophoraceae, Trentepohliaceae, Coleochaetaceae, Chaetosphaeridiaceae and Pleurococcaceae.
i. Plant bodies possess a typical heterotrichous habit. Generally the projecting system is dominant and looks like disc.
ii. Cells are uninucleate with single lamellate parietal chloroplast with one or two pyrenoids.
iii. All or some cells bear a single long sheathed bristle or seta.
iv. The sheath is present in the form of a basal cylinder of mucilage.
v. The growth is always through the apical region.
vi. Asexual reproduction takes place by means of biflagellate zoospores.
vii. Sexual reproduction is oogamous.
4. Order. Oedogoniales
i. Most of the members grow in fresh water. The order is represented by only three genera, Oedogomium, Oedocladium and Bulbochaete.
ii. They are filamentous and the filaments may be branched (Oedocladium and Bulbochaete) or unbranched (Oedogomium).
iii. The plant body is differentiated into apical and basal region.
iv. It consists of cylindrical cells and the cells are longer than breadth.
v. Cells are uninucleate and have reticulate chloroplast with pyrenoids.
vi. Cell division is elaborate and a cap is formed at the upper end of the daughter cell.
vii. Asexual reproduction takes place by pyri- form, multinucleate and multiflagellate zoospores. Flagella are arranged in a ring around the beak-like anterior end.
viii. Sexual reproduction is advanced oogamous type.
ix. Both androspores and antherozoides are multiflagellate.
x. Male gametes are similar to zoospore but smaller in size.
xi. Heterothallic or dioecious species are of two types: macrandrous (where male and female filaments are of normal size) and nannandrous type (where male is very small i.e., dwarf male or nannandrium and the female one is of normal size).
According to Fritsch (1935) the order Oedogoniales contains only one family Oedogoniaceae. The family has only three genera: Oedogomium, Oedocladium and Bulbochaete.
5. Order. Siphonales:
i. Most of the members of Siphonales are marine. A few members are freshwater. Some members grow as epiphytes or endophytes.
ii. Thalloid plant body is variously branched, aseptate and multinucleate i.e., coenocytic.
iii. Plant body may be simple vesicular type (Protosiphon) to much branched filamentous type.
iv. Numerous small and discoid chromato- phores are arranged peripherally inside the thallus.
v. Nuclei are present towards the inner layer.
vi. The characteristic pigments of this order are siphonin and siphonoxanthin.
vii. Presence of siphon-like central vacuole throughout the plant body, which remains filled with sap. Cytoplasm is present between the outer wall and vacuole. The order is named “Siphonales” because of the presence of siphon-like vacuole.
viii. The plant reproduces by all the three means vegetative, asexual and sexual. Vegetative reproduction takes place by fragmentation, asexual reproduction by multiflagellate zoospore, aplanospore or hypnospore and sexual reproduction by oogamy. Rarely they perform iso- and anisogamy.
Fritsch (1935) divided the order Siphonales into 9 families.
viii. Phyllosiphonaceae, and
This classification is also followed by M. O. P. lyenger (1951)
6. Order. Charales
i. Members of this order are distributed throughout the world.
ii. Commonly they are found in fresh water with muddy or sandy bottom and also in water flowing over limestone.
iii. Plants are macroscopic, much branched, and erect and commonly up to 30 cm in length.
iv. The plants are differentiated into nodes and internodes. Some of the nodes bear branches of unlimited growth, those are again divided into nodes and internodes. Each node of the main axis and branch of unlimited growth bear a number of branches of limited growth.
v. Cells are very long, uninucleate and contain many discoid chloroplasts.
vi. Most of the species show cortication in the internodes. The cortex consists of vertically elongated row of cells.
vii. Sexual reproduction is highly advanced, oogamous type.
viii. The male and female reproductive bodies are globule and nucule, respectively. Globule develops many antherozoids and nucule contains only one egg.
ix. Zygote is produced after sexual reproduction. It shows very much elaborate post- fertilization changes. During germination, zygote undergoes meiosis and gradually it forms the plant body.
Fritsch (1935) placed the order Charales under the class Chlorophyceae includes only one family the Characeae having 2 sub families: 1. Nitelleae and 2. Chareae.
Bold and Wynne (1978) placed the order Charales alone under the only class Charophyceae, under the division Charophyta. The order Charales includes only one family Characeae.
Divn. Charophyta: Class. Charophyceae.
H. C. Bold and M. J. Wynne (1978) in their classification took out Chara along with some other genera like Tolypella, Nitella, Nitellopsis, Protochara, Lamprothamnium and Lychnotham- nus from Chlorophyceae and placed them in a separate Division Charophyta. The Charophyta consists of single class Charophyceae; order Charales and family Characeae.
Important characteristics of Characeae:
i. The division Charophyta includes the members of green algae, commonly known as stoneworts.
ii. Plant body shows much elaboration of vegetative structures encrusted with calcium carbonate.
iii. Plant body is erect and consists of elongated, jointed, commonly green main axis bearing branches, differentiated into nodes and internodes.
iv. Each node bears a whorl of lateral branchlets.
v. Asexual reproduction is absent.
vi. Sexual reproduction is of oogamous type.
vii. Antheridia (globule) and oogonia (nucule) show more complexity and elaboration than other Chlorophycean members.
viii. Motile cells are asymmetrical and two flagella are attached in lateral position of an antherozoid.
ix. Sex organs are so large that they can be visible with naked eye.
x. Zygote on germination forms proto- nema (Chara, Nitella) from which vegetative plants are developed.
Economic Importance of Chlorophyceae (Green Algae):
The green algae are not so economically important except a few members.
Among them Chlorella is very important because of its high protein content, presence of vitamins and its use in baking industry in the preparation of cake, pastries etc. It is also used in the preparation of an antibiotic, chlorellin; which is used to control bacteria. It is also used in different physiological experiments.
Another important member, Chara, is very useful to control malaria for its larvicidal properties. It is used as fertiliser and in the preparation of polishes.
Monostroma is used to prepare the common food ‘aonori’ in Japan.
In South India, Green Laver, a kind of food, is prepared from Spirogyra and Oedogonium.
Ulva and Enteromorpha are also eaten’ by some people.
Many members are used as a source of food and O2 for many aquatic animals.