Chlorophyceae: Occurrence, Characteristics and Economic Importance | Phycology

In this article we will discuss about:- 1. Introduction to Chlorophyceae (Green Algae) 2. Occurrence of Chlorophyceae 3. Characteristics 4. Thallus Organisation 5. Economic Importance.

1) 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.

2) 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.

3) Important Characteristics of Chlorophyceae (Green Algae):

• Plant body has a simple construction and shows no differentiation into true root, stem and leaves. For this reason plant body is called a thallus. 
• The cells constituting the thallus are eukaryotic and thus contain all the cell organelles such as the definitely organized nucleus; membrane bound plastids, mitochondria, Golgi bodies, endoplasmic reticulum, and true vesicles.
• Starch is the main reserve food.
• The cell wall is stable and generally has cellulose as the main structural polysaccharide.
• The protoplast is bounded by a thin and semi permeable plasma membrane.
• The cytoplasm possesses many small vacuoles or there is a large central vacuole.
• The pigments are localized in the green plastids known as chloroplasts.
• The main pigments are chlorophyll a and b, but α and β carotenes and xanthophylls are also present.
• The chloroplasts normally contain the pyrenoids surrounded by a starch sheath.
• The shape of chloroplast shows much variation; it is cup shaped, (Chlamydomonas), girdle shaped (Ulothrix), reticulate (Cladophora, oedogonium), stellate, spiral (Spirogyra) or discoid.
• Usually there is a single nucleus in each cell, but the members of Siphonales and Cladophorales are coenocytic. The position, size and shape of the nuclei are quite variable. The nucleoli and chromosomes remain suspended in the granular matrix of the nucleus. In general, there is a single nucleolus per nucleus but in Conjugales, there are several nucleoli in a nucleus.
• Most of the flagellate cells have a photosensitive red eye spot or stigma in the anterior portion, near the flagellar base.
• The motile vegetative or reproductive cells (zoospores and gametes) have two (rarely four) equal, whiplash type flagella inserted ate the anterior end. However, in Oedogoniales the motile cells have a ring or crown of flagella.
• The plant body is unicellular or multicellular. The former may be solitary or colonial. The multicellular forms show a considerable range of variation in the form and structure of the plant body.
• All cells of Chlorophyta contain at least one plastid, though these do not always display photosynthetic pigments. Most of the Chlorophytes are considered to be autotrophic. However, Chlorophytes exhibit a surprising level of nutritional variation.
• Some forms (Prasinophyceans) are known to feed on particles and therefore exhibit phagotrophy and mixotrophy. A few forms are colourless and contain a reduced plastid, they have completely lost their ability to carry out photosynthesis and are thus obligately heterotrophic. Some forms are capable of complementing photosynthesis by uptake and utilization of exogenous carbon i.e., sugars, amino acids etc. (Neilson and Lewin, 1974; Tuchman, 1996), thus they exhibit mixotrophy. Some green algae are photoheterotrophic, i.e., they utilize organic carbon when their photosynthesis becomes limited by the availability of dissolved inorganic carbon (Graham, et al., 1994; Lewitus and Kana, 1994).
• They reproduce by vegetative, asexual and sexual methods. The vegetative propagation takes place by cell division and fragmentation. Akinetes, zoospores and aplanospores are the main asexual spores. Zoospores are often formed during night and are then liberated in the morning.
• Sexual reproduction occurs in all members except some Chlorococcales and Chlorodendrales. It may be isogamous, anisogamous or oogamous.
• The zygote or oospore secretes a thick wall and undergoes a resting period. It germinates under favourable conditions and forms a new thallus.
• The sex organs are always unicellular.
• Zygote generally is the only diploid structure in the life cycle.
• Life history: Most of the members are haploid showing haplontic life cycle. Some members are diploid (Siphonales) showing diplontic life cycle while a few others show isomorphic alternation of generations between haploid and diploid plants (e.g. Cladophora)- called diplohaplontic life cycle. 
• Chlorophytes are better represented in fresh water than in salt water.

4) 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.

5) 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.