Algae

3.1 Introduction to Plant Kingdom

In the previous chapter, we looked at the broad classification of living organisms under the system proposed by Whittaker (1969) wherein he suggested the Five Kingdom classification viz. Monera, Protista, Fungi, Animalia and Plantae. In this chapter we will deal in detail with further classification within Kingdom Plantae popularly known as the 'plant kingdom'.

We must stress here that our understanding of the plant kingdom has changed over time. Fungi and members of the Monera and Protista having cell walls have now been excluded from Plantae though earlier classifications placed them in the same kingdom. So, the cyanobacteria (BGA, blue green algae) that are 'algae' in earlier days, are no longer included here. Of late, cladistics has caused a revision: the plant kingdom is restricted to Eukaryotic, chlorophyll-bearing organisms, including algae, bryophytes, pteridophytes, gymnosperms and angiosperms.

Classification Criteria for Plants

Even within the Plant Kingdom, several classification systems have been proposed. The earlier systems used gross superficial morphological characters such as habit, colour, number of leaves, shape of leaves, etc. Such systems were called artificial classification systems. Examples include Linnaeus's sexual system based on the number of stamens.

Subsequent systems, called natural classification systems, were based on natural affinities among the organisms. They consider the totality of characters — external as well as internal features.

The latest system, phylogenetic classification, is based on evolutionary relationships, supported by DNA evidence. Numerical Taxonomy uses computers and statistical analysis. Cytotaxonomy uses cytological information (chromosome number, structure, behaviour). Chemotaxonomy uses chemical constituents.

3.2 Algae

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (both fresh water and marine) organisms. They occur in a variety of other habitats — moist stones, soils, wood. Some of them also occur in association with fungi (lichen) and animals (e.g., on sloth bear).

Form & Structure

The form and size of algae are highly variable. They range from colonial forms like Volvox and the filamentous forms like Ulothrix and Spirogyra. A few of the marine forms such as kelps, form massive plant bodies.

Reproduction in Algae

Algae reproduce by vegetative, asexual, and sexual methods:

• Vegetative reproduction is by fragmentation. Each fragment develops into a thallus.
• Asexual reproduction is by the production of different types of spores, most common being the zoospores. They are flagellated (motile) and on germination give rise to new plants.
• Sexual reproduction takes place through fusion of two gametes. These gametes can be flagellated and similar in size (as in Chlamydomonas) or non-flagellated but similar in size (as in Spirogyra). Such reproduction is called isogamous.

Fusion of two gametes that are dissimilar in size, as in some species of Chlamydomonas, is termed as anisogamous. Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete is termed oogamous, e.g., Volvox, Fucus.

Economic Importance

• Algae fix at least half of the total CO₂ on earth by photosynthesis. As primary producers of energy-rich compounds which form the basis of food cycles of all aquatic animals, algae are crucial.
• Many species of Porphyra, Laminaria and Sargassum are among the 70-odd species of marine algae used as food.
• Agar, used to grow microbes and in preparation of ice creams and jellies, is obtained from Gelidium and Gracilaria.
• Carrageen, an important commercial product, is obtained from Chondrus crispus (red alga).
• Chlorella and Spirullina are unicellular algae, rich in proteins and used as food supplements even by space travellers.

3.2.1 Three Classes of Algae

The algae are divided into three main classes based on their pigments, stored food, and cell wall:

3.2.1.1 Chlorophyceae (Green Algae)

The members of chlorophyceae are commonly called green algae. The plant body may be unicellular, colonial or filamentous. They are usually grass-green due to the dominance of pigments chlorophyll a and b. The pigments are localised in definite chloroplasts. The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species.

Most of the members have one or more storage bodies called pyrenoids located in the chloroplasts. Pyrenoids contain protein besides starch. Some algae may store food in the form of oil droplets. Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.

Reproduction: Vegetative by fragmentation, asexual by zoospores (flagellated), sexual reproduction shows considerable variation in the type and formation of sex cells (isogamous, anisogamous, or oogamous).

Common examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, and Chara.

3.2.1.2 Phaeophyceae (Brown Algae)

The members of phaeophyceae or brown algae are found primarily in marine habitats. They show great variation in size and form. They range from simple branched, filamentous forms (Ectocarpus) to profusely branched forms as represented by kelps, which may reach a height of 100 metres.

They possess fucoxanthin in their chloroplasts which gives them a brown colour. Food is stored as complex carbohydrates, which may be in the form of laminarin or mannitol. The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin.

The plant body is usually attached to the substratum by a holdfast, and has a stalk (stipe) and leaf-like photosynthetic organ called frond. Vegetative reproduction takes place by fragmentation. Asexual reproduction in most brown algae is by biflagellate zoospores that are pear-shaped and have two unequal laterally attached flagella.

Common examples: Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus.

3.2.1.3 Rhodophyceae (Red Algae)

The members of rhodophyceae are commonly called red algae because of the predominance of the red pigment r-phycoerythrin in their body. Majority of the red algae are marine with greater concentrations found in the warmer areas. They occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates.

The red algae are usually multicellular. Some of them have complex body organisation. The food is stored as floridean starch which is very similar to amylopectin and glycogen in structure.

The red algae usually reproduce vegetatively by fragmentation. They reproduce asexually by non-motile spores and sexually by non-motile gametes. Sexual reproduction is oogamous and accompanied by complex post-fertilisation developments.

Common examples: Polysiphonia, Porphyra, Gracilaria, and Gelidium.

Worked Examples