At present phylogenetic classification systems are the acceptable system among the scientific community. Phylogenetic systems of classification are based on evolutionary relationship between various organisms. This system is based on the assumption that organisms belonging to the same taxa have a common ancestor. We now use information from many other sources too to help resolve difficulties in classification. These become more important when there is no supporting fossil evidence.
Numerical Taxonomy: Numerical Taxonomy is based on all observable characteristics. Number and codes are assigned to all the characters and the data are then processed. In this way each character is given equal importance and at the same time hundreds of characters can be considered.
Cytotaxonomy: Cytotaxonomy is based on cytological information like chromosome number, structure, behaviour, and uses the chemical constituents of the plant to resolve confusions. This system is also used by taxonomists these days.
Habit & Habitat: Algae are simple and thalloid organisms. They are mainly aquatic organisms and live in freshwater and marine habitats. They bear chlorophyll. Some of the algae also occur in association with fungi (as lichen) and animals (e.g. on sloth bear).
Size: Some of them are unicellular, e.g. Chlamydomonas. Some algae live in colonies, e.g. Volvox. Some algae are in filamentous form, e.g. Ulothrix and Spirogyra. Some of the marine forms make massive plant bodies, e.g. kelps.
Reproduction: Vegetative, asexual and sexual methods of reproduction are present in algae.
Vegetative Reproduction: Vegetative reproduction takes place by fragmentation. During vegetative reproduction, a thallus divides into many fragments and each fragment develops into a thallus.
Asexual Reproduction: Asexual reproduction takes place by different kinds of spores; in algae. Zoospores are the most common type of spore. The zoospores are flagellated and hence are motile. They give rise to new plants on germination.
Sexual reproduction: Fusion of gametes is involved in sexual reproduction. There are following types of fusion of gametes in algae:
- Isogamous Fusion: In this case, the gametes are similar in size, e.g. Spirogyra. The gametes can be flagellated or non-flagellated.
- Anisogamous Fusion: In this case, the gametes are dissimilar in size, e.g. some species of Chlamydomonas.
- Oogamous Fusion: In this case, the female gamete is large and non-motile, while the male gamete is small and motile, e.g. volvox and focus.
Economic Importance of Algae:
- Almost 50% of carbon fixation on earth is carried out by algae. Thus, they are among the large producers on the earth; especially in aquatic habitat. About 70 species of marine algae are used as food, e.g. Porpyra, Laminaria and Sargassum.
- Some marine brown and red algae produce large amounts of hydrocolloids. Hydrocolloids are water holding substances and are used commercially.
- Agar; which is obtained from Gelidium and Gracilaria is used as culture medium in laboratories. It is also used in making ice-cream and jelly.
- Chlorella and Spirullina are used as food supplements; even during space travel.
The algae are divided into three main classes, viz. Chlorophyceae, Phaeophyceae and Rhodophyceae.
Characteristics: They are commonly called green algae. Plant body can be unicellular, colonial or filamentous. Dominance of chlorophyll a and b means that chlorophyceae are usually grass green. The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped. Storage bodies; called pyrenoids are found in most of the members. Pyrenoids store protein and starch. Food is also stored in the form of oil droplets in some algae. Cell wall is usually rigid and is composed of an inner layer of cellulose and an outer layer of pectose.
Reproduction: Vegetative reproduction happens by fragmentation or by formation of different types of spores. Asexual reproduction takes place by flagellated zoospores which are produced in zoosporangia. Sexual reproduction can be isogamous, anisogamous or oogamous.
Common Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara
Characteristics: They are also called brown algae. They are found in marine habitats. The brown algae can range from simple branched, filamentous forms to profusely branched forms. Chlorophyll a, c, carotenoids and xanthophyll are the pigments found in phaeophyceae. Colour can vary from olive green to various shades of brown. The colour variation depends on the amount of xanthophyll and fucoxanthin.
Food is stored as complex carbohydrates; in the form of laminarin or mannitol. The cell wall of vegetative cells is composed of cellulose and is covered by a gelatinous coating of algin. Protoplast contains plastids, centrally located vacuole and nucleus.
Plant body is usually attached to the substratum by a holdfast. The plant body has a stalk, stipe and leaf-like fond.
Reproduction: Vegetative reproduction is by fragmentation. Asexual reproduction takes place by biflagellate zoospores. Sexual reproduction can be isogamous, anisogamous or oogamous. Fusion of gametes can happen in water or within the oogonium. Gametes are pyriform (pear-shaped) and bear two laterally attached flagellum.
Common Examples: Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus
Characteristics: These are commonly called red algae. The red pigment, r-phycoerythrin is predominant in rhodopytes. Most of them are marine species and are mainly found in warm areas. They are found in well-lit regions close to the surface of water as well as at great depths. The thallus is multicellular in most of the cases. Food is stored as floridean starch.
Reproduction: Vegetative reproduction is by fragmentation and it is the most prevalent method of reproduction. Asexual reproduction is by non-motile spores. Sexual reproduction is oogamous. Post-fertilisation development is complex.
Common Examples: Polysiphonia, Porphyra, Gracilaria and Gelidium.