Classification of Algae
- 1. Classification of Algae by: Mehwish Manzoor
- 2. General characteristics of Algae They are commonly found in aquatic environments including freshwater, marine and brackish water. They are either motile or non-motile. Some of the motile and non-motile algae may form a colony known as Coenonbium. Algae possess the usual eukaryotic structures - Golgi apparatus, mitochondria, Endoplasmic reticulum and a nucleus. The algae includes both the microscopic unicellular to macroscopic multicellular organisms. Most of the algae are autotrophic characterized by their ability to use the carbon-di- oxide as a carbon source and light as an energy source. They reproduce both sexually and asexually. Algae are photosynthetic eukaryotic organisms.
- 3. Basisfor the classificationof Algae Algae are generally classified on the basis of the following characteristics: feature of the cell walls rties of pigments that contribute to photosynthesis behavior Fritsch classification of algae One of the best known algal classification was proposed by Fritsch who divided them into 11 classes(1945). Classification is based on pigments, flagella and reserve food material. Eleven classes proposed by Fritsch are as follows: 1. Chlorophyceae 2. Xanthophyceae 3. Chrysophyceae 4. Bacillariophyceae 5. Cryptophyceae 6. Dinophyceae 7. Chloromonadineae 8. Euglenineae 9. Phaeophyceae 10. Rhodophyceae 11. Myxophyceae 1980 - Lee used fine structural aspects of organelles such as chloroplast, ER, flagellum, eyespot and nucleus to classify the algae into 6 divisions and 15 classes. The 6 divisions are : Cyanophyta, Glaucophyta, Chromophyta, Rhododphyta, Chlorophyta and Charophyta. Rosowski and Parker (1982) classified algae into 15 classes.
- 4. Van den Hoek (1995) classified algae into 11 divisions: Cyanophyta Prochlorophyta Glaucophyta Rhodophyta Heterokontophyta Haptophyta Cryptophyta Dinophyta Euglenophyta Chloroarachniophyta Cholorphyta Chlorophycophyta – The green algae Chlorophyta is the division of the green algae. They are principally freshwater species. They are also found in sea water, and many other terrestial ecosystems. This division contain around 500 genera and around 8000 species. There are many single celled forms and many colonial types of green algae. Many unicellular green algae are motile by flagella action. Colonial types occur as spheres, filaments or plates. Some species have special structures called holdfasts, which anchor them to submerged objects or aquatic plants. The cells of the chlorophycophyta have a well defined nucleus and usually a cell wall and the chlorophyll and other pigments are in chloroplasts as in higher plants. Majority of the green algae contain one chloroplasts per cell. The cholorplasts contains chlorophyll a and b. Green algae reproduce both sexually and asexually. The cholorplasts are green because they are not masked by any accessory pigments. Because of this character, the members of chlorophyta are called as green algae.
- 5. The chlorophyta has a characteristic set of acessory pigments such as β-carotene and Xanthophylls. Chlamydomonas It is a typical unicellular motile green algae widely distributed in stagnant water, damp soils and even in snow as “Snow algae”. (Chlamydomonas nivalis - found in snow). They reproduce both sexually and asexually. Model organism
- 6. Volvox: Volvox is a colonial green algae. Found in freshwater habitats. It forms spherical colonies of up to 50,000 cells. The individual cells are biflagellate and are morphologically similar to that of Chlamydomonas. Chlorella: It is a green algae which is mainly found in freshwater habitats such as ponds, ditches and lakes.
- 7. It is globular in shape. It is non-motile in nature. Generally used in supplemental food supply - SCP. Ulothrix: It is a filamentous green algae found in freshwater and marine water. They are found in flowing streams. They are attached to the twigs or stones by holdfasts at the bases of the filaments. It reproduces by means of the flagellated zoospores. Each Zoospore may become attached to a solid object and develop in to a filament. Acetabularia: The name, Acetabularia, derives from the Latin word acetabulum, a broad, shallow cup used for dipping bread; the upturned cap of Acetabularia resembles such a cup. For this reason, it is also sometimes called mermaid's wineglass. Each Acetabularia cell is composed of three segments: the "foot" or base which contains the nucleus, the "stalk," and the "cap."
- 8. Rhodopycophyta-The red algae The members of this division are red in colour. Hence they are known as “Red Algae”. Red algae are red because of the presence of the pigment phycoerythrin. They are generally found in freshwater or marine water. The members of this division are generally multicellular though unicellualr forms are known. Some Rhodophytes are also important in the formation of tropical reefs. These reef-building rhodophytes are called coralline algae, because they secrete a hard shell of carbonate around themselves, in much the same way that corals do. In many red algae, asexual reproduction takes place by non motile spores called monospores. Sexual reproduction is oogamous type involving specialised female cells called Carpogonia and specialised male cells called spermatia.
- 9. Several red algae is of economic importance: Carrageenan: It is extracted from the walls of several red algae. It is used as a stabilizer or emulsifier in foods such as ice cream, custards and evaporated milk. Agar, a well-known solidifying agent used in the preparation of microbiological media is extracted from Gelidium and Gracilaria. Gelidium: Gelidium is a genus of Thalloid red algae comprising 124 species. G.amansii - Used in making agar Gracilaria: Gracilaria is a genus of red algae notable for its economic importance in producing agar. It is generally used as a food source for humans and certain fishes such as tangs. Cryptophycophyta-The cryptomonads
- 10. The cryptomonads are a small group of biflagellate organisms. They have two unequal flagella, which arise from the base of the groove. The cells are slipper shaped and occur singly. Some forms have a cellulose wall while others are naked, being surrounded by plasmalemma with a thin granular material on the outside. There are one or two plastids, with or without pyrenoids per cell. Food reserve is stored as a true starch as well as oil. The longer flagellum bears two rows of mastigonemes, while the shorter flagellum bears a single row of mastigonemes. The choloroplasts contain chlorophylls a and c, but their colour is masked by a variety of accessory pigments which occur in different proportions. The pigments include phycocyanin, phycoerythrin, αcarotene and the Xanthophylls. Reproduction is either by means of longitudinal cell division or the formation of zoospores or cysts.
- 11. Cryptomonas: It is a very important member of the cryptophyta division. They are widely distributed in freshwater habitats. Cryptomonas is a dimorphic genus, meaning it could be either protozoan (Cryptomondida) or alga (Cryptophyceae). The cells are usually brownish in color, and have a slit-like furrow at the anterior. They are not known to produce any toxins and are used to feed small zooplankton, which is the food source for small fish in fish farming. Currently there are 26 species of Cryptomonas. Euglenophycophyta-The euglenoids The members of this algal division are commonly called as euglenoids, because the best known genus of the group is Euglena. Most of the euglenophyta are freshwater organisms, but some are marine. There are around 800 species of euglenophytes. They are unicellular flagellates. There are almost always two flagella, but one of these is often very short. The members lack a cell wall but normally are surrounded by an outer layer of pellicle composed of proteins and lipids.
- 12. The euglenoid algae contain chlorophylls a and b like the green algae and typically appear green because the chlorophylls are not masked by any accessory pigments. Of the accessory pigments that are present, the most important are β-carotene, neoxanthin and diadinoxanthin, although echinenone, diatoxanthin and zeaxanthin are also found. This group is known to contain carbohydrate paramylon. Nutrition: A) Photosynthetic: They can supplement photosynthesis by taking up the organic compounds. Eg: Euglena. B)Heterotrophic: There are also many colourless euglenophytes which are completely dependent on heterotrophic nutrition. Eg: Astasia. C)Phagotrophic: Few euglenophytes are phagotrophic and possess a special apparatus for capturing the prey and a cytostome for ingesting them. Eg: Peranema. Euglena: They are both plant-like and animal-like. They are unicellular. They are motile by means of flagellum. They commonly occur in freshwater and salt water. There is no cell wall containing cellulose.
- 13. The outer membrane is an organised periplast. Certain species develop a prominent stigma or eye spot. Contractile vacuoles are also present. Most species of Euglena have photosynthesizing chloroplasts within the body of the cell, which enable them to feed by autotrophy, like plants. In low moisture conditions, or when food is scarce, Euglena forms a protective wall around itself and lies dormant as a resting cyst until environmental conditions improve. All Euglenoids have two flagella rooted in basal bodies located in a small reservoir at the front of the cell. In Euglena, one flagellum is very short, and does not protrude from the cell, while the other is relatively long, and often easily visible with light microscopy. Chrysophyciphyta –The Golden brown algae It is a large group of algae commonly found in the freshwater. Species of chrysophycophyta are predominantly flagellates. Some are amoeboid with the psudopodial extensions of the protoplasm. Most forms are unicellular, but some form colonies. They differ from the green algae in the nature of their pigments, in storing reserve food as oil or chrysolaminarin rather than starch. Their characteristic colour is due to the masking of their chlorophyll by brown pigments. Reproduction is commonly by binnary fission but occasionally isogamous.