In this article we will discuss about:- 1. Characters of Brown Algae 2. Types of Common Brown Algae 3. Economic Importance.
Characters of Brown Algae:
Brown algae are eukaryotic marine algae which possess chlorophyll a, chlorophyll c, abundant fucoxanthin, phycocolloid algin and reserve food in the form of laminarin.
1. Brown algae comprise about 2000 species.
2. Majority of the brown algae are marine.
3. Brown algae generally occur in both tidal and sub tidal regions of colder seas.
4. Unicellular forms are absent.
5. The body consists of a branched filamentous structure in lower forms (e.g., Ectoearpus) and parenchymatous structure in higher forms (e.g., Sargassum, Laminaria, Fucus, Macrocystis).
6. Brown algae include the largest algae. The giant brown algae are called kelps. The largest kelps are Macrocystis (40-100m) and Nereocystis (20—30m).
7. The plant body is often differentiated into holdfast, stipe and lamina (frond).
9. Lamina may be simple or divided variously. Despite its size and complexity of form, lamina of kelp is annual. Lamina (frond) is photosynthetic.
10. The large forms often possess air vesicles or bladders for providing buoyancy.
11. Cell wall contains cellulose, nonglucan saccharides and phycocolloids.
12. Phycocolloids of brown algae are non-sulphated monosaccharide’s. The common ones are alginic acid, fucoidin and fucin. They are copious in species dwelling tidal areas.
Phycocolloids protect the algae from desiccation during low-tide, freezing under low temperature and injury when beaten against rocks.
13. Photosynthetic organelles or chromatophores possess 3-thylakoid lamellae.
14. Photosynthetic pigments include chlorophyll a, chlorophyll с and carotenoids (carotenes and xanthophylls). The brown colour of algae is due to the presence of large amount of xanthophyll called fucoxanthin.
15. Food reserve is laminarin (carbohydrate) and lipid.
16. Cells possess refractile vesicles called fucosan vesicles. The vesicles contain a phenolic chemical named fucosan. Fucosan is colourless inside water but becomes brown or black on exposure to air.
17. Conducting tubes or trumpet hyphae are present in larger brown algae or kelps. The tubes possess sieve septa. They take part in conduction of food materials. The rate is 38- 78cm/hr.
18. Asexual reproduction occurs with the help of both motile and non-motile spores (e.g., neutral spores, tetra spores, mono-spores). The motile spores or zoospores are bi-flagellated and have heterokont flagellation with one smaller whip like smooth flagellum and other larger of tinsel type.
19. Sexual reproduction varies from isogamy, anisogamy to oogamy. In isogamy and anisogamy both the gametes are motile with heterokont flagellation. In oogamy, only the male gametes are motile or flagellate. The female gametes are non-motile.
20. Isomorphic alternation of generations is found in some brown algae, e.g., Ectocarpus, Dictyota. Here both the haploid and diploid generations are present and are similar in structure. In many brown algae, the diploid generation or phase is dominant. The haploid generation or phase is either microscopic or represented by gametes only (e.g. Fucus).
Types of Common Brown Algae:
Some of the common brown algae are:
1. Laminaria:
It is a wide spread kelp or large-sized brown alga popularly called devil’s apron. The size is 1-3 metres. Plant body is sporophyte. It is differentiated into basal holdfast, a near cylindrical stipe and a flattened blade or lamina. Alternation of generations is heteromorphic. Laminaria is a source of food, manure, algin and iodine.
2. Dictyota:
It is a ribbon shaped dichotomously branched marine brown alga that grows in shallow waters. Frond is flat, dichotomously branched. The surface of the frond bears hair and unilocular sporangia. Unilocular sporangia produce haploid tetra spores.
Each tetraspore produces haploid gametophytic thallus that is similar in morphology to sporophytic thallus. Sex organs are borne in clusters or sori. Male sex organs or antheridia produce uniflagellate sperms. Fertilization produces diploid zygote which germinates to produce diploid plant body.
3. Fucus:
It is a leathery flat branched perennial brown alga of small size which has been a source of fodder, manure and algin. Frond is flat and branched both dichotomously and monopodially. A pair of pneumocysts or air bladders occurs in the region of branching. At places the branches contain flask-shaped cavities called conceptacles. Meiosis occurs during gamete formation.
4. Sargassum:
Sargassum is popularly called gulf weed which is used both as fodder and manure. Sarganine is an antibacterial and antifungal extract obtained from it. Both free floating and attached forms occur. Floating forms are common in part of North Atlantic ocean called Sargasso sea where these are menace to shipping. In attached forms, the plant has three parts— holdfast, main axis and laterals.
Main axis bears long laterals on which are borne short laterals or leaves. Pneumocysts or air bladders occur at places in the axils of leaves. In free floating forms, pneumocysts provide buoyancy tor floating while in attached forms they provide buoyancy for keeping the plants upright. Sex organs are borne in two different types of conceptacles.
5. Ectocarpus:
It is a filamentous marine brown alga which has both upright and prostrate regions. Such a growth is called heterotrichous. Upright branches show evection (pushing of parent branch) to give an appearance of dichotomy. Fixation to solid substratum occurs through prostate portion and rhizoids.
Reproduction occurs by fragmentation. The plants can also multiply asexually through the formation of diploid biflagellate zoospores in plurilocular (= neutral) sporangia.
The sporophytic plant body also bears unilocular sporangia in which sporic meiosis occurs and haploid biflagellate meiozoospores are formed. The latter germinate to produce gametophytic thalli. The gametophytes liberate biflagellate gametes from their plurilocular gametangia. The gametes fuse to form diploid zygote that germinates to produce diploid plant.
Economic Importance of Brown Algae:
Some of the major economic importance of brown algae are listed below:
1. Food:
A number of brown algae are used as food in some countries, e.g. Laminaria, Alaria, Macrocystis, Sargassum. The edible brown algae are also used as fodder.
2. Fouling of Ships:
Some brown algae get attached to hulls of ships, e.g. Ectocarpus. Others floating in masses (e.g., Sargassum) have a nuisance value for ships.
3. Iodine:
Fucus and Laminaria are rich source of iodine. Potash is abundant in Macrocystis and Nereocystis.
4. Medicines:
Sodium laminarin sulphate is blood anticoagulant. Laminaria and Ascophyllum have antibiotic properties, while Durvillea has worm expelling or vermifuge properties.
5. Alginic Acid:
It is phycocolloid which is obtained commercially from Laminaria, Macrocystis, Nereocystis, Fucus, Sargassum etc. Alginic acid and its salts are used in obtaining emulsions (ice- creams, ointments, toothpastes, cosmetics, creams, shampoos, etc.); sizing textiles, flame proof plastics, security glass, formation of pills and surgical threads.