What is Brown Algae?
The members of the Division Phaeophyta are commonly known as brown algae, due to the presence of a brown coloured rotenoid, fucoxanthin, in large amounts, which masks the green colour of chlorophyll pigment. But for few fresh water forms, most of the brown algae are marine. They grow in the inter tidal belt and upper littoral region. In tropics, members of Phaeophyta are most abundant in the Sargasso sea of the Atlantic. They are usually found attached to rocks with the help of a discoid or branched holdfast. Many of these algae have air bladders which increase their buoyancy.
General structure of brown algae
The brown algae range in structure from simple to complex parenchymatous forms. Giant kelps (Lominoria) attain a length of more than 50 meters. In such forms, the plant body is differentiated into holdfast, stem-like stipe and flattened leaf- like blades.
Economic value of brown algae
The brown algae have some great economic value. In maritime districts many brown algae which are commonly known as sea weeds, such as species of Fucus, Laminaria, Ascophyllum, Sargassum, etc., are used as fodder. Their fodder value is due to the presence of vitamins and micronutrients in their thalli. A useful product, alginic acid (alginates), is extracted
from the middle lamellae and cell walls of Laminaria, Ascophyllum and Macrocystis. The sanitized alginates are non-toxic and readily form gels. They are used as thickeners and gelling agents in a wide variety of industrial products such as hand creams in cosmetics, emulsifiers in ice cream, polishes, medicines and paints, gelling agents in confectionery and glazes in ceramics. Ash of dry kelps is the source of soda which is used in the manufacture of soap, glassware and alum. Large brown algae are used as organic manure. They are rich in potassium, but have relatively low nitrogen and phosphorus contents.
Cell structure of brown algae
The cell wall of brown algae contains alginic and fucinic acids. The cytoplasm contains many small vacuoles and refractive bodies (fucosan vesicles). The chromatophores are ellipsoidal and discoid and contain chlorophyll a, c, β and c-carotenes, and xanthophylls . They also contain large amount of a brown pigment, fucoxanthin, vhich masks green colour of chlorophyll pigments and that is why these algae appear brown in colour. The main storage product is laminarin. The cells of brown algae are unmucleate with one or more nucleoli. Some common brown algae : A. Sargassum; B. Fucus.
Reproduction of brown algae
In addition to vegetative reproduction by fragmentation of the thallus most brown algae reproduce both by asexual and sexual means. Asexual reproduction is either by biflagellate zoospores or by non-flagellate tetraspores developed in sporangia. Each zoospore bears two unequal flagella which are inserted laterally and sometimes near the posterior end. Sexual reproduction ranges from isogamy to oogamy. In oogamy, fertilization may take place cither after the female gamete or gametes are liberated from the oogonium or when the ovum is retained i a the oogonium. The number of female gametes in an oogonium may be one or eight. The antherozoids are uni-or biflagellate. Most of the members of the group are characterised by a distinct alternation of generations which is usually isomorphic, or sometimes heteromorphic in nature.