- Algae are very useful to man in many ways. About a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis.
- Being photosynthetic, Algae increase the level of dissolved oxygen in their immediate environment.
- Algae are of paramount importance as primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals.
- Among the 70 species of marine algae many species of Porphyra, Laminaria and Sargassum are used as food.
- Agar is obtained from Gelidium and Gracilaria are used to grow microbes and in preparations of ice-creams and jellies.
- Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), e.g., algin (brown algae) and carrageen (red algae) are used commercially.
- Chlorella and Spirullina are unicellular algae, rich in proteins and are used as food supplements even by space travellers.
- The algae are divided into three main classes:
I. Chlorophyceae(Green Algae)
- The Chlorophyceae is commonly called ‘Green algae’.
- Forms-Their plant body may be unicellular, colonial or filamentous.
- They very diverse group, which includes members distributed not only in the sea, but also in freshwater and terrestrial habitats.
- All marine green algae are classified in a common class, called Ulvophyceae and include about 920 species, which are distributed in all seas of the world.
- Pigments-They are usually grass green due to the dominance of pigments chlorophyll a and b.
- These pigments are localised in definite chloroplasts.
- The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species.
- Stored food-Most of the members of Chlorophyceae 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.
- Cell wall-Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.
- Reproduction-Vegetative reproduction usually takes place by fragmentation or by formation of different types of spores.
- Asexual reproduction is by flagellated zoospores produced in zoosporangia.
- The sexual reproduction shows considerable variation in the type and formation of sex cells and it may be isogamous, anisogamous or oogamous.
- Some commonly found green algae are Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara.
- Biological Importance-Main source for aquatic animals, essential source of starch produced via photosynthesis, vital source as well of atmospheric oxygen.
Volvox Chlamydomonas Chara
II. Phaeophyceae(Brown Algae)
- The members of Phaeophyceae or ‘Brown algae’ are found primarily in marine habitats.
- Forms -They show great variation in size and form. They range from simple branched, filamentous forms e.g., Ectocarpus to profusely branched forms e.g., Kelps, which may reach a height of 100 metres.
- Stored food -Food is stored as complex carbohydrates, which may be in the form of laminarin or mannitol.
- Pigments- They possess chlorophyll a, c, carotenoids and xanthophylls. They vary in colour from olive green to various shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin present in them.
- Cell wall-The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin.
- The protoplast contains, in addition to plastids, a centrally located vacuole and nucleus.
- Structure-The plant body is usually attached to the substratum by a holdfast, and has a stalk, the stipe and leaf like photosynthetic organ –the frond.
- The leaf-like structure is referred to as a blade (when it is single and not divided), a frond (when divided), or a lamina (when flattened). The air bladders in the thallus are called pneumatocysts. They increase algal buoyancy.
- Brown algae are mostly marine and macroscopic. They flourish especially in cold ocean waters.
- Reproduction-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.
- Sexual reproduction may be isogamous, anisogamous or oogamous. Union of gametes may take place in water or within the oogonium (oogamous species).
- The gametes are pyriform (pear-shaped) and bear two laterally attached flagella. The common forms are Fucus, Ectocarpus, Dictyota, Laminaria and Sargassum.
- Biological Importance- have alginate in their cell walls, extracted for use as a food thickener, extracted for use as a food thickener, Other species are edible to humans.
Fucus Dictyota Laminaria
III. Rhodophyceae(Red algae)
- Pigment-Rhodophyta are commonly called ‘Red algae’ because of the predominance of the red pigment, r-phycoerythrin in their body.
- Location -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.
- Form-The red thalli of most of the red algae are multicellular.
- Some of them have complex body organisation.
- Stored food -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.
- Cell wall- Their cell wall is two-layered; the outer layer contains agarose and agaropectin whereas the inner layer is chiefly made up of cellulose.
- Reproduction -They reproduce asexually by non-motile spores and sexually by non-motile gametes.
- Sexual reproduction is oogamous and accompanied by complex post fertilisation developments.
- The common members are-Polysiphonia, Porphyra, Gracilaria and Gelidium.
- Biological Importance- Limestone-reef builders of the ecosystem, help build coral reefs by secreting calcium carbonate, harvested for the manufacture of products from agar and/or carrageenan.
Polysiphonia Porphyra Gracilaria
ALGAE – DIVISION
FLAGELLAR NUMBER AND POSITION OF INSERTIONS
Chlorophyll a, b
2,8, equal, apical
Fresh water, brackish water , salt water
Chlorophyll a, c, Fucoxanthin
Cellulose and Algin
Fresh water (rare), brackish water , salt water
Chlorophyll a, d, phycoerythrin
Cellulose, pectin and polysulphate esters
Fresh water (some), brackish water , salt water (most)