- Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic plants.
- They occur in moist stones, soils, and wood or in association with fungi and animals.
Example-1. Lichen is the association of algae with fungi.
- Algae grow on the body of sloth bear.
Fig. sloth bear
- Some algae are unicellular, some exist in colonial or filamentous forms, and a few marine plants form massive plant bodies.
- The algae reproduce by vegetative, asexual, and sexual methods.
- Vegetative reproduction is by fragmentation, where each fragment develops into a thallus.
- Asexual reproduction is by the production of flagellated zoospores which on germination give rise to new plants.
- Sexual reproduction takes place through fusion of two gametes, if the gametes with or without flagella are similar in size they are isogamous, if dissimilar in size they are anisogamous , and if female gamete is large, non-motile and male gamete is smaller, motile, the gametes are termed as
- Isogamous gametes with flagella are seen in Chlamydomonas and without flagella are seen in Spirogyra , anisogamous gametes are seen in in some species of Chlamydomonas, and oogamous gametes are seen in Volvox, Fucus.
- Algae are useful to human beings in the following ways:
- Fix carbon dioxide through photosynthesis and increase the level of dissolved oxygen.
- Primary producers of energy-rich compounds.
- Many species of Porphyra, Laminaria, and Sargassum are used as food.
- Certain marine brown and red algae produce large amounts of hydrocolloids which are used commercially.
- For example- align is produced by brown algae and carrageen is produced by red algae
- Agar, one of the commercial products obtained from Gelidium and Gracilaria are used to grow microbes and in preparations of ice-creams and jellies.
- Chlorella and Spirullina are unicellular algae, rich in proteins and are used as food supplements.
- The algae are divided into three main classes: Chlorophyceae, Phaeophyceae and Rhodophyceae.
- Commonly called as green algae.
- They are usually grass green due to the dominance of pigments chlorophyll a and b.
- Most of the members have one or more storage bodies called pyrenoids located in the chloroplasts, which contain protein besides starch.
- Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.
- Vegetative reproduction usually takes place by fragmentation or by different types of spores, asexual reproduction is by flagellated zoospores produced in zoosporangia, and the sexual reproduction by the formation of sex cells and it may be isogamous, anisogamous or oogamous.
- Some commonly found green algae are: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara.
Fig. Green algae
- Phaeophyceae or brown algae are found primarily in marine habitats.
- They possess chlorophyll a, c, carotenoids and xanthophylls.
- They vary in color from olive green to various shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin present in them.
- Food is stored as laminarin or mannitol.
- The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of align.
- 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.
- Vegetative reproduction takes place by fragmentation, asexual reproduction is by biflagellate zoospores that are pear-shaped and have two unequal laterally attached flagella.
- Sexual reproduction may be isogamous, anisogamous or oogamous where the gametes are pyriform and bear two laterally attached flagella.
- The common forms are Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus.
- Fig. Brown algae
The common members are: Polysiphonia, Porphyra, Gracilaria and Gelidium.
- Rhodophyceae are commonly called red algae because of the predominance of the red pigment, r-phycoerythrin in their body.
- The red thalli of most of the red algae are multicellular.
- The food is stored as floridean starch.
- The red algae usually reproduce vegetatively by fragmentation, reproduce asexually by non-motile spores, and sexually by non-motile gametes.
Fig. Red algae