| Habitat |
Chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic (both freshwater and marine) organisms. They occur in a variety of other habitats: moist stones, soils, and wood. Some of them also occur in association with fungi (lichen) and animals (e.g., on sloth bear). |
Lichen, Marine algae (on sloth bear) |
| Form and Size |
Highly variable, ranging from colonial forms like Volvox and filamentous forms like Ulothrix and Spirogyra. A few marine forms, such as kelps, form massive plant bodies. |
Volvox (colonial), Ulothrix (filamentous), Spirogyra (filamentous), Kelps (massive) |
| Reproduction |
Vegetative: By fragmentation. Each fragment develops into a thallus.
Asexual: By production of different types of spores, the most common being zoospores. They are flagellated (motile) and on germination give rise to new plants.
Sexual: By fusion of two gametes:
- Isogamous: Gametes can be flagellated and similar in size (Ulothrix) or non-flagellated and similar in size (Spirogyra).
- Anisogamous: Fusion of two gametes dissimilar in size (Eudorina).
- Oogamous: Fusion between a large, non-motile female gamete and a smaller, motile male gamete (Volvox, Fucus).
|
Fragmentation, Zoospores, Ulothrix (isogamous), Spirogyra (isogamous), Eudorina (anisogamous), Volvox (oogamous), Fucus (oogamous) |
| Importance |
Algae are useful to man in a variety of ways. At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis. Being photosynthetic they increase the level of dissolved oxygen in their immediate environment. They are of paramount importance as primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals. Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food. Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), e.g., algin (brown algae) and carrageen (red algae) which are used commercially. 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, a unicellular alga rich in proteins, is used as food supplement even by space travellers.
|
Porphyra (food), Laminaria (food), Sargassum (food), Gelidium (agar), Gracilaria (agar), Chlorella (food supplement) |
| Classes |
Divided into three main classes: Chlorophyceae, Phaeophyceae, and Rhodophyceae. |
Chlorophyceae, Phaeophyceae, Rhodophyceae |
