1. ALGAE

2. They are predominantly aquatic, occur both in marine as well as fresh water habitats. However some are terrestrial and grow in moist places. Some grow under very special environmental conditions, like hot water springs, ice and snow, surface of other plants and animals and in symbiotic association (lichen).

7. They are chlorophyll containing simple thalloid, plant body not differentiated into root, stem and leaf, autotrophic organisms. They are of variable size and forms. The size ranges from microscopic unicellular forms like Chlamydomonas to colonial forms like Volvox.

8. Chlamydomonas is a genus of green algae consisting of about 325 species all unicellular flagellates, found in stagnant water and on damp soil, in freshwater, seawater, and even in snow as "snow algae". Chlamydomonas is used as a model organism for molecular biology, especially studies of flagellar motility and chloroplast dynamics, biogeneses, and genetics. One of the many striking features of Chlamydomonas is that it contains ion channels (channelrhodopsins )that are directly activated by light.

9. Volvox is a polyphyletic genus of chlorophyte green algae in the family Volvocaceae. It forms spherical colonies of up to 50,000 cells. They live in a variety of freshwater habitats, and were first reported by Antonie van Leeuwenhoek in Volvox diverged from unicellular ancestors approximately 200 million years ago.
Each mature Volvox colony is composed of numerous flagellate cells. Vascular tissues for water conduction are absent as the entire thalli remain in contact with water.

12. The algal thalli are generally coated by a layer of mucilage which protects them from dessication when exposed to air. They are photoautotrophic in their mode of nutrition and perform photosynthesis due to the presence of chlorophyll. The photosynthetic pigments present in algae are chlorophyll , carotenoids and xanthophylls. Additional accessory pigments occur in specific groups. Red, brown and green algae are distinguished from one another by the nature of their photosynthetic and accessory pigments.

13. classes of algae
Mainly on the basis of their pigments, flagellation, storage products and chemistry of cell wall there are three classes of algae, Chlorophyceae or green algae, Phaeophyceae or brown algae and Rhodophyceae or red algae.

15. Algae reproduce by vegetative, asexual and sexual methods.
Vegetatively they reproduce by fragmentation, in this case the parent body breaks into two or more fragments, each fragment develops into a thallus.

16. Asexual reproduction
they divide by means of motile or non-motile spores. In this case different kinds of spores are produced, which are released from the parent body and on germination gives rise to new plants. The most common spores produced are the zoospores, flagellated motile spores that give rise to new plants on germination.

18. Sexual reproduction
takes place through the fusion of two gametes. Sex organs are non-jacketed and unicellular Sexual reproduction can be :
1.Isogamous- fusion of two gametes which are similar in size, either flagellated or non-flagellated.e.g. Chlamydomonas( gametes are flagellated and similar in size), Spirogyra( gametes are non-flagellated and similar in size).

19. 2.Anisogamous-Fusion of two gametes which are dissimilar in size, some species of Chlamydomonas
3.Oogamous- When a small, motile male gamete fuses with a large non-motile female gamete.e.g. Volvox, Fucus.

23. Economic importance
1.Photosynthesis-At least 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 (as during photosynthesis oxygen is released).It is essential for respiration of aquatic life.
2.Primary are primary producers of energy rich compounds (food).This forms the basis of food cycles of all aquatic plants.

24. Economic importance
3.Food supplements-Many species of Porphyra(Red algae),Laminaria and Sargassum(Brown algae) are among the 70 species of marine algae used as food. Certain unicellular algae like Chlorella and Spirullina are rich in proteins and used as food supplements for space travellers.

27. Economic importance
4.Hydrocolloids-They are water- holding substances. Certain marine brown and red algae produce large amounts of hydrocolloids which are used commercially. Algin and carrageen are hydrocolloids obtained from brown and red algae respectively. Agar, a hydrocolloid is one of the commercial products obtained from Gelidium and Gracilaria( Red algae) . They are used to grow microbes and also in the preparation of ice-cream and jellies.
5.Iodine-Fucus and Laminaria are rich sources of iodine.

28. Susummarymmary

30. Major Harmful effects
1.Parasitic algae- Green algae Cephaleuros is a parasite causing red rust disease in tea and coffee plants.
2.Biofoulers- Some brown algae act as biofoulers, Larger sea weed (Sargassum) float in masses, get entangled to the submerged parts of the ships and slow down their speed.