Algae

-Algae are chlorophyll-bearing, simple, autotrophic and largely aquatic organisms.

-They occur in a variety of habitats like moist damp places .Some of them also occur in a-ssociation with fungi (lichen) and animals.

- They occur as unicellular form like Chlamydomonas , colonial forms like Volvox and the filamentous forms like Ulothrix and Spirogyra.

 -The algae reproduce by vegetative, asexual and sexual methods. Vegetative reproduction is by fragmentation. Asexual reproduction is by spores, the most (zoospores). They are flagellated (motile) and on germination gives rise to new plants. Sexual reproduction takes place gametes and their fusion. (These gametes can be flagellated and similar in size, as in Chlamydomonas . such reproduction is called anisogamous) or non-flagellated  but similar in size , as in Spirogyra . Such reproduction is called isogamous(fusion of gametes similar in size and structure).(Fusion of two gametes dissimilar in size, mostly female gamete  is large and male gamete  is small called oogamy), is less common. e.g., Volvox, Fucus.

Algae are useful to us in many ways:
1. perform carbon dioxide fixation on earth is carried through
photosynthesis.

2. Being photosynthetic they increase the level of dissolved
oxygen in environment.

3. They act as primary producers of energy-rich compounds in  food cycles of all aquatic animals.

4. Many species of algae like  Porphyra, Laminaria and Sargassum ar used as a source of food.

5. Many forms of algae produces hydrocolloids (water holding substances), e.g., algin, carrageen 

6. Agar, produced from  Gelidium and Gracilaria are used in preparations of ice-creams and jellies and also used in salt bridge processes.

7. Carrageenin is used as a clearing agent in liquors.

8. Funori is a glue used as adhesive and in sizing textiles, papers.

9. Chlorella a unicellular alga, rich in proteins is used as food supplement even by
space travellers.

Divisions of Algae and their main characteristics:

Classes	Common Name	Major Pigments	Stored Food	Cell Wall	Flagellar Number and position of Insertions	Habitat
Chlorophycae	Green algae	Chlorophyll a,b	Starch	Cellulose	2-8 equal, apical	Fresh water, brackish water, salt water
Phaeophyceae	Brown algae	Chlorophyll a, c, fucoxanthin	Mannitol, laminarin	Cellulose and algin	2. unequal, lateral	Fresh water (rare) brackish water, salt water
Rhodophyceae	Red algae	Chlorophyll a, d, phycoerythrin	Florideam starch	Cellulose, pectin and poly sulphate esters	Absent	Fresh water (some), brackish water, salt water (most)

The algae are divided into three main classes:

1 Chlorophyceae

2 Phaeophyceae

3 Rhodophyceae

 

1) Chlorophyceae:Green algae

- The plant body may be unicellular, colonial or filamentous.

-They are green due to the presence of pigments chlorophyll a and b. Some algae may store food in the form of oil droplets.
-Cell wall is made up of cellulose and pectose.
-Vegetative reproduction usually takes place by fragmentation.

-Asexual reproduction is by flagellated zoospores.

-The sexual reproduction may be isogamous, anisogamous or oogamous.

Examples of green algae are: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara

Chlamydomonas

•  Found in  habitats rich in ammonia salts.  
• In Chlamydomonas, the cell wall is made up of glycoproteins rather than cellulose.

Spirogyra

• Spirogyra is an unbranched green alga commonly found on  the surface of fresh water ponds. It is popularly known as pond scum or water silk.
• The alga consists of a large number of green cylindrical cells placed end to end in a single row. A cell contains 1 – 16 spirally coiled ribbon shaped chloroplasts.

• It reproduces by three ways:

  (i) Vegetative method: It occurs through fragmentation. Each fragment having one or more cells grows into a complete filament.

  (ii) Asexual reproduction: It occurs by spores.

  (iii) Sexual reproduction: It takes place through conjugation.

Economic importance

Chlorella produces food rich in proteins, fats and vitamins. It also yields an antibiotic chlorellin. Chlorella can be used in prolonged space flights for food, oxygen, disposal of CO₂ and organic matter.

2) Rhodophyceae: Red Algae

• Vegetative reproduction takes place by fragmentation, gemmae, and regeneration.
• A sexual reproduction takes place by spores.
• A sexual reproduction is an advanced type of oogamy. The male sex organ is called spermatangium or antheridium. It produces non flagellate male gamete known as spermatium. The female sex organ is flask shaped and is termed carpogonium.

Economic Importance

Phycocolloids

A number of phycocolloids are extracted for commercial use. These include agar, carrageen, Funori etc

Food

• A number of red algae are edible, e.g., Porphyra (Laver).  Porphyra is cultivated in Japan for commercial use.

Medicines

• Corallina has vermifuge properties while Polysiphonia is antibacterial. Carrageenin fastens blood clotting.

3) Phaeophyceae: Brown Algae

• Vegetative reproduction occurs through fragmentation or, adventitious branches.
• Asexual reproduction occurs with the help of both motile (e.g., zoospores) and non motile spores.
• Sexual reproduction varies from isogamy to oogamy.
• There is no zygotic meiosis in brown algae.

Economic Importance

Food: A number of brown algae are used as food in some countries, e.g., Laminaria, Macrocystis and Sargassum. The edible brown algae are also used as fodder.

Iodine and Potash: Fucus and Laminaria are rich source of iodine. Potesh is used in making soaps,glass and  fertilizers.

Medicines: Sodium laminarin sulphate obtained from Laminaria is an effective blood anticoagulant.