DIVISION - ALGAE

Algae term was given by Linnaeus. The study of algae is called Phycology and the father of phycology is Fristch. He wrote a book Structure and reproduction of algae. The father of Indian phycology is M.O.P. Iyengar.
Algae are multicellular eukaryotic organisms placed in the kingdom Plantae. They are autotrophic i.e. they manufacture their food by photosynthesis. They are found in both fresh and marine water and in many forms like filamentous, colonial. Algae are surrounded by mucilagenous sheath and below the sheath cell wall is present which is made up of cellulose and pectin but mainly made up of cellulose, galactans, mannans and mineral-like calcium carbonate. On the basis of structure, they are thalloid i.e. plant body is not differentiated into root, stem and leaves. The tissue system is also absent in algae. On the basis of nutrition, they are photoautotrophic.They have chloroplast in which photosynthetic pigments are present. classification of algae is mainly based on pigments. Chl-a and 𝛃 carotene are universal pigments of algae. Three types of reproduction found in algae- Vegetative, asexual and sexual reproduction. Vegetative reproduction only by fragmentation and in asexual reproduction zoospores are formed in favourable conditions and Aplanospores, hypnospore and akinete etc. are formed in unfavourable condition. In algae male sex organ is called antheridium and female is called oogonium. The sex organs of algae are unicellular and jacketless. But exceptionally sex organs of green algae Chara are multicellular and jacketed. The male sex organ of Chara is known as globule and female is known as nucule. The plant body of algae is haploid so sexual reproduction takes place through zygotic meiosis. So their life cycle is haplontic. But exceptionally brown algae are diploid. The classification of algae is mainly based on the photosynthetic pigments. In addition to this, cell wall composition and stored food are also the base of classification.

Algae is divided into the following classes :

(1) CHLOROPHYCEAE - Green algae

Green algae are the most advanced algae. It is believed that green algae are the ancestors of the higher plants. Green algae usually have a rigid cell wall made up of an inner layer of cellulose and an outer layer of pectose. Green algae are cosmopolitan in nature. They are found in many forms i.e. unicellular like Chlamydomonas (motile unicellular algae), Chlorella, Acetabularia (largest unicellular plant) and colonial like volvox (motile colony) and multicellular filamentous like Ulothrix and Spirogyra and multicellular thalloid or parenchymatous like Ulva (sea lettuce). Photosynthetic pigments of green algae are chl 'a', chl 'b', 𝛃 carotene and Xanthophyll (Luteaxanthin and violoxanthin). On the basis of pigments, stored food (starch) and cell wall (made up of cellulose and pectose), green algae are considered similar to higher plants. Most of the members of green algae have starch as stored food and some have oil droplets also. One or more pyrenoids are also present in the chloroplast as storage bodies. Pyrenoids contain protein besides starch. Green algae are used as a food because after Spirulina (BGA), Chlorella (green algae) has largest amount of protein. Chlorellin antibiotic is obtained from chlorella. Chlorella is also used as a source of food and oxygen by space travellers. 

(2) PHAEOPHYCEAE - Brown algae

Brown algae also known as Sea weeds. They are found in marine water. They are multicellular filamentous and are the largest in size (upto 100 meter in length). Largest brown algae is Macrocystis. Their plant body is usually attached to substratum by a holdfast and has a stalk (stipe) and leaf like photosynthetic part, frond or lamina, so brown algae known as leafy algae. (e.g. Laminaria). They have Gelatinous coating, Phycocolloids (Algin) protects brown algae against dessication and shocks. Phycocolloids are used in ice cream as thickening agent. Algin used for dentury measurement. Algin is also used in the manufacturing of soap, ice-cream, polish, cream and plastic. Photosynthetic pigments of brown algae are chl 'a', chl 'c', 𝛃 carotene and Fucoxanthin. The amount of Fucoxanthin is more in brown algae due to which these algae are brown in colour. They have stored food Laminarin and mannitol which are derivatives of carbohydrates. e.g. of brown algae - Fucus, Dictyota, Ectocarpus, Sargassum and Laminaria.

(3) RHODOPHYCEAE - Red algae

Red algae are ancient algae. There is no motile stage found in life cycle of red algae and BGA i.e. cilia and flagella are absent. They are mainly found in marine water with greaer concentration found in the warmer areas. But exceptionally Batrachospermum is found in fresh water (river). They are multicellular but exceptionally Porphyridium is unicellular. Their cell wall is complex because made up of cellulose and pectin with polysulphate esters. Some red algae may secrete and deposit calcium carbonate and appear like corals. Their photosynthetic pigments are chl 'a', chl 'd', 𝛃 carotene and Phycobilins (R-phycoerythrin and R-phycocyanin). On the basis of pigments red algae are similar to blue green algae. Colour of red algae changes according to depth in sea this is called as Gaudikov's effect. When red algae are present on the surface of the sea then their colour is blue and when they are at the bottom, their colour is red. At the surface of sea the amount of R-phycocyanin is more while in depth the amount of R-phycoerythrin is more. Penetration power is maximum of violet light. R-phycoerythrin is only pigment to absorb violet light. Due to phycoerythrin red algae are deepest algae. Stored food of red algae are floridean starch which is structurally similar to glycogen and amylopectin. In red algae vegetative reproduction by fragmentation and asexual reproduction by non motile spores. Sexual reproduction in red algae is oogamous and accompanied by complex post fertilization developments. Their female sex organs are called carpogonia and male sex organ is spermatangia. Non motile spore like gametes are formed in spermatangia which are known as spermatia. Agar-Agar phycocolloid is obtained from red algae Gelidium and Gracilaria which is used to prepare culture medium to grow microbes and in preparation of ice creams and jellies. Carrageenin colloid is obtained from red algae Chondrus crispus which is used as gelating agent in food industries (i.e. to make the food item viscous). Capsule medicine is also prepared from carrageenin. Examples of red algae - Harveyella, Porphyra, Gelidium, Gracilaria, Chondrus crispus, Polysiphonia and Batrachospermum.