The algal cells are similar to those of higher plants. The characteristic colour of algae is due to specifIc pigments present in plastids. Algae reproduce by vegetative, asexual and sexual methods. Sexual reproduction varies from simple isogamy to advanced oogamy. Few groups of algae also exhibit a distinct and well-defIned isomorphic and heteromorphic alternation of generations. Algae have been variously classifIed by numerous phycologists, their views always differing. The most simple and practical classifIcation was proposed by British phycologist F.E. Fritsch in 1935.

Algae show a great range of thallus structure-as simple as a single cell of Chlamydomonas and as complicated as an internally differentiated kelp (brown algae).

Green

Green algae are the algae most closely related to plants. They have the same pigments (chlorophyll a and b and carotenoids), the same chemicals in their cell walls (cellulose), and the same storage product (starch) as plants. Photosynthetic reserve is starch that is stored in pyrenoid. Green algae may be unicellular or form filaments, nets, sheets, spheres, or complex mosslike structures. There are both freshwater and marine species. Some species of green algae live on snow, or in symbiotic associations as lichens, or with sponges or other aquatic animals. Edible green algae include Chlorella and sea lettuce. There are at least seventeen thousand species of green algae.

Chlamydomonas

Thallus is unicellular and motile.The cell is usually oval in shape. Sometimes spherical, oblong. The cell is surrounded by a cell wall. It is narrow at its anterior end and broad at the posterior end. Anterior end bears two closely situated flagella.
At the base of each flagellum one contractile vacuole is present. Just near the cell wall, towards the anterio-lateral part of the cell, lies an orange or red coloured spot, called stigma or eye spot. A single cup-shaped chloroplast. The broad portion of the chloroplast has a single pyrenoid.

The cavity is completely filled with the cytoplasm in which lies a single nucleus. Many volutin grains, the main reserve food product, are irregularly distributed in the cytoplasm.

Volvox

Thallus is multicellular, motile and a coenobial colony. Colonies are mostly spherical, rounded or oval in shape. Layer of cells is surrounded by a gelatinous mass which forms the outer and firm limiting layer.

The number of cells in a colony varies from 500-6,500 according to the species. Each cell of the colony is connected with a few of the neighbouring cells by thin and delicate cytoplasmic strands. Each cell is enveloped by an individual gelatinous sheath. All the cells of a colony are typically chlamydomonad in shape, size and structure. Each vegetative cell is biflagellate, motile and ovoid. The two flagella are anteriorly inserted.

A contractile vacuole is situated one each at the base of a flagellum. Cup-shaped chloroplast occupies much of the posterior part in which tuated a single pyrenoid. In the cavity formed by cup-shaped chloroplast lies a single nucleus, surrounded by cytoplasm. Cytoplasm
is rich in volutin grains. Eyespot or stigma occupies anterio-Iateral position.

Chlorella

Single cell represents the thallus. The cell is non-motile. The cell are found singly or sometimes in groups.
A cell is very small. It is spherical to ellipsoidal in shape. A cell has a single cup-shaPed or curved band of chloroplast, parietal in position. A single pyrenoid is generally found (lacking at times).Nucleus lies surrounded by the cytoplasm which fills up most of the cell. The only method of reproduction is by autospores. Two to sixteen autospores are formed by the cell as a result of division.

Spirogyra

Is very common in relatively clear eutrophic water, developing slimy filamentous green masses. In spring Spirogyra grows under water, but when there is enough sunlight and warmth they produce large amounts of oxygen, adhering as bubbles between the tangled filaments. The filamentous masses come to the surface and become visible as slimy green mats. Spirogyra has a cell wall, nucleus, pyrenoid, and spiral chloroplasts.

Spirogyra can reproduce both sexually and asexually. In vegetative reproduction, fragmentation takes place, and Spirogyra simply undergoes intercalary cell division to extend the length of the new filaments. The sexually pocess called conjugation.

Ulva

Individual blades of Ulva can grow to be more than 400 mm (16 in) in size, but this occurs only when the plants are growing in sheltered areas. A macroscopic alga which is light to dark green in colour, it is attached by disc holdfast. Their structure is a leaflike flattened thallus.

DIATOMS AGLES

Diatoms are often regarded as the most beautiful of the algae. Each diatom has a cell wall made of glass that is very finely etched with a species-specific pattern of dots and lines. The patterns on the diatom cell walls are so precise that they were used for years to test the optics of new microscopes. Diatoms are also the most abundant algae in the open ocean and responsible for about one-quarter of all the oxygen gas produced on the earth each year. Diatom populations often bloom in lakes in the spring, providing a major food for zooplankton, forming the base of the aquatic food chain. There are over one hundred thousand species of diatoms.

Organisms are unicellular.The thallus mostly occurs singly or cells maybe united in colonies. A diatom cell is known as a frustule. The wall is composed of two overlapping halves, contains silica (Si). The older part (valve) is known as epitheca which fits closely over the younger part (valve)-hypotheca.

Just inside and close to the cell wall is a cytoplasmic lining. Internal to this is situated a central vacuole. This vacuole is generally interrupted in some forms (e.g. pennate forms) by centrally located prominent band of cytoplasm, in which lies a nucleus. (In centrales this band is absent and nucleus remains near the valve).

Nucleus is very prominent and is round or oval in shape. Chromatophores vary in shape. These are parietal in position. In pennales, chromatophores are richly lobed and perforated and contain many naked pyrenoids. Reserve products are in the form of oils accumulated in large quantities.

BROWN ALGEA

Brown algae are almost exclusively marine and include the largest and most complex seaweeds. Kelp, for example, may be more than 60 meters (200 feet) tall, and forms dense underwater forests off the California coast. Other important brown algae include the rockweeds and Sargassum, for which the Sargasso Sea is named. There are about fifteen hundred species of brown algae. Brown algeas have a lamellar structure. They are attached to the substrate with the help of rhizoids. The reserve substance is laminarin. Contains iodine.

Fucus

Thallus bears many flat strap-like, dichotomously branched blades or wings with smooth or entire margins.
Some species of Fucus bear air bladders within the thallus. These regions of the thalli appear inflated.

Sargassum

Thallus is erect, thalloid and branched. It remains attached to the substratum by a discoid holdfast.
Many branches are flattened along the plane of branching into leaf-like structures called ‘leaves’. Leaves are narrow and their margins are mostly serrate. A few species also show a clear mid-rib.

Macrocystis

Some individuals are so huge that the thallus may grow to up to 60 m (200 ft). The stipes arise from a holdfast and branch three or four times from near the base. Blades develop at irregular intervals along the stipe. M. pyrifera grows to over 45 m (150 ft) long. The stipes are unbranched and each blade has a gas bladder at its base.

Porphyra

Is a genus of coldwater seaweeds that grow in cold, shallow seawater.

Porphyra contains vitamin B12 and one study suggests that it is the most suitable non-meat source of this essential vitamin.

Agar, or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ogonori .

Agar has been used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture media for microbiological work. Agar can be used as a laxative; an appetite suppressant; a vegan substitute for gelatin; a thickener for soups; in fruit preserves, ice cream, and other desserts; as a clarifying agent in brewing; and for sizing paper and fabrics.