The term Algae first coined by Linnnaeus in 1754. Algae is a collective term for all those cholorophyll bearing organisms which are thalloid (non-vascular). Pond scums, stonewarts, sea weeds and the like are collectively known as the algae. It is very difficult to define Algae. According to Fritsch (1935) alga must include all holophytic organisms that fail to reach the higher level of differentiation characteristics of higher plants. G. M. smith defines algae as simple plant with autotrophic mode of nutrition.
Thought the algae have been used for long, but their systemized knowledge started with the invention of microscope in the middle of 17th century.
Habit and Habitat:
The algae are predominantly aquatic and are found in fresh or salt waters. Fresh water forms occur abundantly in ponds, lakes, slow flowing streams and water reservoirs. In habit these may be free swimming, free floating or attached at the bottom in shallow water. Some are terrestrial and grow in wet situations, such as damp soil, damp shaded sides of trees and walls or even rocks. They may live as epiphytes as well as epizoics. Some algae are endophytic and some live in symbiotic relationship with the higher plants.
Most of the marine forms are sea weeds and inhabit vast area of the ocean. The algae which grow attached to the bottom in shallow water along the edges of seas and lakes form benthons.
According to the habitat algae may be classified as:
Aquatic algae
Terrestrial algae
Aerophytes
Cryophytes
Thermophytes
Algae of unusual habitats
Organization of Thallus
Algae exhibit a great diversity in the organisation of the plant body. The simplest forms are motile or non-motile unicells eg Chlorella and Chlamydomonas. In many species cells are grouped into aggregations called colonies eg Volvox and Pediastrum. These assume various forms may be a hollow sphere, flat plate or a filament.
Structure of Algal Cell
The cells constituting the algal thalli are basically of two kinds- Prokaryotic and Eukaryotic.
The prokaryotic cells constitute thalli of cyanophytes (blue-green algae) have a cell wall which contains a specific strengthening component not found in the cell walls of other algae. It is mucopepetide. The DNA material representing the nuclear body consists of fibrils, which may extend throughout the cell or are concentrated in the central part. The mitotic figures are also lacking. The chlorophyll pigment is bound to photosynthetic lamellae or thylakoids, which may be arranged in parallel, layers in the peripheri of the cytoplasm or form a network throughout the cytoplasm. The chloroplasts are absent and so are the mitochondria, golgi body and ER. The ribosomes are present but the nuclear division does not take place by mitosis and no plate formation occurs.
The cells constituting the thalli of all other algae excepting the blue-green algae are called eukaryotic. The eukaryotic cell has the same structure as is typical of the higher plants.
Algal Pigments
The color of the algal thallus which varies in different classes of algae is due to the presence of the presence of definite chemical compound in their cell. These are called pigments. The particular color that a thallus has is due to the predominance of one pigment in a combination of several others. The photosynthetic pigments in algae are of three kinds: Chlorophylls, Carotenoids, Phycobillins (Billiproteins).
Algal Flagella
The motile cells of algae are provided with fine, protoplasmic, whip-like threads, the Flagella. They are extremely fine and hyaline emergencies� of the cytoplasm. In cells possessing firm cell walls, the flagella are connected with the inner cytoplasm through small pores in the cell wall. There is either a single anterior flagellum or flagella occurs in pairs. The flagella on the cell wall may be equal (isokont) or unequal (heterokont). They function as locomotory or propelling structures of the cell. There is granule at the base of each flagellum known as Blepharoplast.
Nutrition
With respect to their nutrition the algae are autotrophic. All or most of the cells of the thallus contain chlorophyll. The green cells can manufacture their own carbohydrate food from carbon dioxide and water through the agency of sunlight. The aquatic forms obtain water and carbon dioxide by osmosis and diffusion from the medium in which they grow. The aerial forms obtain water from damp substratum and carbon dioxide from air.
Reproduction
The primitive algae reproduce only by vegetative method but in the higher forms both asexual and sexual reproduction are of common occurence.
The common method of vegetative reproduction are by simple cell division, fragmentation, Hormogone formation, primary and secondary protonema, tubers, bulbils, amylum starch or by formation of specialized adventitious braches or thalli called propagula.
Asexual reproduction takes place by different types of spores formed in favorable and unfavorable conditions by the division of protoplast. Zoospores are formed in favorable conditions whereas thick walled hypnospores are formed during adverse condition. Other asexual spores are aplanospores, autospores, endospores, neutral spores and monospores. Formation of daughter colonies in Volvox, palmella stage in Chlamydomonas, and gongrosina stage in Vaucheria are specialized method of asexual reproduction in unfavorable condition.
Higher forms reproduce sexually. It is a fusion of two specialized cells known as sex cells or gametes. Sexual reproduction may be isogamous in which two identical gametes fuse to form zygote or heterogamous in which the gametic union take place between two dissimilar gamates having different size and behaviour.
Classification
The primary classification of algae is based on certain morphological and physiological features:
a ) pigment constitution of the cell
b ) chemical nature of stored food material
d ) kind, number, point of insertion and relative length of flagella on the motile cell
e ) chemical composition of wall
f ) presence or absence of definitely organized nucleus in the cell
Corolus Linnaaeus (1754) included algae alongwith lichens in his 25th class Cryptogamia but he did not elaborate further on the classification of lagae.
Vaucher (1803) was first person to propose a system of classification of algae and recognised three groups- Conferves, Ulves, and Tremelles.
Around 1880, Algae along with group of fungi were grouped under thallophyta, a dicision created by Eichler (1836).
F. E. round (1973) recognised the importance of presence or absence of well organized nucleu s in algal cells.
Uses
1. The algae are useful source of commercial products. Four major products derived commercially from them are agar-agar, alginic acid carrageenin, diatomit.
Main source of agar agar in Japan are the thalli of Gellidium, Gracilaria, and Gigartina. Agar agar is of great value in the preparation of food stuffs, medicines, cosmetics, leather and textile industries, used as a laxative, culture medium, baked foods, meat industry and as emulsifier in dairy products.
The chief source of alginic acid are Ascophyllum, Laminaria, Lessonia, Eisenia, Macrocystis and Ecklonia. The alginates are used as thickeners in food industry, cosmetic, and in textile industry as printing pastes. They are of great use in production of plastic, artificial fibres.
The chief source of� carageenin is red alga Chondrus crispus. Its use as a component of tooth paste, deodorants, cosmetics and paints.
2. Algae play an important roe in agriculture. The blue green algae fix the atmospheric nitrogen into soil thus increasing the soil fertility.
3. The are important as a source of food of the fishes, aquatic amphibian, mammals etc. the colonies of Nostoc are boiled and used as food in Brazil. The young stripes of� Laminaria and sporophylls of Alaria are also eaten. Ulva lactusa used in salads and soups.
