Introduction:

Algae (singular, alga) are unicellular microorganisms which encompass chlorophyll and are photosynthetic. Algae are heterogeneous and range from microscopic unicellular forms to macroscopic seaweeds. They are dissimilar from green plants due to their simple reproductive structure for the sexual reproduction. Mostly live in the aquatic environments however many as well thrive as subterranean algae. Algae are of great significance to biologist as single algal cells are complete organisms capable of photosynthesis and the synthesis of other compounds that comprise the cell. The study of algae is termed as phycology.

General Characteristics of Algae:

1) Algae are mostly eukaryotic microorganisms. 

2) They are photosynthetic microorganisms. 

3) Chlorophyll and other pigments are found in the membrane bound organelles termed as chloroplasts. Algae include a discrete nucleus. Other inclusions are starch grains, oil droplets and vacuoles. They have chlorophyll and utilize light energy to produce their chemical energy.  

4) They encompass a broad range of sizes and shapes. Most of the species take place as single cells that might be spherical, rod shaped, club-shaped and spindle-shaped. Others are multicellular and appear in each and every conceivable form, shape and degree of complexity. 

5) In most of the species, the cell wall is thin and rigid cell walls of diatoms are impregnated having silica making them thick and much rigid. 

6) The motile algae like euglena have flexible cell membrane termed as periplasts.

7) They are as well capable to produce oxygen from water.

Occurrence and Distribution of Algae:

Algae are found in numerous places on earth. They take place in great abundance in the seas, ocean, salt lakes, fresh water lakes ponds and streams. Most of them are found in damp soil, on stones, rocks and tree barks.

A few of them are found on animals and plants. Small aquatic forms make up a big part of the free-floating microscopic life in water termed as plankton that is the principal food for aquatic animals comprising such large ones as whales. Plankton is usually considered to be composed of both algae and microscopic animal forms. Phytoplankton is made up of the plants, that is, algal forms and zooplankton is comprised of animal organisms.

Algae are found where there are enough light, moisture and simple nutrients to prolong them.

Several species of algae grow on the snow and ice of Polar areas and mountain peaks, at times taking place in such abundance that the landscape becomes colored by the red pigments in their cells.

At other extreme, a few algae grow in the hot springs at temperatures as high as 55^oC. Some freshwater algae have adapted their metabolism to the high salt concentration found in the brine lakes of the arid South-Western United States.

Several algae are adapted to moist soil, the bark of trees and the surface of rocks, which the algae degrade. The decomposition products are made accessible for soil building and enhancement. Algae are often a dilemma in water supplies as they generate undesirable taste and odor.

Heavy algal growth might form blankets or mats that interfere with the use of some natural waters for the recreational purposes. Such algal mats might act as barriers to the penetration of oxygen into the water; they prevent photosynthesis by prohibiting light from deeper water and this might cause fish and other marine animal to choke.

On the other hand, when scattered in natural waters, algae raise the oxygen concentration via photosynthesis. Heavy growth of some algae decreases hardness of water and eliminates slats which are the cause of blackishness. A few algae are endophytic; which is, they are not free-living however live in other organisms. These algae are widespread in molluscs, protozoa, sponges and corals.

Morphology:

Algae encompass a broad range of sizes and shapes. Most of them are unicellular and might be spherical, club shaped, rod shaped or spindle shaped. Others are multicellular and appear in each and every conceivable forms, shape and degree of complexity comprising membranous colonies, filaments grouped, singly or in clusters having individual strands which might be branched or unbranched tubes.

Algal cells are eukaryotic. Most of them are thin and rigid cell walls; though, the cell walls of diatoms are impregnated having silica threads which build them thick and very rigid.

Algae encompass a discrete nucleus, starch grains on droplets and vacuoles: chlorophyll and other pigments are found in the membrane-bound organelles termed as chloroplasts.

The chloroplast ultra-structure and kind of pigment presents in algae are employed for their categorization, example: green algae, red algae, yellow green algae, golden algae and so on.

Motility:

The motile algae as well termed as the swimming algae have flagella occurring singly or in clusters at the anterior or posterior ends of the cells. A few algae have no means of locomotion and are taken by tides, waves and currents. Some join themselves to the substrate in the body of water where they live and are occasionally broken loose through currents which move them to new locations. In several forms, only the zoospores, the asexual reproductive cells are motile.

Reproduction in Algae:

Algae might reproduce either asexually or sexually. Some of the species are limited to one of these processes. Though, they have complicated life cycles comprising both asexual and sexual means of reproduction.

Asexual Reproduction: 

Asexual reproduction procedures in algae comprise:

a) Purely vegetative binary fission.

b) Production of unicellular spores, most of which, particularly in the aquatic forms contain flagella and are motile, these are termed as zoospores.

In terrestrial kind of algae, non-motile spores or aplanospores are formed; though, some aplanospores can build up into zoospores.

Sexual Reproduction:

All kinds of sexual reproduction are found among the algae. In this method there is a fusion (that is, conjugation) of sex cells termed as gametes to form a zygote. When the gametes are similar, that is, there is no visible sex differentiation. The fusion process is termed as isogamous. Though, when two gametes are dissimilar, the process is termed as heterogamous. In higher algae, the sex cells are distinguished into female and male.

The female egg cell (that is, ovum) is large and non motile, whereas the male gametes (that is, sperm cell) is small and are actively motile. This kind of sexual reproduction is termed as oogamy.

Biological and Economic significance of Algae:

1) Algae as Primary Producers:

Algae form the base or starting of most aquatic food chains due to their photosynthetic activities and are thus termed as primary producers of organic matter.

2) Commercial Product from Algae:

Most of the products of economic value are derived from the algal cell walls. Three of these, agar, alginic acid and carrageenan, are extracted from the algae walls. The other, diatomaceous earth, is comprised of millions of millions of diatom glass walls deposited over time on either fresh-water or the ocean. All three compounds are employed either make gels or to make solution viscous. Carrageenan has been utilized as a stabilizer or emulsifier in foods like ice cream and other milk products. It is as well employed as a binder in tooth-paste or in pharmaceutical goods and also an agent in the ulcer therapy. Carrageenan is as well valuable as a finishing compound in the paper and textile industries, as a thickening agent in sharing creams and lotions and in the soap business.

Agar is famous as solidifying agent in the preparation of the microbiological media. It is achieved from red algae. Species of Gelidium and Gracelaria are employed widely. It is as well significant in the food industry if it is valuable in the manufacture of processed cheese, pudding, jellies, mayonnaise, baking products and canned goods.

In the pharmaceutical business, agar can be utilized as a carrier for a drug. Ointments and lotions can include some agar.

Alginate is as well integrated into cheeses and bakery products, particularly frostings. Other industrial application comprises paper manufacturing, printing of fabrics and paint thickening.

3) Algae as Food:

Most of the species of algae (generally red and brown algae) are employed as food in the Far East. Of the red algae are of the most significant in porphyria; it is utilized as a food in Japan where it is termed as 'mori' and is generally processed into dried sheets.

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