GENERAL CHARACTERISTICS OF ALGAE

INTRODUCTION

Algae (singular, alga) are unicellular microorganisms that have chlorophyll and are photosynthetic. Algae are heterogeneous and range from microscopic unicellular forms to macroscopic seaweeds. They are different from green plants due to their simple reproductive structure for sexual reproduction. Many live in aquatic environments but many also thrive as subterranean algae. Algae are of great importance to biologist because single algal cells are complete organisms capable of photosynthesis and the synthesis of other compounds which constitute the cell. The study of algae is known as phycology. This unit examines the general characteristics, the distribution, the morphology and importance of algae.

General Characteristics of Algae

1. Algae are eukaryotic microorganisms. 
2. They are photosynthetic microorganisms. 
3.  Chlorophyll and other pigments are found in membrane bound organelles known as chloroplasts. Algae contain a discrete nucleus. Other inclusions are starch grains, oil droplets and vacuoles. They contain chlorophyll and utilise light energy to generate their chemical energy. 
4. They have a wide range of sizes and shapes. Many species occur as single cells that may be spherical, rod shaped, club-shaped or spindle-shaped. Others are multicellular and appear in every conceivable form, shape and degree of complexity. 
5. In most species the cell wall is thin and rigid cell walls of diatoms are impregnated with silica making them thick and very rigid. · The motile algae such as euglena have flexible cell membrane called periplasts. 
6.  They are also able to produce oxygen from water. 

 Occurrence and Distribution

Algae are found in many places on earth. They occur in great abundance in the ocean, seas, salt lakes, fresh water lakes ponds and streams. Many are found in damp soil, on rocks, stones and tree barks. Some are found on plants and animals. Small aquatic forms make up a large part of the free-floating microscopic life in water called plankton which is the principal food for aquatic animals including such large ones as whales. Plankton is generally considered to be composed of both algae and microscopic animal forms. Phytoplankton is made up of plants, i.e. algal forms and zooplankton is composed of animal organisms.


Algae are found where there are sufficient light, moisture and simple nutrients to sustain them.

Some species of algae grow on the snow and ice of Polar Regions and mountain peaks, sometimes occurring in such abundance that the landscape becomes coloured by the red pigments in their cells.
At the other extreme, some algae grow in the hot springs at temperatures as high as 550C. Some freshwater algae have adapted their metabolism to the high salt concentration found in the brine lakes of the arid South-Western United States.

Some algae are adapted to moist soil, the bark of trees and the surface of rocks, which the algae degrade. The decomposition products are made available for soil building and enrichment. Algae are often a problem in water supplies because they produce undesirable taste and odour.
Heavy algal growth may form blankets or mats which interfere with the use of some natural waters for recreational purposes. These algal mats may act as barriers to the penetration of oxygen into the water; they prevent photosynthesis by excluding light from deeper water and this may cause fish and other marine animal to suffocate.

On the other hand, when dispersed in natural waters, algae increase the oxygen concentration through photosynthesis. Heavy growth of some algae reduces hardness of water and removes slats which are the cause of blackishness. Some algae are endophytic; that is, they are not free-living but live in other organisms. Such algae are widespread in protozoa, molluscs, sponges and corals.

Morphology

Algae have a wide range of sizes and shapes. Many are unicellular and may be spherical, rod shaped, club shaped or spindle shaped. Others are multicellular and appear in every conceivable forms, shape and degree of complexity including membranous colonies, filaments grouped, singly or in clusters with individual strands which may be branched or unbranched tubes. Algal cells are eukaryotic. Most are thin and rigid cell walls; however, the cell walls of diatoms are impregnated with silica threads which make them thick and very rigid.

Algae have a discrete nucleus, starch grains on droplets and vacuoles: chlorophyll and other pigments are found in membrane-bound organelles known as chloroplasts. The chloroplast ultra-structure and type of pigment presents in algae are used for their classification, e.g. green algae, red algae, yellow green algae, the golden algae, etc.

A very common green alga is spirogyra; a filamentous alga found on the scum that cover ponds are slow moving water.

 Motility

The motile algae also called the swimming algae have flagella occurring singly or in clusters at the anterior or posterior ends of the cells. Some algae have no means of locomotion and are carried by tides, waves and currents. Some attach themselves to the substrate in the body of water where they live and are occasionally broken loose by currents which move them to new locations. In some forms, only the zoospores, the asexual reproductive cells are motile.

Fig. 1: Structure of Spirogyra 

Source: studentxpress.ie 

 Reproduction

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

Asexual Reproduction

Asexual reproduction processes in algae include:

1. purely vegetative binary fission. 
2. production of unicellular spores, many of which, especially in the aquatic forms have flagella and are motile, these are called zoospores. 

In terrestrial types of algae, non-motile spores or aplanospores are formed; however, some aplanospores can develop into zoospores.

 Sexual Reproduction

All forms of sexual reproduction are found among the algae. In this processes there is a fusion (conjugation) of sex cells called gametes to form a zygote. If the gametes are identical, i.e., there is no visible sex differentiation. The fusion process is called isogamous. However, if two gametes are different, the process is called heterogamous. In higher algae, the sex cells are differentiated into male and female.

The female egg cell (ovum) is large and non motile, while the male gametes (sperm cell) is small and are actively motile. This type of sexual reproduction is called oogamy.

Biological and Economic Importance of Algae

1. Algae as Primary Producers Algae form the base or beginning of most aquatic food chains because of their photosynthetic activities and are therefore called primary producers of organic matter. 
2. Commercial Product from Algae Many product of economic value are derived from algal cell walls. Three of these, agar, alginic acid and carrageenan, are extracted from the walls of algae. Another, diatomaceous earth, is composed of millions upon millions of diatom glass walls deposited over time on either fresh water or the ocean. All three compounds are used either make gels or to make solution viscous. Carrageenan has been used as a stabilizer or emulsifier in foods such as ice cream and other milk products. It is also used as a binder in toothpaste or in pharmaceutical products, as well as an agent in ulcer therapy. Carrageenan is also useful as a finishing compound in the textile and paper industries, as a thickening agent in sharing creams and lotions and in the soap industry. 

Agar is well known as solidifying agent in the preparation of microbiological media. It is obtained from red algae. Species of Gelidium and Gracelaria are used extensively. It is also important in the food industry when it is valuable in the manufacture of processed cheese, mayonnaise, pudding, jellies, baking products and canned goods.

In the pharmaceutical industry, agar can be used as a carrier for a drug. Lotions and ointments can contain some agar. About 50 per cent of the ice cream in the U.S. contains alginates which provide a smooth consistency and eliminate ice crystal formation. Alginate is also incorporated into cheeses and bakery products, especially frostings. Other industrial application includes paper manufacturing, the printing of fabrics and paint thickening. Diatomaceous earth is used primarily for filters or filter aids. It is especially suitable because it is not chemically reactive, is not readily compacted or compressed during use and is available in many grades.

 Algae as Food

Many species of algae (mostly red and brown algae) are used as food in the Far East. Of the red algae are of the most important in porphyria; it is used as a food in Japan where it is called “mori”
and is usually processed into dried sheets.