Algae: Definition, Occurrence and Affinities

In this article we will discuss about:- 1. Definition of Algae 2. Occurrence of Algae 3. Flagella 4. Affinities.

Definition of Algae:

We see some bright green or dark green patches growing on the water surface in ponds, tanks, ditches, lakes etc. People call it pond scum. In rainy season play grounds become slippery due to such greening patches growing over it. In the sea we see large brown or red coloured sea weeds floating over it. All these are the bodies of organisms called algae.

The term algae was proposed by Linnaeus in 1753 but he had used this term for those organisms which we call Bryophytes now-a-days.

A. L. de Jussieu (1789) was first to use it for those organisms which we now believe to be algae. Fritsch (1935) defined algae as “the designation algae must include all holophytic organisms, as well as their numerous colourless derivatives, that fail to reach the level of differentiation characteristics of archegoniate plants”.

In simple words we can define algae as the thallose, autotrophic, non-vascular plants having unicelled sex organs and no embryo formation. The branch of botany dealing with the study of algae is called phycology or algology. It is derived from the Greek word phykos which means ‘alga’ or ‘sea weed’.

Occurrence of Algae:

Algae are commonly presumed to be occurring in water and moist places but algae are found in a variety of habitats.

The common places of occurrence of algae are as follows:

(i) Aquatic Algae:

Aquatic algae can be:

(a) Fresh water forms 

(b) Marine forms.

(a) Fresh water forms:

Fresh water forms are found in water of low salinity such as in ponds, lakes, rivers, ditches etc. Cladophora, Vaucheria, Chara and some algae found in slow running water while Spirogyra, Chlamydomonas, Hydrodictyon and Volvox are found in stagnant water.

(b) Marine forms:

The algae found in sea water are called marine algae. Such algae grow in water of high salinity. Marine algae can be macroscopic and very large in size e.g., Macrocystis (70 meters) and Nereocystis (100 meters).

Some other examples of marine algae are:

Viva, Enteromorpha, Sargassum, Fucus, Polysiplionia, Gelidium and Gracilaria etc.

(ii) Terrestrial Algae:

Algae growing on moist soil surface, stones and rocks are terrestrial algae. The algae growing on surface of soil are called saprophytes and the algae growing under the surface of soil are called cryptophytes. Some terrestrial algae grow on moist walls and barks of trees. These algae absorb CO₂ and water from atmosphere.

Some common terrestrial algae are:

Fritschiella, Vaucheria, Chlorella, and Oscillatoria.

(iii) Lithophytic Algae:

Algae growing on surface of rocks and stones are lithophytic e.g., Nostoc, Gloeocapsa.

(iv) Halophytic Algae:

Algae growing in water of high concentration of salts as in salt lakes are halophytic algae e.g., Chlamydomonas ehrenbergii and Dunaliella.

(v) Thermophytic Algae:

The thermophytic algae grow in water of high temperature where other plant forms cannot grow. Some blue green algae are capable of growing at very high temperature because of unorganized nucleus. The thermal algae found in hot water springs are Oscillatoria terebriformis, Heterohormogonium, Synechococcus, Scytonema etc.

(vi) Cryophytic Algae:

Algae occurring in snow and ice are cryophytic algae. These algae impart special colours to snow due to their pigments. Red snow is caused by Haematococcus nivalis and Chlamydomonas nivalis. Green snow is caused by Chlamydomonas yellowstonensis. Purple brown snow is caused by Ancyclonema nordensklioldii. Black snow is caused by Raphidonema.

(vii) Epiphytic Algae:

Algae growing on other algae and plants are called epiphytic algae e.g., Polysiphonia, Oedogonium are found growing on other algae, bryophytes and aquatic angiosperms.

(viii) Epizoic Algae:

Algae growing on other animals are called epizoic algae e.g., Cladophora crisposa grows on snails, Stigeoclonium grows on gills of fishes.

(ix) Endophytic Algae:

Algae growing inside other plants are called endophytic algae e.g., Nostoc is found in thallus of Anthoceros, Anabaena cycadearum is found in coralloid root of Cycas, Anabaena azollae is found in Azolla.

(x) Endozoic Algae:

Algae found inside the body of animals are endozoic algae e.g., Zoo chlorella is found in Hydra and sponges. Some blue green algae are found in respiratory and digestive tracts of animals.

(xi) Parasitic Algae:

Some algae can be found as parasites on plants and animals e.g., Cephaleuros is found on leaves of tea, coffee and mango plants and causes red rust. Polysiphonia fastigiate is semi-parasitic on algae Ascophyllum.

(xii) Symbiotic Algae:

Some algae of Chlorophyceae and Cyanophyceae are found in symbiotic association with other plants. Nostoc and Anabaena make symbiotic association with Anthoceros and coralloid roots of Cycas. Lichens are symbiotic association of algae and fungi.

(xiii) Planktons:

Algae growing on surface of water and found as free floating on surface of water are called planktons. Planktonic algae are mainly members of Chlorophyceae, Cyanophyceae and Bacillariophyceae. When planktonic algae grow fast and increase enormously in number, these algae form water blooms.

Flagella in Algae:

The motile members of algae, zoospores and gametes have one or more flagella which are organs of motion. Flagella are absent in members of Cyanophyceae and Rhodophyceae. Flagella are uniform or thread-like protoplasmic appendages.

All flagella are uniform in their internal structure. Each flagellum is made of two central tubules surrounded by nine peripheral tubules. The structure of 9 + 2 is surrounded by a membrane. In different algal groups flagella differ in number, size, location and types.

The flagella can be 2, 4 or indefinite in number. All flagella of one algae can be equal in size i.e., isokontic (Fig. 1 A, B) or unequal in size i.e., heterokontic. The flagella can be apical, sub-apical and lateral in position (Fig. 1 E).

The flagella can be of following types in algae:

(a) Whiplash or acronematic type:

These flagella do not have hair like appendages and their surface is smooth (Fig. 1 A, B, D, Fig. 2A, B).

(b) Tinsel or pleuronematic type:

These flagella have hair like appendages on their surface. These appendages are called mastigonemes or flimmers.

These can be of following types on the basis of arrangement of mastigonemes.

(i) Pantonematic:

The mastigonemes are arranged in two opposite rows or radially (Fig. 1C, Fig. 1 A, B).

(ii) Pantacronematic:

It is a pantonematic flagellum with terminal fibril (Fig. 1C).

(iii) Stichonematic:

Mastigonemes are present on one side of flagellum (Fig. ID).

Affinities of Algae:

Algae and Fungi:

(a) Similarities:

(i) In both algae and fungi, the plant body is a thalloid i.e., the plant body is not differentiated into root, stem and leaves.

(ii) Algae and fungi both reproduce mainly by formation of spores.

(iii) The sex organs are not bounded by a jacket layer.

(b) Dissimilarities:

(i) Algae mostly grow in sun light while fungi grow in damp shady places.

(ii) Algae contain chlorophyll in the cells hence algae are autotrophic. They manufacture their own food. Fungi do not contain pigments like chlorophyll. Hence, fungi are heterotrophic; they do not manufacture their own food. Fungi are either saprophytic or parasitic.

(iii) Algal cell wall mainly consists of cellulose and fungal cell wall is made of chitin and fungal cellulose.

(iv) In algal cells the reserve food material is mainly starch. In fungal cells the reserve food material is mainly glycogen and oil.

Algae and Bryophytes:

(a) Similarities:

(i) In algae and bryophytes both the plant body is a thalloid i.e., not differentiated into root, stems and leaves.

(ii) The cells in algae and bryophytes both contain pigment chlorophyll; they can manufacture their own food. Plants are autotrophic.

(iii) Both algae and bryophytes reproduce with the help of spores.

(iv) In both cases the cell wall is made up of cellulose.

(v) The reserve food material is starch.

(b) Dissimilarities:

(i) Algae are mostly aquatic (fresh water or marine). Bryophytes are mainly terrestrial. However, only few bryophytes are aquatic.

(ii) In algae plant body does not have pores or stomata. In Bryophytes, pores or stomata are present on body.

(iii) In algae scales are absent on plant body, while bryophytes may have scales on surface.

(iv) In algae the reproduction is vegetative, asexual and sexual. In bryophytes the reproduction is vegetative and sexual only.

(v) In algae the sex organs are not surrounded by jacket layer, while in bryophytes the sex organs are surrounded by jacket layer.