Algae comprise a group of chlorophyll containing thalloid plants of the simplest type, having no true roots, stems, leaves like organs.
The important distinguishing features of algae are as fallows:
- they are mostly chlorophyll containing autotrophic thalloid plants.
- they occur in a variety of habitats, but majority of them are aquatic.
- the plant body does not show differentiation into various tissue systems.
- they have mostly unicellular sex organs without a jacket of sterile cells around them. jacket cells, if present, have different initials. there is a progressive complexity in reproduction.
- embryo is not formed after gametic union.
- they show distinct alternation of generation.
Algae are of universal occurrence and they are found in variety of habitats, such as freshwater, sea water, on snow, on rocks and on/or within the plant and animal bodies. of these, aquatic forms are most common. on the basis of habitats, they may be classified into the fallowing three groups
- Aquatic algae
- Terrestrial algae and
- algae of unusual habitats
Aquatic forms are found in fresh water or in saline water of the sea.
a) Fresh water forms:
these forms occur in fresh water of ponds, pools, lakes, rivers, etc. some fresh water forms like cladophora, odeogonium, ulothrix and chara are found in slow running water where as others like chlamydomonas, volvox etc found in stagnant water.
2. Marine forms
these forms occur in saline water of the sea and are represented by the members of pheophyceae and rhodophyceae. the aquatic algae are either free floating attached to a substratum with the help of an attachment disc, known as holdfast. many free floating algae, together with other similar organisms, form colonies on the surface of water which are called water blooms or phytoplanktons.
during night plankton utilize oxygen dissolved in water and as such submerged aquatic organisms do not get sufficient oxygen for respiration. besides, phytoplanktons also prevent atmospheric oxygen from dissolving in water, thus making the habitat unsuitable for submerged aquatic life.
me frany algae are found attached to rocks along the edges of lakes and seas and these forms are called phytobenthos.
3. Terrestrial algae
some algae are found in terrestrial habitats like soils, rocks, logs, etc. forms like vaucheria, botrydtium, etc which are found on the surface of the soil, are known as saphophytes, where as many blue green algae which occur under the surface of soil, are called cryptophytes.
some algae are also found on tree trunks and moist walls, and they absorb carbon dioxide and water from the atmosphere.
Algae of unusual habitats:
In addition to aquatic and terrestrial habitats, some algae also occur in uncommon habitats.
- Halophytic algae: There are many algae which occur in saline water of seas, but some others can withstand high concentration of salts and occur in salt lakes. They are called halophytes, and include chlamydomonas grow on larger algae or on bryophytes and angiosperems. for example, species of bulbochaete, oedogonium and microspora are found attached to the larger species of cladophora, rhizoclonium and vaucheria. bryophytic and angiospermic flora of rivers and lakes harbour many algal members on their surface. Algae with mucilagenous thalli like chaetophora, oedogonium and zygnema are epiphytic achnanthes, eunotia, synedra, etc.
- Epizoic algae: Many algae grow on animals like snails, fishes and tortoise, and they are known as epizoic. for example, cladophora crispata grows on snails, and species of stigeoclonium are found in the gills of fishes. similarly, species of characiopsis and characium are found on the legs of branchipus.
- Endozoic algae: Algae which occur in the tissues of animals are known as endozoic. for example, species of zoochlorella are found in hydra virdis. beside, several species of the family oscillatoriaceae occur in the respiratory and digestive tracts of vertebrate animals.
- Symbiotic algae: several members of chlorophyceae and cyanophyceae form symbiotic association with fungi, bryphytes, gymnosperms and angiosperms. lichens are important example of symbiotic life and their algal components belong to cyanophyceae or chlorophyceae. colonies of nostoc and anabaena are found in symbiotic association in the thallus of anthoceros and the coralloid roots of cycas.
- Cryophytic algae: Algae growing on ice or snow are called cryophytic. these algae provide attractive colours to snow covered mountains. the alpine and arctic mountains become red due to the growth of haemotococcus nivalis. green snow in europe is due to chlamydomonas yellowstonensis and species of mesotaenium. sometimes snow becomes black to scotiella nivalis. and raphidonema nivalis and raphidonema brevirostri and brownish purple due to ancyclonema nordenskioldii.
- Lithophytic algae: Algae growing on moist rocks and stones are known as lithophytic. many blue green algae like nostoc, rivularia and gloeocapsa commonly grow on moist and shady rocks. scytonema is common on moist walls during rainy season.
- Epiphytic algae: They grow on larger algae or on bryophytes and angiosperms. for example, species of bulbochaete, oedogonium and microspora are found attached to the larger species of cladophora, rhizoclonium and vaucheria. bryophytic and angiospermic flora of rivers and lakes harbour many algal members on their surface. algae with mucilagenous thalli like chaetophora, oedogonium and zygnema are epiphytic on achnanthes, eunotia, synedra, etc.
- Parasitic algae: Algae also grow as parasites on many plants and animals. cephaleuros virescens causes red rust in tea and coffee plantations. phyllosiphon is a parasite on the leaves of arisarum vulgare. polysiphonia fastigiata is a semi parasite on ascophyllum nodosum.
- Thermophytes: Many blue green algae are found in hot water springs. they are able to survive such high temperatures due to the absence of well organised nucleus. since most hot springs are alkaline, some algalogists believe that tolerance to high temperature is also associated with this factor.