Most of the part of the land is covered over either by fresh water or sea water. Besides, several other algae are found in somewhat drier conditions. They are found on the trunks of trees, on telephone wires, on rocks, on walls, in hot springs and in several other unusual habitats.
Here some of the algae have been classified according to their habitats. Special emphasis has been given on the occurrence of fresh water algae.
1. Hydrophytes:
They are more or less completely submerged or free floating on the surface of the water. The hydrophytes may be subdivided into following heads.
(i) Benthophytes:
Several fresh water and marine algae are found in attached condition. The fresh water such as Chara, Nitella, Cladophora, Gongrosira, Chaemosiphon, etc., are found attached to some substratum in the bottom of the water. Almost all of brown algae (Phaeophyceae) are found in attached condition to some substrata in the sea.
(ii) Epactiphytes:
Such algae grow along the shores of lakes and ponds, and may be delimited from benthophytes with some difficulty. The most important fresh water forms are – Oedogonium, Chaetophora, some species of Spirogyra, Mougeotia, some diatoms, Scytonema and Rivularia.
(iii) Thermophytes:
Many algae are reported from hot springs. These algae may tolerate the temperature upto 70°C or more than that. According to Copeland, 53 genera and 153 species of Chroococcaceae may survive upto 84°C. Some Oscillatoriaceae may survive upto 85°C. This supports that Myxophyceae (blue-green algae) are primitive.
(iv) Planktophytes:
The algae which float on the surface of the water are called ‘planktophytes’. They may be of two types, ‘i.e., (a) euplanktophytes (b) tychoplanktophytes.
(a) Euplanktophytes:
They are never attached, and from the very beginning are free floating, e.g., diatoms, Cosmarium, Closterium, Microcystis, Sphaeroplea, Scenedesmus, Pediastrum, Chlamydomonas, Volvox, other Volvocales and some members of Chroococcales. The above given forms are fresh water in habitat.
(b) Tychoplanktophytes:
In the beginning, such algae are attached, but later on they become detached and free floating, e.g., some species of Spirogyra, Zygnema, Cladophora, Oedogonium, Rhizoclonium, Mougeotia, Tribonema, Microspora, Cylindrospermum, Tetraspora, Rivularia, Nostoc, Gloeotrichia, Sargassum, etc.
(v) Halophytes:
The algae occur in saline waters are known as ‘halophytes’. The most striking examples are Dunaliella and Chlamydomonas which occur in salt lakes, the species of Scenedesmus, Aphanocapsa, Pediastrum, Aphanothece, and Oscillatoria are found in saline waters; the species of Enteromorpha are found in inland astuaries; many species of Ulvales, Ulotrichales, Conjugales, and Myxophyceae are found near the sea in astuaries.
(vi) Epiphytes:
Many algae are found upon other living plants and bigger species of algae. Aphanochaete, Bulbochaete, Oedogonium and Microspora, are found as epiphytes upon larger species of Oedogonium, Cladophora, Rhizoclonium, Vaucheria and Hydrodictyon species. Co- leochaete nitellarum is epiphytic upon species of Chara and Nitella.
Some of the species of Coleochaete are epiphytic upon some grasses grown on the banks of the ponds and the hydrophytes such as—Vallisneria, Typha, Ipomoea and several other aquatic plants. Chaetonema is found epiphytic on the mucilaginous masses of Tetraspora and Batrachospermum.
(vii) Epizoophytes:
Certain algae are found on living aquatic animals such as turtles, mollusc shells, fishes, etc. Species of Cladophora grow upon mollusc shells. Protoderma and Basicladia occur on the back of turtles. Characiopsis and Characium occur on the posterior and anterior legs of Branchipus respectively.
2. Edaphophytes:
Such algae are also called terrestrial algae. They are found upon or inside the surface of the earth. They can be (i) saphophytes and (ii) cryptophytes.
(i) Saphophytes:
They are surface algae. Most of the species of Myxophyceae are found upon the surface of the soil. Besides, Mesotaenium, Botrydium, Protosiphon, Oedocladium, Vaucheria, Fritschiella and many others are met with upon the surface of the wet soil.
(ii) Cryptophytes:
Such algae are subterranean in habit and occur inside the soil. The species of Myxophyceae are found in the soil. The species of Nostoc, Anabaena and Euglena have been reported from the paddy fields, where they also fix the atmospheric nitrogen in the soil to enrich the fertility of the fields.
3. Aerophytes:
Such algae are aerial in habitat. They are found upon the trunks of trees, walls, fencing wires, rocks, and animals and so many other aerial substrata.
(i) Epiphyllophytes:
Such algae are epiphytic upon leaves of trees. Species of Trentepohlia are commonly found upon the bark of trees. They also occur upon rocks and fencing wires. They are abundantly found on the fencing wires of Calcutta botanical gardens. Phycopeltis occurs upon Rubus; Phyllosiphon on Arisaema; Rhodochytrium on Asclepias and Solidago.
(ii) Epiphloephytes:
These algae grow on the bark of trees mixed with many mosses and liverworts. Phormidium, Scytonema, Haplosiphon and Schizothrix grow on the bark of trees mixed with liverworts.
(iii) Epizoophytes:
These algae are found even on the bodies of land animals. Certain Chaetophorales are found even on the hairs of sloth.
(iv) Lithophytes:
Many algae grow on the rocks and walls. The species of Scytonema grow on the walls in rainy season and the whole wall becomes black spotted. Vaucheria, Nostoc and many other algae are also found on wet rocks.
4. Cryophytes:
These algae are found on ice and snow. These algal forms cause red snow, green snow, yellow snow, yellowish green snow and violet snow. In European countries, especially in arctic region the green snow is caused by Chlamydomonas, Ankistrodesmus and Mesotaenium; red snow is caused by species of Chlamydomonas. Scotiella, Gloecapsa and diatoms.
5. Symbionts or endophytes:
Many algae grow in symbiotic association of other plants. The most striking example of symbiosis are lichens, here the algae are found in symbiotic association of fungi. Various Myxophyceae, e.g., Chroococcus, Nostoc, Microcystis, Gloeocapsa, Scytonema, Rivularia, etc., have been separated from lichens. Some green algae, e.g., Coccomyxa, Chlorella, Protococcus, Palmella, etc.,
are also found as symbionts in lichens.
Besides, several algae are endophytes in the tissue of other plants. Anabaena azollae is found inside the leaves of Azolla (a pteridophyte). Anabaena cycadae is found in the coralloid roots of Cycas. Nostoc has been reported from the tissues of Anthoceros and Notothylas. Nostoc is found in the leaves of Sphagnum (Bryophyta) and several angiosperms. Chlorochytrium is endophytic inside Lemna, Ceratophyllum and certain mosses.
6. Endozoophytes:
Certain algae occur inside the body of animals. Zooxanthella is found inside fresh water sponges; Zoochlorella is found inside Hydra viridis. According to Langeron about 14 species of Oscillatoriaceae are found in the digestive and respiratory tracts of various vertebrates.
7. Parasites:
Certain algae are parasites upon other plants. The most striking example is Cephaleuros virescens which causes the havoc of tea foliage in Assam and neighbouring areas, called ‘red rust of tea’.
8. Fluviatile algae:
Such algae are found in rapidly flowing waters; Ulothrix occurs in mountain falls. Stigeoclonium, Batrachospermum are reported from the swift running streams of Dehradun and other hilly tracts.
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