They are largely aquatic, either free floating (plankton) or attached (benthos). They occur in other habitats like moist stones, soil, wood etc.
Some of them occur in association with fungi (e.g. lichen) & animals (e.g. on the fur of sloth bear).
On the basis of habitat, algae are of flowing types:
2. Thallus organization:
The algal plant body is a thallus i.e. not differentiated into root, stem & leaves. They vary in form & size. The size ranges from the microscopic unicellular forms, like Chlamydomonas, to multi-cellular forms like Volvox colony, Spirogyra filament. A few brown algae form massive plant bodies (40-60 mt) called kelps or seaweeds. In filamentous & sheet like forms, thallus has 3 parts: holdfast (root-like), stipe (stem-like) and lamina or blade (leaf-like).
The whole algae body is covered by mucilage which protects from desiccation & epiphytic growth.
4. Mechanical Tissue:
Algal lack mechanical tissue as buoyancy holds them erect & their flexibility resist the tides without being torn.
5. Vascular Tissue:
As they live in aquatic medium, no vascular tissues required for water conduction. In some large brown algae (Kelps) trumpet hyphae (food conducting tubes) similar to sieve, tubes of phloem found that carry food from lamina to the hold fast.
6. Algal plastids:
In algae, plastids are without grana & called as rhodoplasts in red algae, phaeoplasts in brown algae & chloroplasts in green algae.
7. Algal pigments:
In algae three classes of photosynthetic pigments found i.e. chlorophylls (5 types), carotenoids (6 types of Carotenes & 20 types of Xanthophylls) & phycobilins (3 types). Chlorophyll-a and β-carotene in all the algal groups.
8. Food reserves:
Food reserves vary from group to group, e.g., starch (green algae), laminarin (brown algae), Floridean starch (red algae), Cyanophycin (BGA), paramylon (in Euglenoids) etc.
9. Vegetative reproduction:
It occurs through fragmentation, fission (e.g. diatoms, desmids), tubers (e.g. Chara), hormogonia (e.g. Nostoc) etc.
10. Asexual reproduction:
It occurs by zoospores, aplanospores, hypnospores autospores, tetraspores, carpospores, exospores, endospores, akinetes etc.
11. Sexual Reproduction:
It is the most advanced form of reproduction which involves the fusion of two specialized haploid cells called gametes to form a diploid zygote or zygospore (2n). Meiosis takes place during germination of zygote and the developing new plant is a haploid gametophyte.
The haploid gametes are produced in a sac-like structure called gametangia. The flagellated gametes, as produced by most algae, are called planogametes & their fusion is called planogamy. Some algae (Spirogyra, diatoms etc.) produced non-flagellate gametes called aplanogametes & their fusion process in called aplanogamy or conjugation.
In algae, sexual reproductions occur during or end of the growing season. When the same algal thallus produces two types of fusing gametes then it is called monoecious or homothallic. But in dioecious or heterothallic species fusing gametes produced by separate thalli.
Based on the size and movement of gametes, following types of sexual reproduction is recognized in algae:
It involves the fusion of gametes which are morphologically similar but physiologically distinct. Such gametes are called isogametes and produced in vegetative cells called gametangia. Isogametes may be planogametes (e.g. Ulothrix, Chlamydomonas) or aplanogametes (e.g. Spirogyra).
It involves the fusion of anisogametes i.e. morphologically & physiologically distinct gametes. The smaller male gamete is more active than the larger female gamete, e.g., Chlamydomonas braunii, Aphanochaete, Phyllobium dimorphum, Enteromorpha intestinalis etc. Pandorina shows incipient anisogamy where fusion takes place between a small and a large gamete.
It is the highest evolved type of sexual reproduction. Oogamy involves the fusion between a smaller motile male gamete and a larger non-motile female gamete. The antheridum (male sex organ) produces motile male gametes called antherozoid or spermatozoid while oogonium (female sex organ) produces non motile female gamete called egg or oospheree.g. Oedogonium,Vaucheria, Chlamydomonas coccifera etc.
12. Life cycle:
Among the sexually reproducing algae, at least 5 main types of life cycle can be seen. These are— haplontic, diplontic, haplo-haplontic (diphasic haplontic), haplo-haplo-haplontic (triphasic haplontic) and diplodiplontic (triphasic diplontic).