Algae represent a large group of different organisms from different phylogenetic groups, representing many taxonomic divisions. In general, algae can be referred to as plant-like organisms that are usually photosynthetic (make their own food due to the presence of chlorophyll) and aquatic and moist places on land; but they do not possess true roots, stems, leaves, vascular tissue and have simple reproductive structures.

         Most of the algae are microscopic, but some are quite large, e.g. some marine seaweeds that can exceed 50m in length. Recently they are classified in the kingdom of protiste, which comprise a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms that have cells with a membrane-bound nucleus. Cyanobacteria are organisms traditionally included among the algae, but they have a prokaryotic cell structure typical of bacteria and conduct photosynthesis directly within the cytoplasm, rather than in specialized organelles.

Types of algae

The main phylogenetic groups of algae are:

• Diatoms:They are single, sometime they do exists in colonies. They are usually yellowish or brownish in colour and are found in fresh- and saltwater, in moist soil, and on the moist surface of plants. Diatoms can occur in a more compact form as a soft, chalky, lightweight rock, called diatomite. Diatomite is used as an insulating material against both heat and sound, in making dynamite and other explosives, and for filters, abrasives, and similar products. Diatoms have deposited most of the earth’s limestone, and much petroleum is of diatom origin.


• Chlorophyta: These organisms commonly known as green algae. Chlorophyta are largely aquatic or marine, a few types are terrestrial, occurring on moist soil, on the trunks of trees, on moist rocks and in snow banks. Various species are highly specialized.


• Euglenophyta: Some euglenoids contain chloroplasts with the photosynthetic pigments; others are heterotrophic and can ingest or absorb their food. Reproduction occurs by longitudinal cell division. Most live in freshwater. The most characteristic genus is Euglena, common in ponds and pools, especially when the water has been polluted by runoff from fields or lawns on which fertilizers have been used. There are approximately 1000 species of euglenoids.



     

• Dinoflagellata: This is the large group of flagellate protistis. Reproduction for most dinoflagellates is asexual, through simple division of cells following mitosis. The dinoflagellates are important constituents of plankton, and as such are primary food sources in warmer oceans.


• Chrysophyta: These algae commonly called golden algae, found mostly in freshwater. Originally they were taken to include all such forms except the diatoms and multicellular brown algae, but since then they have been divided into several different groups based on pigmentation and cell structure. In many chrysophytes the cell walls are composed of cellulose with large quantities of silica. Formerly classified as plants, they contain the photosynthetic pigments chlorophyll a and c. Under some circumstances they will reproduce sexually, but the usual form of reproduction is cell division.


• Phaeophyta: These organisms commonly called brown algae. Many of the world's familiar seaweeds are members of phaeophyta. With only a few exceptions, brown algae are marine, growing in the colder oceans of the world, many in the tidal zone, where they are subjected to great stress from wave action; others grow in deep water. There are approximately 1500 species of phaeophyta.


• Rhodophyta: The photosynthetic organisms commonly known as red algae. Members of the division have a characteristic clear red or purplish color imparted by accessory pigments called phycobilins. There are 4000 known marine species of red algae; a few species occur in freshwater.


• Cyanobacteria: They are often referred to as blue-green algae, even though it is now known that they are not related to any of the other algal groups, which are all eukaryotes. Cyanobacteria may be single-celled or colonial. Depending upon the species and environmental conditions, colonies may form filaments, sheets or even hollow balls. Some filamentous colonies show the ability to differentiate into three different cell types. Despite their name, different species can be red, brown, or yellow; blooms (dense masses on the surface of a body of water) of a red species are said to have given the Red Sea its name.