The word algae represents 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 and aguatic, but do not have true roots, stems, leaves, vascular tissue and have simple reproductive structures. They are distributed worldwide in the sea, in freshwater and in moist situations on land. Most are microscopic, but some are quite large, e.g. some marine seaweeds that can exceed 50 m in length.
The algae have chlorophyll and can manufacture their own food through the process of photosynthesis. 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.
Almost all the algae are eukaryotes and conduct photosynthesis within membrane bound structure called chloroplasts, which contain DNA. The exact nature of the chloroplasts is different among the different lines of algae.
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: unicellular organisms of the kingdom protista, characterized by a silica shell of often intricate and beautiful sculpturing. Most diatoms exist singly, although some join to form colonies. They are usually yellowish or brownish, and are found in fresh- and saltwater, in moist soil, and on the moist surface of plants. Fresh-water and marine diatoms appear in greatest abundance early in the year as part of the phenomenon known as the spring bloom, which occurs as a result of the availablity of both light and (winter-regenerated) nutrients. They reproduce asexually by cell division. When aguatic diatoms die they drop to the bottom, and the shells, not being subject to decay, collect in the ooze and eventually form the material known as diatomaceous earth. 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. The surface mud of a pond, ditch, or lagoon will almost always yield some diatoms.
Chlorophyta: division of the kingdom of protista consisting of the photosyntetic organism commonly known as green algae. The various species can be unicellular, multi-cellular, coenocytic (having more than one nucleus in a cell), or colonial. Chlorophyta are largely aguatic 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: small phylum of the kingdom protista, consisting of mostly unicellular aguatic algae. 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: large group of flagellate protistis. Some species are heterotrophic, but many are photosynthetic organisms containing chlorophyll. Various other pigments may mask the green of these chlorophylls. Other species are endosymbionts of marine animals and protozoa, and play an important part in the biology of coral reefs. Other dinoflagellates are colorless predators on other protozoa, and a few forms are parasitic. 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. Many forms are phosphorescent; they are largely responsible for the phosphorescence visible at night in tropical seas. There are approximately 2000 species of dinoflagellates.
Chrysophyta: large group of eukariotyes 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: phylum of the kingdom protista consisting of those organisms commonly called brown algae. Many of the world's familiar seaweeds are members of phaeophyta. Like the chrysophytes brown algae derive their color from the presence, in the cell chloroplasts, of several brownish carotenoid pigments, as fucoxathin. 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: phylum of the kingdom protista consisting of 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. The red algae are multicellular and are characterized by a great deal of branching, but without differentiation into complex tissues. Most of the world's seaweeds belong to this group. Although red algae are found in all oceans, they are most common in warm-temperate and tropical climates, where they may occur at greater depths than any other photosynthetic organisms. Most of the coralline algae, which secrete calcium carbonate and play a major role in building reefs, belong here. Red algae are a traditional part of oriental cuisine. There are 4000 known marine species of red algae; a few species occur in freshwater.
Cyanobacteria: phylum of prokaryotic aguatic bacteria that obtain their energy through photosynthesis. 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. There are two main sorts of pigmentation. Most cyanobacteria contain chlorophyll a, together with various proteins called phycobilins, which give the cells a typical blue-green to grayish-brown colour. A few genera, however, lack phycobilins and have chlorophyll b as well as a, giving them a bright green colour.
Unlike bacteria, which are heterotrophic decomposers of the wastes and bodies of other organisms, cyanobacteria contain the green pigment chlorophyll (as well as other pigments), which traps the energy of sunlight and enables these organisms to carry on photosynthesis. Cyanobacteria are thus autotrophic producers of their own food from simple raw materials. Nitrogen-fixing cyanobacteria need only nitrogen and carbon dioxide to live: they are able to fix nitrogen gas, which cannot be absorbed by plants, into ammonia (NH3), nitrites (NO2) or nitrates (NO3), which can be absorbed by plants and converted to protein and nucleic acids.
Cyanobacteria are found in almost every conceivable habitat, from oceans to fresh water to bare rock to soil. Cyanobacteria produce the compounds responsible for earthy odors we detect in soil and some bodies of water. The greenish slime on the side of your damp flowerpot, the wall of
your house or the trunk of that big tree is more likely to be cyanobacteria than anything else. Cyanobacteria have even been found on the fur of polar bears, to which they impart a greenish tinge. In short, Cyanobacteria have no one habitat because you can find them almost anywhere in the world.
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