Cyanobacteria is any of a large, heterogeneous group of prokaryotic, principally photosynthetic organisms. Cyanobacteria resemble the eukaryotic algae in many ways, including morphological characteristics and ecological niches, and were at one time treated as algae, hence the common name of blue-green algae. Algae have since been reclassified as protists, and the prokaryotic nature of the blue-green algae has caused them to be classified with bacteria in the prokaryotic kingdom Monera.
these membrane-bound organelles are carried out in prokaryotes by various parts of the cell, such as the cell membrane. Some cyanobacteria, especially planktonic forms, have gas vesicles that contribute to their buoyancy. Chemical, genetic, and physiological characteristics are used to further classify the group within the kingdom. Cyanobacteria may be unicellular or filamentous. Many have sheaths to bind other cells or filaments into colonies.
Cyanobacteria contain only one form of chlorophyll, chlorophyll a, a green pigment. In addition, they contain various yellowish carotenoids, the blue pigment phycobilin, and, in some species, the red pigment phycoerythrin. The combination of phycobilin and chlorophyll produces the characteristic blue-green colour from which these organisms derive their popular name. Because of the other pigments, however, many species are actually green, brown, yellow, black, or red.
Most cyanobacteria do not grow in the absence of light (they are obligate phototrophs); however, some can grow in the dark if there is a sufficient supply of glucose to act as a carbon and energy source.
In addition to being photosynthetic, many species of cyanobacteria can also “fix” atmospheric nitrogen–that is, they can transform the gaseous nitrogen of the air into compounds that can be used by living cells. Particularly efficient nitrogen fixers are found among the filamentous species that have specialized cells called heterocysts. The heterocysts are thick-walled cell inclusions that are impermeable to oxygen; they provide the anaerobic (oxygen-free) environment necessary for the operation of the nitrogen-fixing enzymes. In Southeast Asia, nitrogen-fixing cyanobacteria often are grown in rice
paddies, thereby eliminating the need to apply nitrogen fertilizers.
Cyanobacteria range in size from 0.5 to 60micrometres, which represents the largest prokaryotic organism. They are widely distributed and are extremely common in fresh water, where they occur as members of both the plankton and the benthos. They are also abundantly represented in such habitats as tide pools, coral reefs, and tidal spray zones; a few species also occur in the ocean plankton. On land, cyanobacteria are common in soil down to a depth of 1 m (39 inches) or more; they also grow on moist surfaces of rocks and trees, where they appear in the form of cushions or layers.
Cyanobacteria flourish in some of the most inhospitable environmentsknown. They can be found in hot springs, in cold lakes underneath 5 m of ice pack, and on the lower surfaces of many rocks in deserts. Cyanobacteria are frequently among the first colonizers of bare rock and soil. Various types of associations take place between cyanobacteria and other organisms. Certain species, for example, grow in a mutualistic relationship with fungi, forming composite organisms known as lichens.
Cyanobacteria reproduce asexually, either by means of binary or multiple fission in unicellular and colonial forms or by fragmentation and spore formation in filamentous species. Under favourable conditions, cyanobacteria can reproduce at explosive rates, forming dense concentrations called blooms. Cyanobacteria blooms can colour a body of water. For example, many ponds take on an opaque shade of green a result of overgrowths of cyanobacteria, and blooms of phycoerythrin-rich species cause the occasional red colour of the Red Sea. Cyanobacteria blooms are especially common in waters that have been polluted by nitrogen wastes; in such cases, the overgrowths of cyanobacteria can consume so much of the water’s dissolved oxygen that fish and other aquatic organisms perish. Two spicies, which fall under this category, are Anabaena and Microcystis.
Grass Green Algae are members of the division Chlorophyta, comprising about 6,000 species. The photosynthetic pigments (chlorophylls a and b, carotene, and xanthophyll) are in the same proportions as those in seed and other higher plants. The typical green algal cell, which can be motile or nonmotile, has a central vacuole, pigments contained in plastids that vary in shape in different species, and a two-layered cellulose and pectin cell wall. Food is stored as starch in pyrenoids (proteinaceous cores within the plastids). Green algae, variable in size and shape, include single-celled (Chlamydomonas, desmids), colonial (Hydrodictyon, Volvox), filamentous (Spirogyra), and tubular (Actebaularia, Caulerpa) forms. Sexual reproduction is common, with gametes that have two or four whiplike flagella. Asexual reproduction is by cell division (Protococcus), motile and nonmotile spores (Ulothrix, Oedogonium), and fragmentation.
Most green algae occur in fresh water, usually attached to submerged rocks and wood or as scum on stagnant water; there are also terrestrial and marine species. Free-floating microscopic species serve as food and oxygen sources for aquatic organisms. Green algae are also important in the evolutionary study of plants; the single-celled Chlamydomonas is considered similar to the ancestral form that probably gave rise to land plants. Six spicies, which fall under this category, include Cyclotella, Spirogyra, Ulothrix, Cladophora, Closterium and Chlorella.
Golden Algae are simple organisms constituting the phylum Chrysophyta of the kingdom Protista . About 10,000 species exist of these mostly single-celled freshwater and marine algae . The chrysophytes are characterized by their yellowish xanthophyll pigments, which mask the green of the chlorophyll that is also present. Most have cell walls containing silica or calcium. Three classes are widely recognized (sometimes as separate phyla): Chrysophyceae, the yellow-brown algae; Xanthophyceae, the yellow-green algae; and Bacillariophyceae, the diatoms. They are characterized by the pigment fucoxanthin and oil droplets as the food-reserve Sexual reproduction is rare; a sexual reproduction is by the formation of motile and nonmotile spores and by cell division. Five species that are in this category include Tabellaria, Flagilaria, Melosira, Asterionella and Stephanodiscus.