Cyanobacteria, commonly known as blue-green algae, are members of the Kingdom Monera. Members of this kingdom lack a nuclear region clearly defined by a membrane or envelope and are thus known as prokaryotes. Members of the blue-green algae are among the oldest fossils ever found, dating back to over three billion years.
Cyanobacteria are photosynthetic unicellular or colonial algae. They contain chlorophyll a and other accessory pigments, but these are not located in distinct chloroplasts. Instead, they are distributed throughout the cell on flat organelles called thylakoids. Accessory pigments in this group include b- carotenes, xanthophylls, and phycobilins. One type of phycobilin is phycocyanin, which gives the characteristic bluish-green color, and hence their common name. Their cell wall is composed of mucopeptides. Many species also have a gelatinous sheath around their cells. Some species live as single-celled individuals like the genus Sphaerocystis, while others grow in colonies like the genera Anabaena and Oscillatoria. Sexual reproduction is rare. Most blue-green algae simply divide asexually by binary fission, or divide by fragmentation of the filaments if they are colonial.
Cyanobacteria are a successful group because they grow in a diversity of habitats. Some species grow on moist tree bark and soil. A few genera like Nostoc develop a mutualistic relationship with their fungal symbionts and grow as lichens. Many species of blue-green algae grow in freshwater and marine habitats.
Some genera such as Aphanizomenon and Anabaena are able to produce ammonia from nitrogen gas using the enzyme nitrogenase, a process known as nitrogen fixation. This process is sometimes done in specialized enlarged cells called heterocysts. Other species form the characteristic coloration seen in the upper spray regions of intertidal zones. Those species that live in shallow coastal areas can form dense mounds known as stromatolites. Some species proliferate in the near-boiling temperatures of thermal hot springs, while others tolerate the frigid temperatures found in Antarctic lakes. More commonly, many species of blue-green algae form dense mats floating on the surface of eutrophic lakes in the summer months. They are able to float on the surface using specialized gas vacuoles. Such species are often considered to be indicators of eutrophication. Spirulina is an example of blue-green algae that commonly grows in dense blooms in African lakes. Filter-feeding fish such as Tilapia thrive on consuming this alga. Humans have also found them to be a valued health food because they are high in protein, vitamins, and linolenic acid. Aquaculturists in Mexico, Israel, and California have learned to cultivate Spirulina. Some species of blue-green algae produce a toxin that can inhibit filter-feeding zooplankton such as Daphnia. These toxins consist of various types of alkaloids, polypeptides, pteridines, and lipopolysaccharides, which can be lethal to cattle at high concentrations. There have even been reported cases of humans becoming sick after drinking this water. The bad taste in residential drinking water can often be attributed to the chemicals geosmin and methyl-isoborneol (MIB) released by some blue-green algae and fungi. These toxins are released when the cells die and decay.
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