cyanobacteria(redirected from cyanobacterium)
Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
A large and heterogeneous group of photosynthetic microorganisms, formerly referred to as blue-green algae. They had been classified with the algae because their mechanism of photosynthesis is similar to that of algal and plant chloroplasts; however, the cells are prokaryotic, whereas the cells of algae and plants are eukaryotic. The name cyanobacteria is now used to emphasize the similarity in cell structure to other prokaryotic organisms. See Algae, Cell plastids
All cyanobacteria can grow with light as an energy source through oxygen-evolving photosynthesis; carbon dioxide (CO2) is fixed into organic compounds via the Calvin cycle, the same mechanism used in green plants. Thus, all species will grow in the absence of organic nutrients. However, some species will assimilate organic compounds into cell material if light is available, and a few isolates are capable of growth in the dark by using organic compounds as carbon and energy sources. Some cyanobacteria can shift to a different mode of photosynthesis, in which hydrogen sulfide rather than water serves as the electron donor. Molecular oxygen is not evolved during this process, which is similar to that in purple and green photosynthetic sulfur bacteria. The photosynthetic pigments of cyanobacteria include chlorophyll a (also found in algae and plants) and phycobiliproteins. See Chlorophyll, Photosynthesis
Cyanobacteria are extremely diverse morphologically. Species may be unicellular or filamentous. Both types may aggregate to form macroscopically visible colonies. The cells range in size from those typical of bacteria (0.5–1 micrometer in diameter) to 60 μm.
When examined by electron microscopy, the cells of cyanobacteria appear similar to those of gram-negative bacteria. Many species produce extracellular mucilage or sheaths that promote the aggregation of cells or filaments into colonies.
The photosynthetic machinery is located on internal membrane foldings called thylakoids. Chlorophyll a and the electron transport proteins necessary for photosynthesis are located in these lipid membranes, whereas the water-soluble phycobiliprotein pigments are arranged in particles called phycobilisomes which are attached to the lipid membrane.
Several other types of intracellular structures are found in some cyanobacteria. Gas vesicles, which may confer buoyancy on the organisms, are often found in cyanobacteria that grow in the open waters of lakes. Polyhedral bodies, also known as carboxysomes, contain large amounts of ribulose bisphosphate carboxylase, the key enzyme of CO2 fixation via the Calvin cycle. Several types of storage granules may be found.
Cyanobacteria can be found in a wide variety of fresh-water, marine, and soil environments. They are more tolerant of environmental extremes than are eukaryotic algae. For example, they are the dominant oxygenic phototrophs in hot springs (at temperatures up to 72°C or 176°F) and in hypersaline habitats such as may occur in marine intertidal zones.
Cyanobacteria are often the dominant members of the phytoplankton in fresh-water lakes that have been enriched with inorganic nutrients such as phosphate. It is now known that high population densities of small, single-celled cyanobacteria occur in the oceans, and that these are responsible for 30–50% of the CO2 fixed into organic matter in these environments. About 8% of the lichens involve a cyanobacterium, which can provide both fixed nitrogen and fixed carbon to the fungal partner. See Lichens, Phytoplankton
Cyanobacteria are thought to be the first oxygen-evolving photosynthetic organisms to develop on the Earth, and hence responsible for the conversion of the Earth's atmosphere from anaerobic to aerobic about 2 billion years ago. This development permitted the evolution of aerobic bacteria, plants, and animals. See Bacteria, Prebiotic organic synthesis
a term used primarily in the microbiological literature since the 1970’s to denote the cyanophates (blue-green algae). The basis for introducing the term was a number of similarities between the Cyanobacteria and other prokaryotes, namely, bacteria. Both groups have similarities in their genetic structure, ribosomal and photosynthetic apparatus, and cell wall, having common chemical components, such as murein in the cell wall and poly-β-hydroxybutyrate as a reserve, and similar genetic properties.