- Also called:
- cyanobacteria
- On the Web:
- Rutgers Cooperative Extension - Cyanobacteria (Blue-Green Algae) in Waterways (Nov. 07, 2026)
Blue-green algae
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What are blue-green algae?
Blue-green algae, also known as cyanobacteria, are a large and diverse group of photosynthetic prokaryotic organisms. They were once classified as algae but are now grouped with bacteria in the kingdom Monera due to their prokaryotic nature.
Like other prokaryotes, cyanobacteria lack membrane-bound organelles. They contain chlorophyll a, carotenoids, and phycobilins, which give them their characteristic blue-green color, though they can also be green, brown, yellow, black, or red. Many can also fix atmospheric nitrogen, converting it into usable compounds.
Cyanobacteria are widespread in various environments, including fresh water, tide pools, and soil, and can even thrive in extreme conditions like hot springs and cold lakes. They reproduce asexually and can form dense concentrations called blooms.
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Where can blue-green algae be found?
Blue-green algae, also known as cyanobacteria, are widespread and common in various habitats. They are frequently found in fresh water as plankton and benthos, as well as in tide pools, coral reefs, and tidal spray zones. A few species can also be found in the ocean plankton.
On land, blue-green algae are common in soil down to 1 m (39 inches) or more. They also thrive on moist surfaces of rocks and trees, forming cushions or layers. They can flourish in extreme environments such as hot springs, cold lakes under ice, and desert rocks. They are often among the first colonizers of bare rock and soil and can form mutualistic relationships with fungi, creating lichens.
Under favorable conditions, blue-green algae can reproduce rapidly, forming dense concentrations called blooms, which can color bodies of water. These blooms are especially common in waters polluted by nitrogen wastes, where they can deplete oxygen and harm aquatic life.
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How do blue-green algae get their energy?
Blue-green algae, also known as cyanobacteria, are mostly photosynthetic organisms that use chlorophyll a, a green pigment, to capture light energy from the Sun. They also contain carotenoids and phycobilins, which contribute to their color and also help in light absorption.
Like plants and eukaryotic algae, cyanobacteria use water as an electron source in photosynthesis, which generates oxygen. Many cyanobacteria cannot grow without light, but some can grow in the dark if glucose is available as a carbon and energy source. Cyanobacteria utilize light-absorbing pigments organized in photosystems within intracellular thylakoids to convert solar energy into chemical energy.
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Why are blue-green algae sometimes called cyanobacteria?
Blue-green algae are also known as cyanobacteria because they are prokaryotic organisms that share similarities with eukaryotic algae. They possess only one form of chlorophyll, chlorophyll a, a green pigment; carotenoids, which are various yellowish pigments; and phycobilin, a blue pigment. The combination of phycobilin and chlorophyll gives them a characteristic blue-green color.
Cyanobacteria were once classified as algae but were reclassified as bacteria in the kingdom Monera due to their prokaryotic nature. Cyanobacteria were likely the first oxygen-producing cells and are believed to have greatly increased the oxygen content of the atmosphere.
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How do blue-green algae help the environment?
Blue-green algae, also known as cyanobacteria, are photosynthetic organisms that play a vital role in the environment. They can convert atmospheric nitrogen into compounds usable by living cells, reducing the need for synthetic nitrogen fertilizers. They are also among the first colonizers of bare rock and soil and can form mutualistic relationships with fungi to form lichens. In some parts of the world, they are harvested for human and animal consumption.
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What problems can occur when blue-green algae grow too much in lakes and ponds?
When blue-green algae, or cyanobacteria, experience rapid growth, several problems can arise in lakes and ponds.
Dense concentrations of blue-green algae, known as algal blooms, can block sunlight from reaching underwater plants, which inhibits photosynthesis and can lead to the death of these plants. Additionally, these blooms can clog the gills of fish, causing suffocation.
Furthermore, the decomposition of large algal blooms can deplete water oxygen levels, resulting in the death of fish and other aquatic animals. Some species of blue-green algae produce toxins, which can render shellfish unsafe for human consumption and harm birds and marine mammals. These toxins can also cause skin and lung irritation in humans.
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Blue-green algae, 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.
Like all other prokaryotes, cyanobacteria lack a membrane-bound nucleus, mitochondria, Golgi apparatus, chloroplasts, and endoplasmic reticulum. All of the functions carried out in eukaryotes by these membrane-bound organelles are carried out in prokaryotes by the bacterial 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.
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Most cyanobacteria do not grow in the absence of light (i.e., 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 60 micrometres, 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 environments known. 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.
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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 as 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.