The term blue algae is commonly used for the blue-green algae of the phylum Cyanobacteria, also known as blue-green bacteria, cyanobacteria or Cyanophyta. They have traditionally been viewed as algae since they live in water and carry out photosynthesis, but recent works typically exclude them from the algae group and view them as photosynthesising bacteria instead. Blue algae are more closely related to other bacteria than to algae. Unlike algae, blue algae (cyanobacteria) have cell walls containing peptidoglycan (a polymer consisting of sugars and amino acids) and no membrane-bound organelles. Another notable difference is that a blue algae contains a single circular chromosome and carries out photosynthesis using thylakoid membranes instead of chloroplast.
Blue algae can survive in virtually all types of habitat as long as there is moist and sunlight. A majority of the known species inhabits freshwater, but you can find plenty of blue algae in the ocean and in brackish environments as well. Only a tiny amount of water is necessary and this is why you can come a cross blue algae in the damp fur of a sloth as well as on the surface of a temporarily moistened rock in the desert.
An interesting feature with the blue algae is its capability of living inside other organism. Several species of blue green algae are endosymbionts, i.e. they will live inside the body or cells or another organism. The host organism gains from this relationship since the blue algae will provide it with energy from the photosynthesis. Blue green algae can for instance be found inside certain plants, lichens, sponges and protists.
Blue algae is normally blue-green in colour, but can also be reddish brown. The name cyanobacterium is derived from the Greek word for “blue”: κυανός. Some species of blue algae will change their colour depending on the colour of the light to make photosynthesis more efficient. Red light will lead to an increased production of the substances that make the algae green, while green light will lead to an increased production of the substances that makes the algae red.
Some types of blue algae live alone, while others form colonies. A colony can for instance look like a blue-green sheet on the water, long thread-like filaments or a hallow ball. Some colonies are advanced enough to produce more than one type of cells and this is naturally a very big step in terms of evolution. Some advanced colonies can produce so called akinetes to survive harsh environmental conditions such as drought. The akinetes are highly resistant spores that can stay dormant for long periods of time until the environment becomes more favourable for blue algae again.
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