Algae can exist as single cells, an example of which is Chlamydomonas, or joined together in chains like Spirogyra or made up of many cells, for instance Rhodymenia (red seaweed).
Filaments of the alga Spirogyra
Green algae can make their own food through a process of photosynthesis. They are at the beginning of the food chain and are known as primary producers.
© Eric Grave / Science Photo Library
Chlamydomonas is a unicellular green alga. It is motile and has two tail-like flagella that it uses for locomotion.
© Microfield Scientific Ltd / Science Photo Library
Algal blooms lining the shores (light green).
These blooms occur as a result of a change in the nutrient levels of the river. Contamination by sewage or fertilisers can increase the water's mineral content, which accelerates the growth of all plants, particularly aquatic algae.
© Bernhard Edmaier / Science Photo Library
A selection of diatoms
Diatoms are single-celled photosynthetic algae. Their cell walls contain a hard substance called silica.
© Steve Gschmeissner / Science Photo Library
Dulse (red) seaweed - Rhodymenia palmata
Rhodymenia palmata is an edible alga. Dulse is very popular in Ireland, where it is often mixed with potatoes and butter, adding a salty, savoury bite to fried potato champ.
© Dr Keith Wheeler / Science Photo Library
Most algae live in fresh or sea water where they can either be free-floating (planktonic) or attached to the bottom. Some algae can grow on rocks, soil or vegetation as long as there is enough moisture. A few algae form very close partnerships with fungi to form lichens. Unusual algal habitats are the hairs of the South American Sloth and Polar bears.
All algae contain a pigment called chlorophyll a (other types of chlorophyll such as b, c and / or d may also be present) and they make their own food by photosynthesis. The chlorophyll is contained in the chloroplasts and gives many algae their green appearance. However some algae appear brown, yellow or red because in addition to chlorophylls they have other accessory pigments that camouflage the green colour.
Diatoms a type of algae, are found floating in the phytoplankton of the seas. Their cell walls contain a hard substance called silica. When the diatoms die they sink to the floor. Their soft parts decay and the silica cell wall remains. Over time the pressure of the seawater pushes the silica together to form one large layer. This silica is mined from the seabed, crushed and used in abrasives and polishes such as toothpaste.