Algae are aquatic organisms that carry out photosynthesis. They live in fresh and salt water, and can also live in or on plants and in the soil if there is enough moisture. Many algae consist of a single cell and can be as tiny as 1 micrometre (0.00004 in) in diameter, but many join together to form filaments, spirals, or other groupings. Most seaweeds are also algae, including the giant kelp, which can grow to over 60 metres (200 ft) long. Through the process of photosynthesis, algae produce much of the world’s oxygen and absorb a great deal of carbon dioxide, which as most people are now aware, is a greenhouse gas.
Photosynthesis is the process carried out by the green pigment chlorophyll. In this process carbon dioxide and water are turned into simple sugars (used for food) and oxygen (released into the water), using sunlight as the energy source. It is the process by which plants and algae grow and develop, and the chlorophyll is the reason most plants and many algae are green. Some algae contain other pigments as well as chlorophyll, and these algae range from golden to brown and red in colour.
As in plants, the chlorophyll in photosynthetic algae is contained in bodies within the cell called chloroplasts. These may have originally been photosynthetic bacteria that entered into a symbiotic relationship with algae, in a similar way to mitochondria, which are the energy plants in animal cells.
The photosynthesis carried out by algae produces much of the world’s oxygen, some of which is used by fish and other sea creatures, and some of which escapes into the air. It also transforms carbon dioxide dissolved in water into simple sugars that are used as food by the algae themselves. In turn, the algae become food for the other creatures above them in the food chain. In this way the photosynthetic algae are a fundamental source of food on the planet, being autotrophic, which means they create their own food from sunlight, water and CO2.
Algae populations sometimes increase quickly and their population explodes in a condition known as algal bloom. One well-known example is the red algae called dinoflagellates which can bloom and turn the sea red. Algal blooms can kill fish because large numbers of the algae die and sink to the bottom, where oxygen is used in breaking them down. So much oxygen can be used that there is very little oxygen left for fish to breathe. (Incidentally, the blue-green algae which also bloom are actually bacteria and not algae at all.)
The algae are diverse and wide-ranging. They are loosely grouped into the Kingdom Protista, but their classification is still a subject of contention since new information is constantly being discovered, especially through DNA analysis. Some of the large algae such as giant seaweeds are also sometimes classified as plants. Whatever their classification, however, the algae are extremely important as the producers of oxygen and in turning carbon dioxide and water into food for themselves and all the myriad creatures above them in the food chain.