Algae

Algae are a large and varied group of photosynthetic organisms, mostly aquatic, that range from single-celled plankton to giant kelp tens of meters long. They are not a single evolutionary group, because the lineages called green, red and brown algae are separately related to other organisms, and cyanobacteria, sometimes called blue-green algae, are in fact bacteria.

Green algae are the ancestors of land plants, and red algae such as dulse and laver are used as food. Brown algae include the large seaweeds giant kelp, bull kelp and bladderwrack. Diatoms and other single-celled forms make up much of the phytoplankton. Algae lack true roots, stems and leaves, and they reproduce by spores and by gametes as well as by simple division.

Algae carry out about half of the photosynthesis on Earth and supply much of the oxygen in the air, forming the base of most aquatic food webs. Their activity moves carbon into the deep sea through the biological pump. Many live in partnership with others: the algae inside corals are the reason for coral bleaching when they are lost.

Excess nutrients cause rapid growth known as algal blooms, a feature of eutrophication, and some blooms release toxins or use up oxygen as they decay. People use algae for food, fertilizer, thickeners such as agar and carrageenan, and as sources of oils and fuel in biotechnology. Algae are grown in a bioreactor for such products.