Phycology is the scientific study of algae. Phycology is a branch of life science and often is regarded as a subdiscipline of botany. Algae are important plants as primary producers in aquatic ecosystems. Most algae are eukaryotic, photosynthetic organisms that live in a wet environment. They are distinguished from the higher plants by a lack of true roots, stems or leaves. They do not flower. Many species are single-celled and microscopic (including phytoplankton and other microalgae); many others are multicellular to one degree or another, some of these growing to large size (for example, seaweeds such as kelp and Sargassum). Phycology includes the study of prokaryotic forms known as blue-green algae or cyanobacteria. A number of microscopic algae also occur as symbionts in lichens. These are very large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelp (large brown alga), that may grow up to 50 meters in length. Algae carry out almost half of the photosynthesis on earth - they fix 105,000,000,000 metric tonnes of carbon per year, and produce every second breath of oxygen. Algae are primary producers that support a diverse range of ecosystems: e.g. algae are essential to the growth of coral reefs, they form gigantic undersea kelp forests, and they dominate the marine phytoplankton. Alga products are used in insulating materials, bricks, scouring powder, and filters. Certain species are used in sewage-oxidation ponds. Other kinds of algae on the other hand are distinct from green plants and from each other in having different and unrelated accessory pigments. These pigments are responsible for the ways different algae absorb light, providing advantage to each individual type of alga to compete best at a water depth where its preferred wavelength is perhaps strongest.