Seaweeds are multicellular marine plants which grow on inorganic nutrients (N, P, CO2…) and with light. In opposition to microalgae, which are unicellular plants, most of the seaweed is sedentary biomass. Seaweed generally grows attached to inert substrates (lithophyte) or on other seaweeds (epiphyte). Seaweeds can cover wide territories, often described as forests and compared to terrestrial systems. The need of light restricts the colonization of seaweed to the near shore. Indeed, seaweed is generally found in the littoral zone, from the high water mark until just below shorelines which are permanently submerged (30m deep).
Seaweeds present different types of structures, such as simple filaments measuring a few centimeters to fronds (leaves) measuring up to 20m long, as well a variety of colors. The different pigments in seaweed have allowed early scientists to class these plants into three categories: Brown algae, Green Algae and Red algae. Later, these three groups also happened to match evolution lineages which emerged from living organisms, other than bacteria. A total of 6000 red algal species, 1800 brown algal species and approximately 1200 green algal species are listed throughout the world.