The red algae are a large group of mostly multicellular, marine algae, including many notable seaweeds. Most of the coralline algae, which secrete calcium carbonate and have played a major role in building certain reefs, belong here. Red algae are a traditional part of some oriental cuisine, for instance as nori, and are used to make certain other products like agar and food additives.
The chloroplasts of red algae are bound by a double membrane, so presumably were acquired by direct endosymbiosis of cyanobacteria. Like most cyanobacteria, they are pigmented with chlorophyll a and various proteins called phycobilins, which gives them a distinctive red colour. The only other groups with primary chloroplasts are the green plants and glaucophytes, and it has been proposed that these groups all share a common origin. Most other algae have chloroplasts taken secondarily from one of these forms, and in particular a number of groups appear to have acquired them from red algae: heterokonts, haptophytes, cryptomonads, and dinoflagellates. These chloroplasts typically have chlorophylls a and c lack phycobilins, but show genetic similarities to those of this group.
Red algae have mitochondria with flat cristae, and undergo closed mitosis. Unlike most other algae, no flagella are found in any member of the group. Unicellular forms typically live attached to surfaces rather than floating among the plankton, and both the larger female and smaller male gametes are non-motile, so that most have a low chance of fertilization. Cell walls are made out of cellulose and thick gelatinous polysaccharides, which are the basis for most of the industrial products made from red algae.