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The red algae (Rhodophyta,, from Greek: ῥόδον (rhodon) = rose + φυτόν (phyton) = plant, thus red plant) are a large group of mostly multicellular, marine algae, including many notable seaweeds. Most of the coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong here. Red algae such as dulse and nori are a traditional part of European and Asian cuisine and are used to make other products like agar, carrageenans and other food additives.

Biology
Many red algae have multicellular stages but these lack differentiated tissues and organs. Unlike most other algae, no cells with a flagellum 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. They have 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.



The chloroplasts of red algae are bound by a double membrane, like those of green plants; both groups (Archaeplastida) probably share a common origin. Their plastids formed by direct endosymbiosis of a cyanobacteria, and in red algae are pigmented with chlorophyll a and various proteins called phycobiliproteins, which are responsible for their reddish color. Other reddish algae are not classified as red algae but as Chromista which are hypothesied to have acquired their chloroplasts from red algae through endosymbiosis.

Taxonomy
The diverse eukaryotes that constitute the red algae have been the focus of numerous recent molecular surveys and remain a rich source of undescribed and little known species for the traditional taxonomist. Molecular studies place the red algae in the Archaeplastida (Plantae sensu lato); however, supraordinal classification has been largely confined to debate on subclass vs. class level status for the two recognized subgroups, one of which is widely acknowledged as paraphyletic. This narrow focus has generally masked the extent to which red algal classification needs modification.

Below are two valid published taxonomies of the red algae, although neither necessarily has to be used, as the taxonomy of the algae is still in a state of flux. Note also that there is continued scientific debate as to whether the Rhodophyta should be included in the Kingdom Protista or the Kingdom Plantae. These two classification systems, which place the red algae in the plant kingdom, are shown in the table below.

Species
There are around 4,100 known species, with nearly all of them marine, and only 200 that live in freshwater. Some examples are:
 * Atractophora hypnoides
 * Gelidiella calcicola
 * Lemanea
 * Palmaria palmata
 * Schmitzia hiscockiana
 * Chondrus crispus
 * Mastocarpus stellatus

Red Algae as food
Several species are used as food. Dulse (Palmaria palmata) and Porphyra are perhaps the best known.