phylum Rhodophyta
Most of the world's seaweeds belong to this group. Members of the division have a characteristic clear red or purplish color imparted by accessory pigments called phycobilins, unique to the red algae and the cyanobacteria. The chloroplasts of red algae are believed to be derived from cyanobacteria that formed an ancient symbiotic relationship with the algae. The coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong here.
Red algae
Red algae are red because of the presence of the pigment phycoerythrin; this pigment reflects red light and absorbs blue light. Because blue light penetrates water to a greater depth than light of longer wavelengths, these pigments allow red algae to photosynthesize and live at somewhat greater depths than most other "algae". Some rhodophytes have very little phycoerythrin, and may appear green or bluish from the chlorophyll and other pigments present in them. Red algae are ecologically significant as primary producers, providers of structural habitat for other marine organisms, and their important role in the primary establishment and maintenance of coral reefs. Some red algae are economically important as providers of food and gels. For this reason, extensive farming and natural harvest of red algae occurs in numerous areas of the world. Gracilaria is a genus of Rhodophyta notable for its economic importance as an agarophyte, as well as its use as a food for humans and various species of shellfish.