Of the approximately 6000 species, most red algae are marine; only a few occur in freshwater. Rhodophytes are usually multicellular and grow attached to rocks or other algae, but there are some unicellular or colonial forms. They do not have flagellated cells, are structurally complex, and have complex life cycles divided into three phases. Many red algae feature pit connections between the cells, and their cell walls include a rigid component composed of microfibrils and a mucilaginous matrix. Agar and carrageenin are two red algal mucilages that are widely used for gelling and thickening purposes in the food and pharmaceutical industries.
Rhodophytes contain chlorophyll a which is masked by phycobilin pigments bound to proteins. The chloroplasts in red algae resemble Cyanobacteria both biochemically and structurally. Food reserves are stored outside of the chloroplasts as Floridean starch.
The coralline red algae deposit calcium carbonate in their cell walls, making them particularly tough and stony. They are often abundant, ecologically important, and widespread from the arctic regions to the tropics and play an important role in building tropical reef communities. Coralline red algae can form an algal ridge that absorbs wave energy and thereby protects the more delicate organisms that inhabit the sheltered lagoons and back-reef habitats.
Research scientist and curator Dr. Walter H. Adey has extensively collected the crustose corallines throughout the world since the 1960's, most notably in the North Atlantic, Caribbean and Hawaii. The specimens from the northwest Atlantic, north Norway and Iceland have been curated and inventoried during 2000-2001 with the assistance of Sue Lutz (Research and Curatorial Assistant). The specimens from Japanese regions, as well as the remaining European collections, are currently being curated and inventoried.
Some red algae are of economic importance, either as food (Porphyra) or as producers of secondary products (Gelidium, Gracilaria, Chondrus, etc.) used in the food and drug industries. (See additional comments under Economic Uses of Algae.)
Additional introductory information about this algal group may be found at the University of California - Museum of Paleontology.