The Rhodophyceae, or red algae, comprise the only class in the division Rhodophyta. The Rhodophyceae are probably one of the oldest groups of eukaryotic algae. The red algae are most likely directly descended from a cyanome in the Glaucophyta (see Chapter 3). It is likely that the first red alga evolved into an ecological niche that was unoccupied by cyanobacteria, the only extant photosynthetic alga that evolved oxygen. This ecological niche would have been in waters with a pH less than 5, which, for some unknown reason, cyanobacteria are not able to inhabit (Brock, 1973). Indeed, modern phylogenetic studies utilizing nucleic-acid sequencing have shown that Cyanidium, an alga that lives in acidic waters, is probably the oldest extant red alga (Oliveira and Bhattacharya, 2000).

The Rhodophyceae lack flagellated cells, have chlorophyll a, phycobiliproteins, floridean starch as a storage product, and thylakoids occurring singly in the chloroplast.

A majoity of seaweeds are red algae, and there are more Rhodophyceae (about 4000 species) than all of the other major seaweed groups combined. Although marine red algae occur at all latitudes, there is a marked shift in their abundance from the equator to colder seas. There are few species in polar and subpolar regions, where brown and green algae predominate, but in temperate and tropical regions they far outnumber these groups. The average size of the plants also differs according to geographical region. The larger species of fleshy red algae occur in cool–temperate areas, whereas in tropical seas the Rhodophyceae (except for massive calcareous forms) are mostly small, filamentous plants. The Rhodophyceae also have the ability to live at greater depths in the ocean than do members of the other algal classes. They live at depths as great as 200 m, an ability related to the function of their accessory pigments in photosynthesis. About 200 species of Rhodophyceae occur in freshwater, where they do not reach as great a size as the red seaweeds (Skuja, 1938). The majoity of freshwater red algae occur in running waters of small to midsized streams (Sheath and Hambrook, 1988). Few red algae occur at currents of less than 30 cm s −. This fast flow probably favors red algae because loosely attached competitors are washed out and because of a constant replenishment of nutrients and gases.