(botany) A class of algae belonging to the division or subphylum Rhodophyta.

A large class of plants, commonly called red algae, coextensive with the division Rhodophycota. Most red algae are found in the ocean, growing on rocks, wood, other plants, or animals in the intertidal zone and to depths limited by the availability of light. A few genera and species occur in fresh water, and these are usually found in rapidly flowing, well-aerated, cold streams. Some, however, grow in quiet warm water, while a few are subaerial. Most red algae are photosynthetic, but some grow on other algae with varying degrees of parasitism. Approximately 675 genera and 4100 species are recognized. See also Algae.

Rhodophyceae are characterized by a unique combination of biochemical, reproductive, and ultrastructural features. The primary photosynthetic pigment is chlorophyll a. Water-soluble tetrapyrrolic compounds called phycobilins serve as accessory photosynthetic pigments. The chief food reserve, floridean starch, is a branched polymer of glucose similar to amylopectin of green plants. It occurs as granules in the cytoplasm. Multinucleate cells are common. Rhodophyceae are distinctive among eukaryotic algae in their lack of flagella, a feature shared among major groups only by the chlorophycean order Zygnematales. Some unicellular forms and many spores and male gametes of multicellular forms are capable of gliding or feeble ameboid motion. Unicellular red algae, which may form mucilaginous colonies, are considered primitive. Most red algae have multicellular thalli of microscopic or macroscopic size, including individual filaments, blades, and complex plants of distinctive form produced by the interplay of filamentous systems. See also Corallinales; Zygnematales.

Two subclasses of Rhodophyceae are traditionally recognized: the Bangiophycidae and the Florideophycidae. Classification within the Bangiophycidae is based largely on vegetative and asexual reproductive features, while that of the Florideophycidae is based primarily on details of the development of the female reproductive system and carposporophyte, secondarily on vegetative features. Rhodophyceae seem most closely related to Cyanophyceae in their use of chlorophyll a and phycobilins as photosynthetic pigments and the absence of flagella. They probably did not evolve directly from Cyanophyceae, however, but from a colorless, nonflagellate, eukaryotic ancestor that acquired pigments from an endosymbiotic blue-green alga.

The most salient feature of red algae is their beauty, which has drawn admiration from generations of seaside visitors. Their greatest significance, however, is their role in the formation of coral reefs, the Corallinales being responsible for cementing together various animal and algal components. Of more apparent economic importance is their use as food, a centuries-old tradition of maritime peoples in many parts of the world. See also Agar; Carrageenan; Reef.