Rhodophyta , phylum (division) of the kingdom Protista consisting of the photosynthetic organisms commonly known as red algae . 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.

Of the approximately 4,000 known species of red algae, nearly all are marine; a few species occur in freshwater. Although red algae are found in all oceans, they are most common in warm-temperate and tropical climates, where they may occur at greater depths than any other photosynthetic organisms. The red algae are multicellular and are characterized by a great deal of branching, but without differentiation into complex tissues. The red algal cell wall has a firm inner layer containing cellulose and a mucilaginous or gelatinous outer layer. Cells may have one or more nuclei, depending on the species. Cell division is by mitosis . The red algae are remarkable in that they are not flagellated; none has motile cells of any kind.

Cells of the Rhodophyta possess chloroplasts that, in addition to the phycobilins, contain chlorophyll a, carotenes, and xanthophylls. At great ocean depths, where the wavelength of light available for photosynthesis is very different from that in shallow water, the phycobilins become more active than the chlorophylls in absorbing light; this fact may explain the ability of red algae to exist at depths of up to 879 ft (268 m). The carbohydrate reserves of red algae are in the form of floridean starch, a specialized glucose polymer of different structure than the starch of plants.

The life cycle of the red algae is extremely complex, involving one haploid phase and two diploid phases. Most marine red algae have soft and delicate bodies, or thalli; however, the coralline algae have thalli that become strongly calcified and contribute significantly to the growth of coral reefs in tropical seas. Because of the permanent nature of the structures that they produce, coralline algae have a rich fossil record that extends back as far as 700 million years. Commercial agar , used as a culture medium for bacteria and other organisms as well as for other purposes, is produced from several genera of red algae. The so-called Irish moss is the source of carrageenin, a substance widely used as a stabilizing agent in emulsions and in ice cream.

Bibliography: See H. C. Bold and M. J. Wynne, Introduction to the Algae: Structure and Reproduction (1985); C. A. Lembi and J. R. Waaland, Algae and Human Affairs (1988); C. van den Hoek, Algae: an Introduction to Phycology (1994).

Rhodophyta

The red algae phylum Rhodophyta synthesizes a class of water-soluble pigments termed phycobilins, known to be produced only by another algae, the Cryptomonads. There are approximately 6,000 species of Rhodophyta. Some of them are unicellular species that grow as filaments or membrane-like sheet cells, and some multicellular coralline species deposit calcium carbonate inside and around their cell walls, which are very similar in appearance to pink and red corals. Some Rhodophyta have an important role in coral-reef formation in tropical seas due to the deposits of calcium carbonate crystals they release in the environment, and are therefore termed coralline algae.

Rhodophyta are ancient algae whose fossil remains are found under the form of coralline algal skeletons in limestone deposits of coral reef origin dating back to the Precambrian Era. They use the blue spectrum of visible light to accomplish photosynthesis that allows them to live in deep waters, storing energy under the form of Floridean starch. They make mostly chlorophyll-a, and the pigments alpha and beta-carotene, phycoerythrin, as well as others similar to those made by Cyanobacteria, such as allophycocyanin and r-phycocyanin. The cell walls are made mainly of cellulose (but some species use xylan), and colloidal substances, such as agars and carageenan; and the cells may be multinucleated. The Floridean starch, a carbohydrate molecule consisting of 15 units of glucose, is kept free in the cytoplasm , whereas in other algae it is attached to the chloroplast . Some species are consumed by humans such as the Japanese nori (Porphyra ) and others are utilized as components in processed food and by the pharmaceutical industries, such as Chondrus, and Gelidium.

See also Blue-green algae; Petroleum microbiology; Protists; Xanthophylls

Rhodophyta (red algae) A group of Eukarya which are mostly red in colour; no flagellated cells are formed, and the storage product is a type of starch known as floridean starch. Sexual reproduction tends to be complicated. Red algae may be unicellular, but most are filamentous or membranaceous. The majority occur in the sea, but some are freshwater or terrestrial. The group includes 1 class (Rhodophyceae) with many orders and more than 4000 species. The red seaweeds are more numerous than green and brown seaweeds in temperate and tropical regions, but less numerous in colder regions. Several red seaweeds yield useful products (e.g. agar, algin, and carragheen). The red algae are believed to be one of the oldest groups of eukaryotic algae.

Rhodophyta (red algae) A phylum of algae that are often pink or red in colour due to the presence of the pigments phycocyanin and phycoerythrin. Members of the Rhodophyta may be unicellular or multicellular; the latter form branched flattened thalli or filaments. They are commonly found along the coasts of tropical areas. Sexual reproduction is by means of carpospores (see carpogonium).