Rhodophyta (redirected from rhodophytes)

Rhodophyta (rōdŏf`ətə), 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).

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Rhodophyta [rō′däf·əd·ə]

(botany)

The red algae, a large diverse phylum or division of plants distinguished by having an abundance of the pigment phycoerythrin.

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Rhodophyta 

a division of algae; red algae.

The red algae include unicellular and multicellular forms. All are characterized by the presence in the chromatophores (in addition to chlorophyll a, carotene, and xanthophyll) of chlorophyll d, R-phycoerythrin (a red pigment), and R-phycocyan (a blue-green pigment); by the absence of motile flagellate stages (zoospores and spermatozoa); and by a unique sexual process. The thalli are filamentous, bushy, or lamellar; in some, they are divided into stems and leaflike organs. In a number of species, the thalli are hardened because of calcium deposits (Lithothamnion). The color of the red algae varies from dark red (almost black) to pink, yellowish, and, (occasionally) steel blue. Reserve nutrients include red-algal starch, floridoside, and mannoglycer-ate.

The division Rhodophyta comprises 600 genera (3,750 species). It is divided into two classes: Bangiophyceae and Florido-phyceae. Asexual reproduction proceeds by means of nonmotile spores, which form in sporangia singly (monospores) or in groups of four (tetraspores). Sexual reproduction proceeds by the process of oogamy. Nonmotile gametes, called spermatia, form in the male sex organs, or spermatangia. The spermatia are carried in the water to the female sex organs, or carpogonia. After fertilization, the contents of the carpogonium, in the Bangiophyceae, break up into haploid carpospores; in the Floridophyceae, the carpogonium yields filamentous multicellular processes, on each end of which forms a carpospore. Alternatively, threads emerge from the carpogonium that join together with special large (auxiliary) cells of the thallus, to which are transferred the nuclei that form during the division of the fertilized egg of the carpogonium (one to each). The nuclei within the auxiliary cells do not merge; these cells, in turn, yield filamentous processes bearing carpospores.

Rhodophyta are predominantly marine, often flourishing at greater depths than green and brown algae, owing to the presence of phycoerythrin, which is apparently capable of utilizing green and blue light rays (which penetrate deeper in water than others) for photosynthesis. Less often, the red algae are found in fresh waters and soil. Fossil Rhodophyta have been found in Cretaceous deposits. Ahnfeltia, Gelidium, Phyllophora, and Fur-cellaria are of the greatest practical value, yielding jelling substances (agar, agaroid, and carrageenin). Some members of the group, such as Porphyra, are used as food.

REFERENCES

Kursanov, L. I. Burye i krasnye vodorosli. Moscow, 1927.
Zinova, E. S. “Vodorosli Iaponskogo moria: Krasnye vodorosli (Rhodophyceae).” Trudy Tikhookeanskogo komiteta, 1940, vol. 5.
Zinova, A. D. Opredelitel’ krasnykh vodoroslei severnykh morei SSSR. Moscow-Leningrad, 1955.
Kylin, H. Die Gattungen der Rhodophyceen. Lund, 1956.

IU. E. PETROV