The members of the Division Rhodophyta are commonly known as red algae, due to the presence of a water soluble red pigment—r-phycoerythrin, which completely covers the green colour of chlorophyll a. It is a large group with over 5,000 species, mostly marine. Red algae have the ability to live at greater depths in the ocean than do the other algal groups. They exhibit a high degree of epiphytism and parasitism with considerable specificity. The epiphytic species have normal form and pigmentation, but parasitic species show much reduction in their form and pigmentation. Polysiphonia lanosa, Ceratocolax hartzii and species of Harveyella, etc. are some parasitic species found growing on the members of other groups of algae.
Economic importance of red algae
Some red algae are of considerable economic importance. Species of Porphyra and Rhodymenia are used as food in coastal regions. Agar-agar, an important industrial product, is obtained from the species of Glelidium, Gracilaria, Chondrus and Gigartina. The greatest use of agar is in food, pharmaceutical and cosmetic industry. It is also used as stiffening agent in culture media. Another important industrial product, called carrageenan, obtained from the cell walls of the species of Chondrus and Gigartina is extensively used in balance of emulsions in paints and cosmetics. It is also used as a clearing agent in alcohol and sugar industry. So, the red algae has its greet importance to modern civilization.
The soft and slimy thalli of many red algae are the objects of beauty. The thallus ranges from simple unicellular to complex multiaxial forms . Some species are coralline; they secrete and deposit calcium carbonate over their walls and form coral-like structures. The photosynthetic pigments of red algae are chlorophyll a and d, α-and β-carotene, xanthophylls and biliproteins such as r-phycoerythrin and r-phycocyanin. The main storage product is floridean starch, occurring in the form of granules in the cytoplasm.
Reproduction of red algae
Red algae are an ancient group with no flagellated cells formed at any stage in their life cycle. Reproduction is mainly by asexual and sexual methods. Sexual reproduction is oogamous with or without elaborate post-fertilization stages, which depend whether reduction division of the diploid zygotic nucleus is delayed or takes place immediately after its formation. The male reproductive organ, called spermatangium, is a very simple structure which bears a single one-celled, uninucleate, non-motile male gamete, called spermatium . The female reproductive structure is carpogonium; it has a long neck-like structure and a bulbous base where the female nucleus is lodged. The red algae have very intricate post-fertilization stages and diploid phase. The earlier part of diploid phase is entirely reliant on the gametophytic individual and the latter part is represented by an independent sporophyte which may or may not be different from the gametophyte morphologically. The sporophyte bears tetrasporangia and tetraspores. Reduction division takes place during the sporogenesis in the sporangium of the independent sporophyte. Such forms exhibit isomorphic to heteromorphic alternation of generations.