The red coloration of the rhodophytes is due to the pigments phycoerythrin and phy-cocyanin, which mask the chlorophylls present, in this case a and d. The biggest single difference between the red algae and the other groups we have looked at so far is that they lack flagella at any stage of their life cycle. Thus they are completely lacking in any motile forms, even in the reproductive stages; the gametes rely on being passively dispersed. Almost all the red algae are multicellular marine species, inhabiting habitats ranging from shallow rock pools to the ocean's deeps.
Life cycles vary considerably, and may be quite complex, with variations on the alternation of generations theme. Several species of the more primitive red algae reproduce
asexually by releasing spores into the water. These attach to an appropriate substrate and mature into an adult.
Red algae are the source of several complex polysaccharides of commercial value. Agar and agarose are used in the laboratory in microbial growth media and electrophoresis gels respectively, whilst carrageenan is an important thickening agent in the food industry. In addition, Porphyra species are cultivated in Japan for use in sushi dishes.
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