15 Red Algae

Red algae are probably best known for their economic and ecological importance. For example, Porphyra yezoensis and some other species are grown in mariculture operations for use as human food, and Kappaphycus is among several genera that are cultivated or harvested for extraction of gel-forming agar, agarose, and carrageenan (see Chapter 4). Gelling polysaccharides, which play essential ecological roles in red algae, are widely used for laboratory cell-culture media, nucleic acids research, and food processing. Red algae are also increasingly appreciated as sources of compounds that evolved as defensive responses to microbial or herbivore attack that also have potential uses in human medicine.

Though several genera of red algae occur in freshwaters, thousands of red algae occur on tropical and temperate marine shores, where they play important ecological roles. For example, certain calcified red algae known as corallines help to build and maintain coral reefs, which harbor diverse organisms. Giant clams, banded coral shrimp, and clownfish are just a few of the fascinating animals that rely on reef habitats. By forming hard, flat sheets that consolidate and stabilize reef crests, coralline algae protect reefs from wave damage. For this reason, coralline red algae are regarded as keystone organisms, species whose decline (like the removal of the keystone from an arch) could cause the collapse of a larger-scale structure, in this case the loss of entire biotic communities. Fossil evidence indicates that coralline red algae have been playing this important role for hundreds of millions of years. In this chapter, we survey the evolutionary history, cell biology, body forms, intriguingly complex reproduction, ecology, and diversity of the red algae.