One of the most important of all algal substances is agar, produced most abundantly by the red alga Gelidium. This substance, which has the consistency of gelatin, is used around the world in laboratories and medical institutions as a solidifier of nutrient culture media for the growth of bacteria. When other nutrients are added to it, it can also be used to solidify culture media for the growth of both plant and animal cells. Full-sized plants have been induced to develop from pollen grains sown on nutrient agar cultures. Orchid tissues are cultured commercially on such media and induced to grow into full-sized plants (as discussed in the section "Micropropagation" in Chapter 14), and its use in making the capsules containing drugs and vitamins is now worldwide. It is also used as an agent in bakery products to retain moistness, as a base for cosmetics, and as an agent in gelatin desserts to promote rapid setting.

Purple Laver
Figure 18.28 Beds of purple laver ("nori") ready to harvest in shallow marine water at Sendai, Japan. (Courtesy I. A. Abbott)

Chapter 18

Current research involving red algae and other seaweeds indicates they contain a number of substances of potential medicinal value. More than 20 seaweeds have been used in preparations designed for the expulsion of digestive-tract worms, control of diarrhea, and the treatment of cancer. Some have shown considerable potential as antibiotics and insecticides. Chemical relatives of DDT have been found to be produced by certain red algae. The sea hare and other marine animals feed on such algae, and it is possible that such animals may degrade (break down) the DDT-like compounds to simpler substances—a feat unknown among land animals.

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