phycology, the study of algae, a large heterogeneous group of chiefly aquatic plants ranging in size from microscopic forms to species as large as shrubs or trees. The discipline is of immediate interest to humans because of algae’s importance in ecology. Certain algae, especially planktonic (i.e., floating or drifting) forms, constitute a vital segment of food chains. In coastal regions many large species of algae are supplementary food sources for humans. In industry some algae are sources of commercially valuable substances such as iodine, agar, carrageenan, alginic acid, and potash. Other alga products are used in insulating materials, bricks, scouring powder, and filters. Certain species are used in sewage-oxidation ponds.