Alga is a term used to refer to a large group of aquatic organisms, including SEAWEED, diatoms, KELP, AGAR, desmids, nostoc, and spirogyra. SEAWEED is a type of algae that grows in the ocean and can be used for food or fertilizer. Diatoms are microscopic single-celled algae that are found in both fresh and salt water. KELP is a type of large brown algae that grows in shallow coastal waters. AGAR is a gelatinous substance derived from certain types of algae and is used in the laboratory for culturing bacteria. Desmids are single-celled green algae that live in freshwater ponds and lakes. Nostoc is a type of blue-green algae that forms colonies in moist soil and on rocks. Spirogyra is a type of filamentous green algae that is found in freshwater ponds and streams.