A polysaccharide that is a major constituent of the cell walls of certain red algae (Rhodophyceae), especially members of the families Gigartinaceae, Hypneaceae, Phyllophoraceae, and Solieriaceae. Extracted for its suspending, emulsifying, stabilizing, and gelling properties, it is one of three algal polysaccharides of major economic importance, the others being agar and alginate.